NATL INST OF STAND S TECH

A111D7 ESfibfifl

o

<n Q

J

NBS SPECIAL PUBLICATION 260-77

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

tandard Reference Materials:

Application of

Some Metal SRM'sas

Thermometric Fixed Points

•QC

100

.157

No ,260-7;

1982
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act ol Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Chemical Physics —

Analytical Chemistry — Materials Science

THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Manufacturing

Engineering — Building Technology — Fire Research — Chemical Engineering2

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303.
Standard Reference Materials: National Bureau of standards

Library, E-01 Admin. Bldg.

Application of Some Metal SRM's

as Thermometric Fixed Points

OCT * 5 1982

An Llu

* Ll 6 7

no

George T. Furukawa

John L. Riddle

William R. Bigge

Earl R. Pfeiffer

Temperature and Pressure

Measurements and Standards Division

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

^ of

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued August 1982
Library of Congress Catalog Card Number: 82-600578

National Bureau of Standards Special Publication 260-77

Natl. Bur. Stand. (U.S.), Spec. Publ. 260-77, 140 pages (Aug. 1982)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1982

For Sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402.

Price $6.00

(Add 25 percent for other than U.S. mailing.)
PREFACE

Standard Reference Materials (SRM's) as defined by the

National Bureau of Standards are we 1 1 -characteri zed materials

produced in quantity and certified for one or more physical

or chemical properties. They are used to assure the accuracy

and compatibility of measurements throughout the Nation.

SRM's are widely used as primary standards in many diverse

fields in science, industry, and technology, both within the

United States and throughout the world. They are also used

extensively in the fields of environmental and clinical analysis. In many applications, traceability of quality control

and measurement processes to the national measurement system

are carried out through the mechanism and use of SRM's. For

many of the Nation's scientists and technologists it is

therefore of more than passing interest to know the details

of the measurements made at NBS in arriving at the certified

values of the SRM's produced. An NBS series of papers, of

which this publication is a member, called the NBS Special

Publication - 260 Series , is reserved for this purpose.

This 260 Series is dedicated to the dissemination of

information on different phases of the preparation, measurement, certification and use of NBS-SRM's. In general, much

more detail will be found in these papers than is generally

allowed, or desirable, in scientific journal articles. This

enables the user to assess the validity and accuracy of the

measurement processes employed, to judge the statistical

analysis, and to learn details of techniques and methods

utilized for work entailing the greatest care and accuracy.

These papers also should provide sufficient additional information not found on the certificate so that new applications

in diverse fields not foreseen at the time the SRM was originally issued will be sought and found.

Inquiries concerning the technical content of this paper

should be directed to the authors. Other questions concerned

with the availability, delivery, price, and so forth will

receive prompt attention from:

Office of Standard Reference Materials

National Bureau of Standards

Washington, D.C. 20234

George A. Uriano, Chief

Office of Standard Reference Materials

i i i
OTHER NBS PUBLICATIONS IN THIS SERIES

Catalog of NBS Standard Reference Materials

(1981-83 edition), R. W. Seward, ed., NBS

Spec. Publ. 260 (November 1981).

Michaelis, R. E., and Wyman, L. L. Standard

Reference Materials: Preparation of White Cast

Iron Spectrochemical Standards. NBS Misc.

Publ. 260-1 (June 1964). COM 74- 1 1 06 1 **

Michaelis, R. E., Wyman, L. L., and Flitsch, R., Standard Reference Materials: Preparation of

NBS Copper-Base Spectrochemical Standards.

NBS Misc. Publ. 260-2 (October 1964). COM74-

11063**

Michaelis, R. E., Yakowitz, H., and Moore, G. A., Standard Reference Materials: Metallographic

Characterization of an NBS Spectrometric LowAlloy Steel Standard. NBS Misc. Publ. 260-3

(October 1964). COM74-1 1060**

Hague, J. L. Mears, T. W., and Michaelis, R. E., Standard Reference Materials: Sources of

Information, NBS Misc. Publ. 260-4 (February

1965). COM74-11059

Alvarez, R., and Flitsch R., Standard Reference

Materials: Accuracy of Solution X-Ray Spectrometric Analysis of Copper-Base Alloys. NBS

Misc. Publ. 260-5 (March 1965). PB168068**

Shultz, J. L, Standard Reference Materials:

Methods for the Chemical Analysis of White

Cast Iron Standards, NBS Misc. Publ. 260-6

(July 1975). COM74-1 1068**

Bell, R. K., Standard Reference Materials:

Methods for the Chemical Analysis of NBS

Copper-Base Spectrochemical Standards. NBS

Misc. Publ. 260-7 (October 1965). COM74-

11067**

Richmond, M.S., Standard Reference Materials:

Analysis of Uranium Concentrates at the

National Bureau of Standards. NBS Misc. Publ.

260-8 (December 1965). COM74-1 1066**

Anspach, S. C, Cavallo, L. M. Garfinkel, S. B. Hutchinson, J. M. R., and Smith, C. N., Standard Reference Materials: Half Lives of Materials

Used in the Preparation of Standard Reference

Materials of Nineteen Radioactive Nuclides

Issued by the National Bureau of Standards

NBS Misc. Publ. 260-9 (November 1965).

COM74-1 1065**

Yakowitz, H., Vieth, D. L., Heinrich, K. F. J., and

Michaelis, R. E., Standard Reference Materials:

Homogeneity Characterization on NBS Spectrometric Standards II: Cartridge Brass and LowAlloy Steel, NBS Misc. Publ. 260-10 (December

1965). COM74-1 1064**

Napolitano, A., and Hawkins, E. G., Standard

Reference Materials: Viscosity of Standard

Lead-Silica Glass, NBS Misc. Publ. 260-11

(November 1966). NBS Misc. Publ. 260-11**

Yakowitz, H., Vieth, D. L., and Michaelis, R. E., Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric Standards III: White Cast Iron and Stainless Steel Powder Compact, NBS Misc. Publ. 260-12

(September 1966). NBS Misc. Publ. 260-12**

Spijkerman, J. L., Snediker, D. K., Ruegg, F. C,

and DeVoe, J. R., Standard Reference Materials: Mossbauer Spectroscopy Standard for the Chemical Shift of Iron Compounds, NBS Misc.

Publ. 260-13 (July 1967). NBS Misc. Publ.

260-13**

Menis, O., and Sterling, J. T., Standard Reference

Materials: Determination of Oxygen in Ferrous

Materials - SRM 1090, 1091, and 1092, NBS

Misc. Publ. 260-14 (September 1966). NBS Misc.

Publ. 260-14**

Passaglia, E., and Shouse, P. J. Standard Refer- ence Materials: Recommended Method of Use

of Standard Light-Sensitive Paper for Calibrating Carbon Arcs Used in Testing Textiles for Colorfastness to Light, NBS Misc. Publ. 260-15

(June 1967). (Replaced by NBS Spec. Publ.

260-41.)

Yakowitz, H., Michaelis, R. E., and Vieth, D. L., Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric Standards IV: Preparation and Microprobe Characterization of W-20% MO Alloy Fabricated by

Powder Metallurgical Methods, NBS Spec.

Publ. 260-16 (January 1969). COM74-1 1062**

Catanzaro, E. J., Champion, C. E., Garner, E. L., Marinenko, G., Sappenfield, K. M.,and Shields,

W. R. Standard Reference Materials: Boric

Acid; lsotopic and Assay Standard Reference

Materials, NBS Spec. Publ. 260-17 (February

1970). Out of Print

1* V
Geller, S. B., Mantek, P. A., and Cleveland, N.

G., Standard Reference Materials: Calibration of NBS Secondary Standard Magnetic Tape

(Computer Amplitude Reference) Using the

Reference Tape Amplitude Measurement

"Process A, "NBS Spec. Publ. 260-18 (November

1969). (See NBS Spec. Publ. 260-29.)

Paule, R. C, and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressure of Gold (Certifica- tion of Standard Reference Material 745). NBS

Spec. Publ. 260-19 (January 1970). PBI90071**

Paule, R. C, and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressures of Cadmium and

Silver, NBS Spec. Publ. 260-21 (January 1971).

COM74-1 1359**

Yakowitz, H., Fiori, C. E., and Michaelis, R. E., Standard Reference Materials: Homogeneity

Characterization of Fe-3 Si Alloy, NBS Spec.

Publ. 260-22 (February 1971). COM74-1 1357**

Napolitano, A., and Hawkins, E. G., Standard

Reference Materials: Viscosity of a Standard

Borosilicate Glass, NBS Spec. Publ. 260-23

(December 1970). COM71-00157**

Sappenfield, K. M., Marineko, G., and Hague, J. L., Standard Reference Materials: Comparison

of Redox Standards, NBS Spec. Publ. 260-24

(January 1972). COM72-50058**

Hicho, G. E., Yakowitz, H., Rasberry, S. D., and

Michaelis, R. E., Standard Reference Materials:

A Standard Reference Material Containing

Nominally Four Percent Austenite, NBS Spec.

Publ. 260-25 (February 1971). COM74-1 1356**

Martin, J. F., Standard Reference Materials:

National Bureau of Standards-US Steel Corportion Joint Program for Determining Oxygen and

Nitrogen in Steel, NBS Spec. Publ. 260-26

(February 1971). 85 cents* PB 81176620

Garner, E. L., Machlan, L. A., and Shields, W. R., Standard Reference Materials: Uranium

Isotopic Standard Reference Materials, NBS

Spec. Publ. 260-27 (April 1971). COM74-

11358**

Heinrich, K. F. J., Myklebust, R. L., Rasberry, S. D., and Michaelis, R. E., Standard Reference

Materials: Preparation and Evaluation of

SRM's 481 and 482 Gold-Silver and GoldCopper Alloys for Microanalysis, NBS Spec.

Publ. 260-28 (August 1971). COM7 1-50365**

Geller, S. B., Standard Reference Materials: Cali- bration of NBS Secondary Standard Magnetic

Tape (Computer Amplitude Reference) Using

the Reference Tape Amplitude Measurement

"Process A-Model 2," NBS Spec. Publ. 260-29

(June 1971). COM71-50282

Gorozhanina, R. S., Freedman, A. Y., and

Shaievitch, A. B. (translated by M. C. Selby),

Standard Reference Materials: Standard

Samples Issued in the USSR (A Translation

from the Russian). NBS Spec. Publ. 260-30 (June

1971). COM71-50283**

Hust, J. G., and Sparks, L. L., Standard Reference

Materials: Thermal Conductivity of Electrolytic Iron SRM 734 from 4 to 300 K, NBS Spec. Publ.

260-31 (November 1971). COM71-50563**

Mavrodineanu, R., and Lazar, J. W., Standard

Reference Materials: Standard Quartz Cuvettes,

for High Accuracy Spectrophotometry, NBS

Spec. Publ. 260-32 (December 1973). 55 cents* SN003-003-01213-1

Wagner, H. L., Standard Reference Materials:

Comparison of Original and Supplemental

SRM 705, Narrow Molecular Weight Distri- bution Polystyrene, NBS Spec. Publ. 260-33

(May 1972). COM72-50526**

Sparks, L. L., and Hust, J. G., Standard Reference

Materials: Thermoelectric Voltage, NBS Spec.

Publ. 260-34, (April 1972). COM72-50371**

Sparks, L. L., and Hust, J. G., Standard Refer- ence Materials: Thermal Conductivity of

Austenitic Stainless Steel, SRM 735 from 5 to 280 K, NBS Spec. Publ. 260-35 (April 1972.) 35 cents* COM72-50368**

Cali, J. P., Mandel, J., Moore, L. J., and Young, D.

S., Standard Reference Materials: A Referee

Method for the Determination of Calcium in Serum, NBS SRM 915, NBS Spec. Publ. 260-36

(May 1972). COM72-50527**

Shultz, J. 1. Bell., R. K. Rains, T. C, and Menis,

O., Standard Reference Materials: Methods of

Analysis of NBS Clay Standards, NBS Spec.

Publ. 260-37 (June 1972). COM72-50692**

Richmond, J. C, and Hsia, J. J., Standard Refer- ence Materials: Preparation and Calibration of

Standards of Spectral Specular Reflectance,

NBS Spec. Publ. 260-38 (May 1972). COM72-

50528**

Clark, A. F., Denson, V.A., Hust, J. G., and

Powell, R. L., Standard Reference Materials

The Eddy Current Decay Method for Resistivity

Characterization of High-Purity Metals, NBS

Spec. Publ. 260-39 (May 1972). COM72-50529**
McAdie, H. G., Garn, P.D., and Menis, O., Standard Reference Materials: Selection of Thermal

Analysis Temperature Standards Through a Cooperative Study (SRM 758, 759, 760), NBS

Spec. Publ. 260-40 (August 1972.) COM72-

50776**

Wood, L. A., and Shouse, P. J., Standard

Reference Materials: Use of Standard LightSensitive Paper for Calibrating Carbon Arcs Used in Testing Textiles for Colorfastness to Light, NBS Spec. Publ. 260-41 (August 1972)

COM72-50775**

Wagner, H. L. and Verdier, P. H., eds.. Standard

Reference Materials: The Characterization of

Linear Polyethylene, SRM 1475, NBS Spec.

Publ. 260-42 (September 1972). COM72-

50944**

Yakowitz, H., Ruff, A. W., and Michaelis, R. E., Standard Reference Materials: Preparation and

Homogeneity Characterization of an Austenitic

Iron-Chromium-Nickel Alloy, NBS Spec. Publ.

260-43 (November 1972). COM73-50760**

Schooley, J. F., Soulen, R. J., Jr., and Evans, G. A.,

Jr., Standard Reference Materials: Preparation

and Use of Superconductive Fixed Point

Devices, SRM 767, NBS Spec. Publ. 260-44

(December 1972). COM73-50037**

Greifer, B., Maienthal, E. J. Rains, T. C, and

Rasberry, S. D., Standard Reference Materials:

Powdered Lead-Based Paint, SRM 1579, NBS

Spec. Publ. 260-45 (March 1973). COM73-

50226**

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and

Electrical Resistivity Standard Reference Materials: Austenitic Stainless Steel, SRM's 735 and

798, from 4 to 1200 k, NBS Spec. Publ. 260-46

(March 1975). SN003-003-0 1278-5

Hust, J. G., Standard Reference Materials: Electrical Resistivity of Electrolytic Iron, SRM 797,

and Austenitic Stainless Steel, SRM 798, from 5 to 280 K, NBS Spec. Publ. 260-47 (February

1974). COM74-50176**

Mangum, B. W., and Wise, J. A., Standard Refer- ence Materials: Description and Use of Precision

Thermometers for the Clinical Laboratory,

SRM 933 and SRM 934, NBS Spec. Publ. 260-48

(May 1974). 60 cents* SN003-003-0 1278-5

Carpenter, B. S., and Reimer, G. M., Standard

Reference Materials Calibrated Glass Standards for Fission Track Use, NBS Spec. Publ.

260-49 (November 1974). COM74-51185

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials:

Electrolytic Iron, SRM's 734 and 797 from 4 to 1000 K, NBS Spec. Publ. 260-50 (June 1975).

$1.00* SN003-003-0 1425-7

Mavrodineanu, R., and Baldwin, J. R., Standard

Reference Materials: Glass Filters As a Standard

Reference Material for Spectrophotometry;

Selection; Preparation; Certification; Use-SRM

930, NBS Spec. Publ. 260-51 (November 1975).

$1.90* SN003-003-01481-8

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials

730 and 799, from 4 to 3000 K, NBS Spec. Publ.

260-52 (September 1975). $1.05* SN003-003-

01464-8

Durst, R. A., Standard Reference Materials:

Standardization of pH Measurements, NBS

Spec. Publ. 260-53 (December 1975, Revised).

$1.05 SN003-003-01551-2

Burke, R. W., and Mavrodineanu, R. Standard

Reference Materials: Certification and Use of

Acidic Potassium Dichromate Solutions as an

Ultraviolet Absorbance Standard, NBS Spec.

Publ. 260-54 (August 1977). $3.00* SN003-003-

01828-7

Ditmars, D. A., Cezairliyan, A., lshihara, S., and

Douglas, T. B., Standard Reference Materials:

Enthalpy and Heat Capacity; Molybdenum

SRM 781, from 273 to 2800 K, NBS Spec. Publ.

260-55 (September 1977). $2.20* SN003-003-

01836-8

Powell, R. L., Sparks, L. L., and Hust, J. G.,

Standard Reference Materials: Standard

Thermocouple Materials, Pt.67: SRM 1967, NBS

Spec. Publ. 260-56 (February 1978). $2.20*

SN003-003-0 18864

Cali, J. P. and Plebanski, T., Guide to United

States Reference Materials, NBS Spec. Publ.

260-57 (February 1978). $2.20* PB 277173

Barnes, J. D., and Martin, G. M., Standard Refer- ence Materials: Polyester Film for Oxygen Gas

Transmission Measurements SRM 1470, NBS

Spec. Publ. 260-58 (June 1979) $2.00* SN003-

003-02077

Chang, T., and Kahn, A. H. Standard Reference

Materials: Electron Paramagnetic Resonance

Intensity Standard; SRM 2601, NBS Spec.

Publ. 260-59 (August 1978) $2.30* SN003-

003-01975-5

vi
Velapoldi, R. A., Paule, R. C, Schaffer, R., Velapoldi, R. A., Paule, R. C, Schaffer, R.,

Mandel, J., and Moody, J. R., Standard Refer- Mandel, J., Machlan, L. A., Garner, E. L., and

ence Materials: A Reference Method for the Rains, T. C, Standard Reference Materials: A

Determination of Sodium in Serum, NBS Spec. Reference Method for the Determination of

Publ. 260-60 (August 1978). $3.00* SN003-003 Lithium in Serum, NBS Spec. Publ. 260-69 (July)

01978-0 1980). $4.25* SN003-003-02214-4

Verdier, P. H., and Wagner. H. L., Standard Refer- Marinenko, R. B., Biancaniello, F., Boyer, P. A., ence Materials: The Characterization of Linear Ruff, A. W., DeRobertis, L., Standard Reference

Polyethylene (SRM 1482, 1483, 1484), NBS Materials: Preparation and Characterization of Spec. Publ. 260-61 (December 1978). $1.70* an Iron-Chromium-Nickel Alloy for MicroSN003-003-02006-1 analysis, NBS Spec. Publ. 260-70 (May 1981).

Soulen, R. J., and Dove, R. B., Standard Reference $2.50* SN003-003-02328-1

Materials: Temperature Reference Standard Seward, R. W., and Mavrodineanu, R., Standard

for Use Below 0.5 K (SRM 768). NBS Spec. Reference Materials: Summary of the Clinical Publ. 260-62 (April 1979). $2.30* SN003-003- Laboratory Standards Issued by the National

02047-8 Bureau of Standards, NBS Spec. Publ. 260-71

Velapoldi, R. A., Paule, R. C, Schaffer, R. (November 1981). $6.50* SN003-003-0238 1-7 Mandel, J., Machlan, L. A., and Gramlich, J. W., Reeder, D.J., Coxon, B., Enagonio, D., Christensen,

Standard Reference Materials: A Reference R. G., Schaffer, R., Howell, B. F., Paule, R. C,

Method for the Determination of Potassium in Mandel, J., Standard Reference Materials: SRM

Serum. NBS Spec. Publ. 260-63 (May 1979). 900, Antiepilepsy Drug Level Assay Standard,

$3.75* SN003-003-02068 NBS Spec. Publ. 260-72 (June 1981). $4.25*

Velapoldi, R. A., and Mielenz, K. D., Standard SN003-003-02329-9

Reference Materials: A Fluorescence Standard Interrante, C. G., and Hicho, G. E., Standard ReferReference Material Quinine Sulfate Dihydrate ence Materials: A Standard Reference Material

(SRM 936), NBS Spec. Publ. 260-64 (January Containing Nominally Fifteen Percent Austenite

1980). $4.25* SN003-003-02 148-2 (SRM 486), NBS Spec. Publ. 260-73 (January

Marinenko, R. B., Heinrich, K. F. J., and Ruegg, 1982). $2.75* SN003-003-02386-8

F. C, Standard Reference Materials: Micro- Marinenko, R. B., Standard Reference Materials:

Homogeneity Studies of NBS Standard Reference Preparation and Characterization of K-411 and

Materials, NBS Research Materials, and Other K-414 Mineral Glasses for Microanalysis: SRM

Related Samples. NBS Spec. Publ. 260-65 470. NBS Spec. Publ. 260-74 (in press).

(September 1979). $3.50* SN003-003-021 14-1 Weidner, V. R., Hsia, J. J., Standard Reference

Venable, W. H., Jr., and Eckerle, K. L., Standard Materials: Preparation and Calibration of First Reference Materials: Didymium Glass Filters for Surface Aluminum Mirror Specular Reflectance

Calibrating the Wavelength Scale of Spectro- Standards (SRM 2003a), NBS Spec. Publ. 260-75

photometers (SRM 2009, 2010, 2013). NBS Spec. (in press).

Publ. 260-66 (October 1979). $3.50* SN003-003- Hicho, G. E. and Eaton, E. E., Standard Reference

02127-0 Materials: A Standard Reference Material

Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel, Containing Nominally Five Percent Austenite

J., Murphy, T. J., and Gramlich, J. W., Standard (SRM 485a), NBS Spec. Publ. 260-76 (in press).

Reference Materials: A Reference Method for the Furukawa, G. T., Riddle. J. L., Bigge, W. G., and

Determination of Chloride in Serum, NBS Spec. Pfieffer, E. R., Standard Reference Materials:

Publ. 260-67 (November 1979). $3.75* SN003- Application of Some Metal SRM's as Thermo003-02136-9 metric Fixed Points, NBS Spec. Publ. 260-77 (in Mavrodineanu, R. and Baldwin, J.R., Standard press).

Reference Materials: Metal-On-Quartz Filters as a

* Send order with remittance to Superintendent

Standard Reference Material for Spectrophoto- of Documents, US Government Printing Office

metry-SRM 2031, NBS Spec. Publ. 260-68 Washington, DC 20402. Remittance from

(April 1980). $4.25* SN003-003-02 167-9 foreign countries should include an additional

one-fourth of the purchase price for postage. ** May be ordered from: National Technical

Information Services (NTIS). Springfield

Virginia 22151.

vii
TABLE OF CONTENTS

PAGE

1. Introduction 2

2. Zinc Freezing Point 12

2.1 Zinc Samples . 12

2.2 Graphite Cells for Containing Metal s 16

2.3 Filling Graphite Cells with Zinc 23

2.4 Assembly of Freezing-Point Cells 28

2.5 Freezing-Point Furnace 28

2.6 Preparation of Zinc-Point Freeze 32

3. Tin Freezing Point 38

3.1 Tin Samples 38

3.2 Graphite Cells for Tin 46

3.3 Filling Graphite Cells with Tin 46

3.4 Tin Freezing-Point Cells 46

3.5 Freezing-Point Furnace for Tin 49

3.6 Preparation of Tin-Point Freeze 49

4. Aluminum Freezing Point .... 54

4.1 Aluminum Samples 54

4.2 Graphite Cells for Containing Aluminum 57

4.3 Filling Graphite Cells with Aluminum 57

4.4 Aluminum Freezing-Point Cells 66

4.5 Freezing-Point Furnace for Aluminum 67

4.6 Preparation of Aluminum-Point Freeze ^2

v i i i
TABLE OF CONTENTS (continued)

PAGE

5. Cadmium Freezing Point 76

5.1 Cadmium Sample . . . 76

5.2 Graphite Cells for Cadmium 82

5.3 Filling Graphite Cells with Cadmium ......... 82

5.4 Cadmium Freezing-Point Cells 82

5.5 Freezing-Point Furnace for Cadmium 82

5.6 Preparation of Cadmium-Point Freeze 83

6. Mercury Triple Point 87

6.1 Mercury Samples 87

6.2 Borosilicate Glass Cells for Mercury . 90

6.3 Filling Borosilicate Glass Cells with Mercury .... 90

6.4 Realization of Mercury Triple Point by a Freezing Method . 93

6.5 Realization of Mercury Triple Point by a Melting Method 98

7. Summary 103

8. References 104

9. Appendices 107

ix
LIST OF TABLES

TABLE MO. PAGE

I. Defining Fixed Points of the IPTS-68 3

II. Secondary Reference Points 5

III. Effect of Pressure on the Temperatures of Some

Defining Fixed Points and Secondary Reference

Points of the IPTS-68 10

IV. Emission Spectrochemical Analysis of the Zinc

Samples (From the Assay Certificates of the

Supplier) 13

V. Emission Spectrochemical Analysis of the Zinc

Samples (NBS Analysis) 14

VI. Mass Spectrochemical Analysis of the Zinc Samples 15

VII. Neutron Activation Analysis of the Zinc Sample 17

VIII. Emission Spectrochemical Analysis of the Tin Samples

(From the Assay Certificates of the Supplier) 39

IX. Emission Spectrochemical Analysis of the Tin Samples

(NBS Analysis and Supplier Analysis) 40

X. Mass Spectrochemical Analysis of the Tin Samples from

SRM-741 (Lot HPM 6779) 43

XI. Neutron Activation Analysis of the Tin Samples 45

XII. Identification of Tin Samples by Bar Number, Residual

Resistivity Ratios, and Designation of Assembled TinPoint Cells 47

XIII. Chemical Analyses of the Aluminum Samples 56

XIV. Emission Spectrochemical Analysis of the Cadmium

Samples (From the Assay Certificate of the Supplier). ... 77

XV. Mass Spectrochemical Analysis of Cadmium Specimens

from SRM-746 (Lot HPM 7541 ) 78

XVI. Emission Spectrochemical Analysis of the Mercury

Sample (SRM-743) 89

x
LIST OF FIGURES

FIGURE NO. PAGE

1. Graphite crucible for containing metal samples used in

freezing-point measurements 19

2. Graphite lid and thermometer well (threaded type) for

crucible of Fig. 1 20

3. Graphite lid and thermometer well (slide-through type)

for crucible of Fig. 1 21

4. Borosil icate-glass tube used to house the graphite

crucible assembly with metal (tin, cadmium, or zinc)

for freezing-point measurements 22

5. Arrangement for filling the graphite crucible with metal

sample by induction heating 24

6. Graphite funnel used to fill graphite crucible with

metal sample and graphite heat shunt used in assembling

metal freezing-point cell 25

7. Apparatus and method for installing graphite lid and

thermometer well in the graphite crucible containing

the molten, metal freezing-point sample 26

3. Apparatus for inserting graphite thermometer well into

the graphite crucible containing the molten metal sample . 27

9. Freezing-point cell of zinc, tin, or cadmium 29

10. Schematic of furnace used with zinc- and tin-point cells . 30

11. Consecutive measurements of the resistance of an SPRT in

a single zinc freeze (SRM-740) 35

12. Freezing curve of zinc (SRM-740) 36

13. Comparison of tin freezing-point temperatures of samples

with different residual resistance ratios: R(0°C)/R(4. 2K) 48

14. Consecutive measurements of the resistance of an SPRT

in a single tin freeze (SRM-741 ) 51

15. The freezing-curve of tin (SRM 741) 53

16. Graphite crucible used to contain aluminum samples in

freezing-point measurements 58

xi
FIGURE NO .

LIST OF FIGURES (continued)

PAGE

17. Graphite lid and thermometer well (slide-through type)

for the crucible of Fig. 16 59

18. Aluminum-point cell sealed in fused-quartz envelope

(Type A cell) 60

19. Aluminum-point cell in fused-quartz envelope with

Inconel casing (Type B cell) 62

20. Aluminum-point cell with fused-quartz casing (Type C

cell) 64

21. Schematic arrangement of the high vacuum, argon-gas

purification and handling, and pressure-measurement

system for aluminum-point measurements .... 68

22. Schematic of furnace used with aluminum-point cells. ... 70

23. Typical freezing curves of five aluminum samples,

starting approximately fifteen minutes after the

insertion of an SPRT in the cell 75

24. The freezing curve of cadmium (SRM-746) 84

25. Consecutive measurements of the resistance of an SPRT in the same cadmium freeze (SRM-746) 86

26. Borosil icate-glass cell used to contain mercury

samples for triple-point measurements 91

27. Arrangement used for filling mercury cells by vacuum distillation 92

28. Mercury triple-point cell assembly • 94

29. Dry Ice - ethyl alcohol bath used in freezing

experiments with mercury 95

30. Freezing curves of six mercury cells 97

31. Consecutive measurements of the resistance of an SPRT in two mercury freezes (measurement sets B and C)

and in one mercury melt (measurement set A) [SRM-743] ... 99

32. Freezing (F) and melting (M) curves obtained with

mercury samples in borosil icate-glass (BS) and stainlesssteel (SS) cells 100

33. Plastic-foam container used in melting experiments

with mercury 101

xii
APPENDICES

CERTIFICATES AND ANALYSES

OF STANDARD REFERENCE MATERIALS

AND RESEARCH MATERIALS

PAGE

A. SRM-740 Zinc, Defining Freezing-Point Standard

(419.58°C) of the IPTS-68. 108

B. SRM-43h Zinc, Freezing-Point Standard (419. 5°C). HO

C. SRM-683 Zinc. Ill

D. SRM-42g Tin, Freezing-Point Standard (231 .967°C). H3

E. SRM-741 Tin, Defining Freezing-Point Standard

(231.9681°C) of the IPTS-68. 114

F. SRM-44f Aluminum, Freezing-Point Standard (660. 3°C). H 6

G. Research Materials, RM-1R and RM-1C. H?

H. SRM-746 Cadmium. 121

I. SRM-743 Mercury, Freezing-Point Standard (-38.841°C).

xi i i

Application of Some Metal SRM's as Thermometric Fixed Points

George T. Furukawa, John L. Riddle, William R. Bigge, and Earl R. Pfeiffer

Temperature and Pressure Measurements and Standards Division

Center for Absolute Physical Quantities

National Measurement Laboratory

National Bureau of Standards, Washington, DC 20234

Abstract

Equipment and procedures are described for the realization of

liquid-solid phase equilibrium states of some pure metals as thermometric

fixed points for platinum resistance thermometry. The design and techniques

for assembling fixed-point cells using Standard Reference Materials

(SRM) 740 (zinc), 43h (zinc), 42g (tin), 741 (tin), 44f (aluminum), 746

(cadmium), and 743 (mercury) are given and the results of the measurements

using the purer of these metal SRM's are analyzed and evaluated. The

reproducibility of temperature measurements with these metal SRM's is

shown to be about + 0.1 mK.

Key words: Aluminum point; cadmium point; check thermometers; freezing

point; melting point; mercury point; phase equilibrium; standard platinum

resistance thermometer (SPRT); thermometric fixed point; tin point;

triple point; zinc point.

Disclaimer

In order to adequately describe materials and experimental procedures,

it was occasionally necessary to identify commercial products by

manufacturer's name or label. In no instance does such identification

imply endorsement by the National Bureau of Standards nor does it imply

that the particular products or equipment is necessarily the best

available for that purpose.

1
1 . Introduction

The current internationally adopted temperature scale is the International

Practical Temperature Scale of 1968 (Amended Edition of 1975) [36]1

,

henceforth in this publication to be designated as IPTS-58. The IPTS-68

is based on the assigned values of the temperatures of specified equilibrium

states of pure substances (i.e., defining fixed points) and on standard

instruments calibrated at these temperatures. Between the fixed-point

temperatures, interpolation equations relate the indications of the

standard instruments to temperature values of the IPTS-68. The platinum

resistance thermometer is the specified standard instrument from 13.81 K (-259.34°C) to 903.89 K (630.74°C), the platinum-10% rhodium alloy

versus platinum thermocouple from 903.89 K to 1337.58 K (1064. 43°C), and

the optical pyrometer above 1337.58 K. (A platinum resistance thermometer

that meets the IPTS-68 specifications for a standard will, henceforth,

be designated as a standard platinum resistance thermometer or simply as

a SPRT.)

The assigned values of temperature of the thirteen defining fixed

points of the IPTS-68 are listed in Table I; nine substances are involved.

In addition to the defining fixed points, the IPTS-68 document [36]

lists thirty-one other equilibrium states designated as secondary reference

points which involve twenty-nine substances, four of these substances

being used also for the equilibrium states of the defining fixed points.

Recently, the Consultative Committee for Thermometry (a subcommittee of

the International Committee of Weights and Measures) published an extended

list of sixty-one secondary reference points involving thirty-six different

substances [4]. This extended list is given in Table II. Where more

than one value of temperature was given for the equilibrium state, an average value is given in Table II. The vapor pressure versus temperature

relations given for some of the substances in the compilation are omitted

from Table II.

The secondary reference points are useful in certain measurement

situations, e.g., for checking the calibration of SPRT's and secondary

thermometers, particularly in the temperature range where they are to be

used. The secondary reference points may be used also for direct

calibration of thermometers, if certain limitations are recognized. If

an SPRT is calibrated using secondary reference points to obtain the

coefficients of the specified form of the IPTS-68 interpolation equation,

the interpolation equation should be expected to deviate somewhat from

an interpolation equation that would be obtained if the SPRT were calibrated

according to the IPTS-68 specifications. The temperature value that is

assigned to a secondary reference point is usually an average of a number of measurements with different SPRT's calibrated in terms of the

IPTS-68 and of a number of realizations of the reference point. The

uncertainties of calibration using secondary reference points will

include, in addition to the local uncertainties of measurements and

Figures in brackets indicate the literature references listed at the

end of this publication.

2
Table I

Defining Fixed Points of the IPTS-689

Substance and , Assigned value of the

equil i brium state IPTS-68 temperature

T V + or

t , L

e-r^ , I

1 "J Ql

1 5 . o 1

e-H^, BP to3,33u.o Pa )

1 / . U4£ ore n no -<:bo . 1 (Jo

a n nd e-r^, dP 90 9R -COtl 9^9 R7 . 0/

d e No R D 5 Ne , or 97 1 n? -lHD 9dfi . D&R UHO

n td U^, IP 3t .001 91R 7R.Q

f A*< TD Ar , IP 0"? 7 QR IRQ "5 n;

U^j LP on i rr 1 R9 Q£9

r^U , IP C 1 o . ID nU . mU 1

H

2 0, BP 373.1 5 100

Sn, FP9 505.1181 231 .9681

Zn, FP 692.73 419.58

Ag, FP 1235.08 961 .93

Au, FP 1337.58 1064.43

a Except for the triple points and the equilibrium hydrogen point at

17.042 K, the assigned values of temperature are for equilibrium

states at a pressure of 101,325 Pa (1 standard atmosphere). In

the realization of the fixed points, small departures from the

assigned temperatures occur as a result of small deviations from

the specified pressure at the point of immersion of the thermometer.

Adjustment is made for these small temperature differences.

^TP = triple point, BP = boiling point, CP = condensation point,

FP = freezing point, and e-H

2 = equilibrium hydrogen.

C Hydrogen has two molecular configurations (nuclear spin arrangements),

designated ortho and para. The equilibrium composition is temperature

dependent. Equilibrium hydrogen means that the hydrogen has its

equilibrium composition at the relevant temperature.

3
Table I (continuation)

Fractionation of isotopes or impurities dictates the use of boiling

points (vanishingly small vapor fraction) for hydrogen and neon,

and condensation point (vanishly small liquid fraction) for oxygen.

Q The neon used should have the normal isotopic composition: 0.905

mol 20 Ne, 0.092 mol 22 Ne, and 0.0027 mole 2l Ne.

^The triple point of argon may be used as an alternative to the

condensation point of oxygen.

g The freezing point of tin (f = 231.9292°C) may be used as an alternative to the boiling point of water.

4
Table II

Secondary Reference Points 3

K rl Equilibrium State ' IPTS-68 Temperature

T,K t,°C

n-H

2

, TP 13.957 -259.1 93

n-H

2

, BP 20.397 -252.753

0

2

, TR(a-B) 23.867 -249.283

Ne, TP 24.561 -248.589

o

2

, tr(b-y) 43.801 -229.349

N

2

, TP 63.146 -210.004

N

2

, BP 77.344 -195.806

Ar, BP 87.294 -185.856

CH

4

, TP 90.686 -182.464

Kr, TP 115.770 -157.380

Kr, BP 119.800 -153.350

Xe, TP 161 .388 -111 .762

Xe, BP 165.060 -108.090

co

2

, SP 1 94.674 -78.476

C0

2

, TP 216.580 -56.570

Hg, TP 234.308 -38.842

Hg, FP 234.314 -38.836

Ice pointc 273.15 0

Phenoxybenzene, TP 300.011 26.861

Ga, MP 302.921 29.771

Benzoic acid, TP 395.52 122.37

In, FP 429.784 156.634

Bi, FP 544.592 271 .442

Cd, FP 594.258 321 .108

5
I

Table II (continuation)

Pb, FP 600.652 327.502

Hg, BP 629.81 356.66

S, BP 717.824 444.674

Cu-Al eutictic, MP 821 .41 548.26

Sb, FP 903.905 630.755

Al, FP 933.607 660.457

Cu-Ag eutectic, FP 1052.75 779.60

Cu, FP 1358.03 1084.88

Fe, RT of M 1670 1397

Ni, FP 1728 1455

Co, FP 1768 1495

Fe, MP 1808 1535

Pd, FP 1827 1554

Zr, RT of M 1940 1667

Pt, FP 2042 1769

Zr, MP 2128 1855

Rh, FP 2236 1963

A1

2 0

3

, MP 2327 2054

Nb, RT of M 2425 2152

Mo, RT of M 2531 2258

Ir, FP 2720 2447

Nb, MP 2746 2473

Mo, MP 2896 2623

W, MP 3695 3422

a The temperatures listed are the best values at the time of compilation.

^The equilibrium states in the table are at a pressure of 101,325 Pa

(1 standard atmosphere) except for the triple points.

6
Table II (continuation)

c The ice point is a very close approximation to the temperature defined

as being 0.01 K below the triple point of water.

^TP = triple point, BP = boiling point, SP = sublimation point,

FP = freezing point, MP = melting point, TR = solid phase transition

point, RT of M = radiance temperature of the melting metal, and

n-H? = normal hydrogen (the high temperature equilibrium composition of

ortho and para hydrogen, closely approached at room temperatures,

consisting of a mixture of 75% orthohydrogen and 25% parahydrogen)

7
realizations of the secondary reference points, the errors of assignment

of temperature values to the secondary reference points and the differences

in the SPRT's. Also, in such calibrations the range of the temperatures

of the secondary reference points used is, in general, less than the

range of the defining fixed points of the IPTS-68; hence, the resulting

interpolation equation that is obtained is expected to be of lower

accuracy than the interpolation equation from a regular calibration.

Any extrapolation of the equation beyond the range of the calibration

points should be expected to give even larger errors. (See reference

[31] for analysis of propagation of errors when temperatures are extrapolated

above the zinc point, even though the calibration is in accordance with

the IPTS-68.)

For thermometers (other than SPRT's) that are calibrated using the

fixed points, the interpolated values of temperature between the calibration

points should be expected to be different from those of the SPRT's,

again depending upon the range of interpolation. The calibration of

such thermometers should be at small enough temperature intervals so that the temperature derivative of the thermometer scale is close enough

to that of the SPRT for the application. Industrial type platinum

resistance thermometers, because of their construction (which is not

strain free or free of possible chemical contamination), should be

expected to behave differently from SPRT's.

The large number of substances given in Tables I and II shows that

a wide variety of pure substances is needed for accurate thermometry.

Some of these substances are available from the National Bureau of

Standards - Office of Standard Reference Materials (NBS-OSRM). Among

the thermometric fixed points, the realization of liquid-solid phase

equilibrium states is relatively less complicated than the liquid-vapor

phase equilibrium states because the effect of pressure on the temperature

of the former is relatively smaller. Metals have the additional advantage

of having high thermal conductivity which promotes rapid temperature

equilibrium. This paper deals with the application of some of the

available metal standards that are suitable for the calibration and

testing of SPRT's.

A number of high-purity metals suitable for thermometric freezingpoint (or melting-point) standards can be obtained as Standard Reference

Materials (SRM) from the NBS-OSRM [39]. The calibration or testing of

SPRT's usually requires relatively large samples (about 130 to 180 cm^)

in the freezing-point cells [16,25]. The SPRT must be immersed sufficiently

in the sample to provide measurements that are independent of the ambient

temperature. Moreover, with certain AC bridge [6] or DC current comparator

bridge [22], the SPRT (of R(0°C)=25.5 ohm) can give measurement resolutions

that correspond to better than 0.01 mK with systems that have high

thermal stability. At these high measurement resolutions, not only must

the immersion of the SPRT be adequate but also an accurate information

on the relative location of the SPRT resistor coil and the liquid surface

of the freezing-point cell must be available to correct for the hydrostatic head with an accuracy consistent with the measurement precision.

8
Similarly, wherever applicable, adjustments for variations in the barometric pressure must also be made or, better still, the pressure in the

freezing-point cell be controlled to 1 atm. Table III lists the effect

of pressure and hydrostatic head on the freezing point for some of the

substances.

There is a practical limitation on the height (depth of immersion)

to which the cell containing the metal could be made. However, the

apparent depth of immersion in the thermometric cell can be increased by

suitably tempering the thermometer stem above the cell to a temperature

close to that of the cell [13,27,31]. The adequacy of immersion of the

SPRT should always be tested by measuring the thermometer resistance at

two or more depths of immersion in the cell (e.g., at maximum immersion

and at a few cm less). The temperature readings must be consistent with

the difference due to the effect of hydrostatic head of the liquid

metal. With a relatively large sample and precise temperature control

at about 1 K below the freezing point, total freezing times of 14 to 20

hours can be achieved using a relatively simple tube furnace to contain

the cell. Longer freezing times can be obtained by controlling the

furnace temperature closer than 1 K below the freezing temperature.

This paper describes the procedures that have been employed, in the

Platinum Resistance Thermometry Laboratory of the NBS, in the assembly

of freezing-point cells with metal standards from the NBS-OSRM for use with long-stem type (or immersion type) SPRT's. In particular, procedures

are described for assembling cells of zinc, tin, aluminum, cadmium, and

mercury. However, similar procedures can be employed in assembling

cells of indium, lead, antimony, bismuth, silver, gold, and other metals

that can be melted in graphite crucibles. [Gallium melting-point standard

(SRM-1968), designed for calibration of miniature temperature probes,

such as thermistors and thermocouples, is available from the NBS-OSRM

packaged in plastic holders suitable for immediate application using a thermostated bath [38].]

The zinc point and the tin point are defining fixed points of the

IPTS-68; they are used regularly for the calibration of SPRT's in the

Platinum Resistance Thermometer Laboratory (henceforth, to be designated

simply as PRTL) of the NBS. The aluminum point and the mercury point are being considered as additional fixed points to better define the

future new IPTS. The cadmium point has been selected for verifying the

SPRT calibrations at the triple point of water, the zinc point, and the

tin point. The accuracy of the calibration can be checked within a few

tenths of a millikelvin from the measurements at the cadmium point.

Although the realization of the indium point is not described in this

report, it will be investigated in the near future for application in

the verification of the SPRT calibrations. This report describes

procedures used in the PRTL of the NBS for realizing the freezing points

of the metals. The observed freezing curves of the metals and the

results of repeated measurements of SPRT's during a single freeze of the

metal (except aluminum) are shown.

9
Table III

Effect of Pressure on the Temperatures of Some

Defining Fixed Points and Secondary Reference Points of the IPTS-68

Substance Equil ibrium Pressure Coefficient

d Equilibrium

State

a Temperature

T,K

mK per

std atm

mK per meter

of 1 i qui d

e-H

2

, TP 13.81

b

34 0.25

0

2

, TP 54.361

b

12 1.5

Ar, TP 83.798 b

25 3.3

H

2 0, TP 273. 16

b -7.5 -0.7

Sn, FP 505.1181 b

3.3 2.2

Zn, FP 692. 73

b

4.3 2.7

Ag, FP 1235. 08

b

6.0 5.4

Au, FP 1 337 . 58

b

6.1 10

Ne, TP 24.551

c 16 1.9

Hg, FP 234.314° 5.4 7.1

Ga, TP 302. 9241

d -2.0d -1.2d

In, FP 429.784 4.9 3.3

Bi, FP 544.592 -3.5 -3.4

Cd, FP 594.258° 6.2 4.8

Pb, FP 600.652° 8.0 8.2

Sb, FP 903.905° 0.85 0.5

Al, FP 933.607° 7.1

e 1.6e

TP = triple point, FP = freezing point at 101,325 Pa (1 standard

atmosphere)

.

b Defining fixed point of the IPTS-68.

From reference [ 36 ]

.

d

From reference [ 23 ].

From reference [ 13 ].

10
In the paper, the application of the zinc standard is discussed

first in the following sequence:

1) the description of the metal standard,

2) the graphite cell to contain the zinc,

3) the filling of the crucible with zinc,

4) the zinc freezing-point cell,

5) the description of the furnace for carrying out the freezingpoint measurements,

6) the procedure for carrying out the freezing-point measurements,

and,

7) the results of the freezing-point measurements.

The other metal standards that require graphite cells are then described

in the order: tin, aluminum, and cadmium. References are made to the

descriptions given for zinc, or for any other metal that was discussed

earlier, wherever the equipment or the procedures are identical or sufficiently similar. The application of the mercury standard, which

follows an entirely different procedure from those of the other metals,

is described in full separately.

11
2 . Zinc Freezing Point

The state of thermodynamic equilibrium between solid and liquid

zinc at 1 standard atmosphere of pressure is a defining fixed point of

the IPTS-68 with an assigned value of 419.58°C (692.73 K) ; henceforth,

it will be referred to as the freezing point of zinc or, simply, the

zinc point. Because of the greater reproduci bil i ty and ease of realization

of the zinc point over the sulfur point (normal boiling point of sulfur),

the use of the zinc point was recommended in place of the sulfur point

in the text revision of the International Practical Temperature Scale of

1948 [34]. In the IPTS-68 [36] the zinc point replaced the sulfur point

and the sulfur point was placed among the secondary reference points.

Prior to 1966 the sulfur point was maintained at the NBS for the calibration

of SPRT's (see reference [33]); since 1966 the zinc point has been

maintained instead.

2 . 1 Zinc Samples

Zinc freezing-point samples are available from the NBS-OSRM in two

different levels of purity; one sample designated SRM-740 (see Appendix

A) is nominally 99.9999 percent pure and the other sample designated

SRM-43h (see Appendix B) is nominally 99.999 percent pure. The SRM-740

zinc (supplier lots HPM 581 5, 6003, and 6071 ) 2 is of exceptionally high

purity; it is used at the NBS to realize the defining fixed point, i.e.,

the freezing point of pure zinc, as specified on the IPTS-68. The

material was prepared by double vacuum distillation of commercial

electrolytic grade zinc, the yield being approximately 70 percent. The

purified material was then melted in a graphite crucible and poured into

graphite boats for zone refining. Twenty zone refining passes were made

in air after which the "leading" and the "trailing" ends of the bar were cropped, the yield from this process being approximately 60 percent.

The zone refined bars were homogenized and finally, to degassify the

material, were given a gradient freeze under hydrogen gas. The gradient

freeze process involved melting the sample in a fused quartz boat by

heating in a furnace with eight zones; when completely melted, the heat

to the zones was turned off sequentially, one per hour. Approximately 3 cm were cropped from each end of the bars. The final bars were semicylindrical (5 cm across the flat) and about 68 cm long.

Assay values, based on emission spectrochemical analysis, that

accompanied the zinc bars from the supplier (from which SRM-740 samples were prepared by cutting and cleaning) are listed in Table IV. Emission

spectrochemical analysis at NBS of two specimens from lot HPM 5815 is

summarized in Table V. Table VI summarizes mass spectrochemical analyses

on specimens from lots HPM 5815 and 6003. The emission spectrochemical

analyses from the NBS and from the supplier are in good agreement.

The NBS-OSRM obtained from the supplier 458 kg of nominally 99.9999

percent zinc in three lots: HPM 5815, 6003, and 6071. The lot HPM 5815

consisted of 33 bars weighing a total of 164 kg, lot HPM 6003 of 32 bars

weighing 158 kg, and lot HPM 6071 of 27 bars weighing 136 kg (NBS

Order No. S-440757-66, Contract No. CST-.122).

12
Table IV

Emission Spectrochemical Analysis of the Zinc Samples

(From the Assay Certificates of Supplier,

by Analysis of the "Master Sample")

Sample (Lot HPM)

El ement 581

6003^

(in ppm)

6071

Ca 0.1 <0.1

Cu <0.1 0/1 <0.1

Fe 0.2 0.3 0.3

Mg 0.1 <0.1 <0.1

Pb 0.2 0.3

Sb 0 ... 1 0.1 0.1

Si 0.1 0.1 0.1

a Date: October 29, 1965.

b

Date: December 23, 1965.

c Date: January 20, 1966.

13
Table V

Emission Spectrochemical Analysis of the Zinc Samples 9

(SRM-740, Lot HPM 5815, Bars 1 and 33)

Impurity elements detected and estimated concentrations in ppm:

Ag <0.1 , Cd <0.1 , Cu <0.1

Fe 0.2, Mg <0.1

Presence of Si questionable, if present, does not exceed 0.2.

Estimated limits of detection in ppm and the elements sought and not

detected

<0.1 Be

0.2 Bi, Pb, Ca, Cr, Co, Mn, Ni , Sn, V

0.5 Al , Au, Ca, B, Ge, In

1 .0 Sb

a Analysis by R. Alvarez of Spectrochemical Analysis Section, National

Bureau of Standards, (March 7, 1966).

14
Table VI

Mass Spectrochemical Analysis of the Zinc Samples

(SRM 740, Lots HPM 5815 and 6003)

5815

1A2

Lot HPM

Bar and

Specimen No.

Detection

Limit b 0.0052

(Atomic ppm)

5815

1B2

0.0054

5815

33A2

0.0076

5815

33B2

6003

34T2

0.0190 0.0090

Elements Found (ppm by weight)

6003

34L2

0.0046

Pb <0.04 <0 04 <0.04 0.6 <0.07 <0.04

W <0.08 <0 06 0.3 <0.1 <0.06 0.3

Cd <0.04 <0 04 0.05 <0.1 0.9 0.04

Ni 0.08 0 05 0.06 0.3 0.08 0.02

Fe 0.06 0 04 0.05 0.07 0.06 0.05

Cr 0.05 0 01 0.01 0.03 0.02 0.01

Cg

c 0.003 0 004 0.006 0.006 0.001 0.007

K

c 0.009 0. 016 0.03 0.03 0.01 0.03

ci- 0.03 0 4 0.1 0.2 0.06 0.05

Al; 0.007 0 005 0.004 0.01 0.01 0.004

Na 0.009 0 01 0.01 0.03 0.01 0.03

Limits of Detection of Elements that Interfere.

These elements were not detected at these limits (ppm by weight)

Au 23 5 7 5 18 7 Pt 0.6 0.1 0.2 0.1 1 0.1

Ta 400 3 200 20 80 450

Ba 0.7 0.07

Cs 0,6 0.9

Nb 0.2 0.2 0.1 0.1 0.3 0.1

Se 0.04 0.04 0.1 0.07 0.16 0.06

Ga 2.5 1.6 0.6 70 2 2 Cu 400 40 13 1300 20 5 Co 0 o Q1 0.08 0.03 0.02 0.1 0.01

0.03 0.04

Ti 0,2 0.1 S 100 20 13 70 30 40

Si 0.1 0.1 0.05 0.4 0.3 0.06

Mg 0.002 0.002 0.001 0.001 0.0003 0.001

0 0.5 3 6 20 1 0.05

N 30 20 15 50 10 20

C 1.5 0.7 1 2 1.5 1 Li 0.0001 0.0001 0.0001 0.0004 0.0001 0.0001

Analysis was performed by P.J. Paulsen and P.E. Branch, Analytical

Chemistry Division, National Bureau of Standards (March 17, 1966).

bTo convert the atomic ppm detection limit to ppm by weight detection limit

for any element, multiply by the ratio of the atomic weight of the element

to that of zinc. Elements not listed are at a concentration below the

atomic ppm detection limit given.

c These elements appeared to be surface contamination.

15
However, the mass spectrochemical analysis shows elements such as Ca,

Fe, Mg, and Pb to be less than that found by the emission spectrochemical

analyses. The results of the neutron activation analysis given in Table

VII are consistent with those of the emission and mass spectrochemical

analyses. R.L. Powell (at the time with the Cryogenic Properties of

Solids Section of the NBS at Boulder, Colorado) found the residual

resistivity ratios (RRR, ratio of the resistivity at 0°C to that at

4.2K) of samples from the opposite ends of two bars, on which the emission

spectrochemical analyses were also made, to range from 33,000 to 38,000

[29]. From the results of the various analyses the purity of the SRM740 zinc has been estimated to be better than 99.9999 percent.

The SRM-740 zinc samples are available in 350-g units. They were

cut "dry" from the final bars with a carbide-tipped cutter in such a manner that only the carbide tip came into contact with the zinc bar.

The bars were handled using polyethylene gloves and were held in the

milling machine through a polyethylene cover. After cutting, the samples

were etched in high-purity dilute nitric acid, rinsed with distilled

water, and then air dried. Finally, they were sealed in polyethylene

bags. (See SRM-740 certificate, Appendix A.)

Depending upon the graphite crucible used for the zinc-point cell,

the mass of zinc required to properly fill the crucible was either 1210 g or 1160 g. The graphite crucible is filled with enough metal so that

the upper surface of the liquid metal, when a few degrees above the

melting point, would be between 1 and 5 mm below the lower surface of

the crucible lid with the well inserted. When the 350-g SRM-740 units were used to fill the cells, three units plus a suitable fraction of the

fourth unit were needed. Thus, additional cutting of the zinc was required. In other cases the samples for the zinc-point cells were cut

directly from the SRM-740 source bars. In both cases the cut samples were cleaned as described above.

The zinc bars for SRM-43h (Appendix B) were prepared by casting

commercial electrolytic grade zinc in a manner similar to that described

for the SRM-740 zinc. The material was received from the supplier in

three lots (HPM 6576, 6577, and 6592) totaling approximately 454 kg.

The semi-circular bars were cut and cleaned to form the 350-g SRM-43h

units. The zinc from these lots is issued also as SRM-683. (See SRM683 certificate for information on the purity of SRM-43h zinc, Appendix C.)

2.2 Graphite Cells for Containing Metals

Graphite is sufficiently chemically inert to contain many metals

that melt above 100°C (In the case of mercury, which melts near -39°C,

both borosilicate glass and type 304 stainless steel cells are being

used; see Section 6.2). The graphite for fabricating the crucibles and

assembled parts comprising the cells must be highly pure, sufficiently

dense (at least 1.76 g/cm )

, and free of voids and cracks. After

fabrication, the graphite parts must be cleaned of metal impurities that

may have been introduced during the machining process. There are a number of

16
Table VII

Neutron Activation Analysis of the Zinc Sample9

Element Lot HPM

b

(in ppm)

Cd <0.16

Au <0.006

Ag <0.10

Na <0.004

Analysis by G.R. Smith, Analytical Chemistry Division, National

Bureau of Standards (March 1, 1966).

^Analysis report does not differentiate whether the sample was HPM

5815, HPM 6003, or HPM 6071.

a

17
fabricators of graphite equipment in the United States that can provide

the graphite and machine and clean the cell parts ready for filling.

The finished graphite cell should be handled as being very fragile

and it should be thoroughly inspected for defects before filling. It is

difficult to detect defects, such as cracks and voids, in the graphite.

At elevated temperatures minute openings in the graphite may become more

pronounced. On occasions molten metal has seeped through crucibles that

otherwise appeared perfectly sound. A graphite cell in which one metal

leaked through might have been perfectly suitable for another metal that

melts at a lower temperature or has a higher viscosity. Hence, during

filling and during use, one should be prepared for the possible leakage

of hot liquid metal through the graphite. Before filling, the graphite

crucible and associated parts should be first baked in high vacuum at

least 200°C above the melting point of the metal to be filled to remove any volatile impurities.

The dimensions of graphite crucibles, lids, and thermometer wells

that are used at the NBS are shown in Figures 1, 2, and 3 (also, in

Figures 16 and 17). The outside diameter of the crucible has been

standardized to 43.7 ± 0.1 mm so that the crucible can be inserted

inside cylindrical glass holders that are nominally 45 to 48 mm inside

diameter.

The crucible shown in Figure 1 has been used with both thermometer

well and lid assemblies of Figures 2 and 3. The screw threads of the

lid and thermometer well of Figure 2 have been cut so that the two can

be "snugly" assembled. The enlarged radius below the screw threads on the thermometer well serves to strengthen the well near its junction

with the lid. (Although more graphite must be machined, the lid and

thermometer well could be fabricated as one piece instead of two pieces

as shown in Figures 2 and 3.)

Above 200°C or so, the graphite must be protected from air to avoid

oxidation. The protective holder for the graphite cell is designed so that it could be handled with the cell still hot and also could contain

inert gas at a pressure slightly above that of the ambient. The protecti

enclosure for the graphite cell could be stainless steel or a high

chromium-nickel alloy, such as Inconel 3

. However, to avoid possible

contamination of the metal sample inside the graphite cell by particles

of the metal holder that could become loose, a borosilicate glass or fused-quartz glass is used at the NBS to contain the graphite cell.

Figure 4 shows the dimensions of the borosil icate-glass holder used with

graphite cells containing zinc, tin, or cadmium.

Certain commercial equipment, instruments, or materials are identified

in this paper in order to adequately specify the experimental procedure.

In no case does such identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply that the material

or equipment identified is necessarily the best available for the purpose

18
DIMENSIONS

IN mm

21.8 RADIUS

CRUCIBLE

Figure 1. Graphite crucible for containing metal samples used in freezing

point measurements.

19
5/8" x 18 10 3

THREAD

3.2

RADIUS

5/8" x 18

THREAD

LID

DIMENSIONS

IN mm

WELL

Figure 2. Graphite lid and thermometer well (threaded type for crucible

of Fig. 1.

20
h—34.9—

LID

MjH

6.4 1 1 ih-10.3

T

£

1 tP-f—

227.0

231.8

DIMENSIONS

IN mm

I U.50

4.8

12.70

WELL

Figure 3. Graphite lid and thermometer well (slide-through type) for

crucibl e of Fig. 1

21
130

460

270

46.28 ± 0.05

3

f

T

50.0 ± 0.2

50.27 ± 0.02

DIMENSIONS

IN mm

50.0 ± 0.2

BOROSILICATE-GLASS TUBE

Figure 4. Borosil icate-gl ass tube used to house the graphite crucible

assembly with metal (tin, cadmium, or zinc) for freezingpoint measurements (see Fig. 9). The tube was made from precision

bore tubing and ground on the outer surface to the indicated diameter.

22
2.3 Filling Graphite Cells with Zinc

Figure 5 shows the bell-jar equipment arrangement that was used to

fill the graphite crucibles with zinc samples. The graphite funnel, C,

used for holding the zinc to be melted into the crucible is shown in

detail in Figure 6. Since zinc has a relatively high vapor pressure

(about 20 Pa [28]) at its melting point, about 0.3 atm pressure of

purified argon gas was introduced into the bell jar space while the zinc

sample was being melted into the graphite crucible. The graphite crucible

and the funnel with the zinc sample (see Figure 5) were first evacuated

at about 200°C for an hour; then argon gas was introduced and the induction

heating power was raised to melt the zinc into the crucible as quickly

as possible to minimize the evaporation loss of zinc. A 5-kW, 450-kHz

generator was employed for the induction heating power source. After

the start of melting, about 15 minutes were usually required to completely

melt the zinc sample in the funnel into the crucible. (In spite of the

presence of argon gas and rapid melting of zinc into the crucible, a "mirror" of zinc formed on the bell jar wall.) The filled crucible was then allowed to cool nearly to room temperature, the bell jar was filled

to atmospheric pressure with purified argon gas, and the graphite crucible

containing the now solidified zinc sample was transferred into a special

borosilicate tube for inserting the graphite thermometer well and lid

assembly (see Figure 7 and Figure 8).

The procedure used for the installation of the lid and thermometer

well into the filled crucible depended upon the design of the lid - thermometer well assembly. Where the thermometer well was attached or attachable firmly to the lid (see Figure 2), the arrangement shown in

Figure 7 was used. (This graphite cell design was used exculsively with

tin and zinc samples). The filled crucible was slipped into the special

borosilicate glass tube and the lid-thermometer well assembly was positioned

on a push-rod holder as shown in the figure. The glass tube assembly

was thoroughly purged with helium gas and then lowered into a tube

furnace held at a temperature 15 to 20°C above the melting point of the

metal. (The furnace to be employed for the freezing-point measurement

itself was used for this purpose). When the metal was completely

melted, the glass tube was raised out of the furnace and the thermometer

well and lid assembly was smoothly and quickly pushed into place. To

make the assembly operation easy, the outside diameters of the crucible

and lid were made the same (see Figures 1 and 2) and the inside diameter

of the glass tube in the section near the top of the crucible was reduced

to only slightly larger (about 0.01 mm) than the outside diameter of the

crucible. Hence, this reduced section of the glass tube served to guide

and correctly position the lid to cover the crucible. The lid was designed to fit the crucible closely to minimize the possibility of

later contamination. Finally, the assembled graphite cell was allowed

to cool slowly and later assembled as a freezing-point cell.

For the graphite cell design, in which the thermometer well slipped

freely through the center hole in the lid (see Figure 3), the arrangement

shown in Figure 8 was used to assemble the thermometer well and lid with

23
•7.6 cmFigure 5. Arrangement for filling the graphite crucible with metal

sample by induction heating.

A. Borosil icate-glass envelope.

B. Metal sample.

C. Graphite sample holder and funnel.

D. Graphite crucible.

E. Induction heater coils.

F. "0" ring seal

.

G. Borosil icate-glass stand.

H. Slot for pumping out and gas purging.

I. Connection to vacuum and purified argon supply.

24
%

-?

I

15.9

—47.6 —

i

h28.6-*|

H4.8

L1J

9.9

i

C3Z4.8

15.9

HEAT SHUNT

DIMENSIONS

IN mm

V

70.0-

60.0

105'

/

130

120

T

20.0

|—29.5—

|

FUNNEL

Figure 6. Graphite funnel used to fill graphite crucible with metal

sample (see Fig. 5) and graphite heat shunt used in assembling

metal freezing-point cell (see Fig. 9).

25
A

560 mm

153 mm

560 mm

244

mm

A. Stainless-steel pusher rod.

B. Gas seal with silicone rubber. Permits

linear motion of the pusher rod (A).

C. Inlet for purified helium gas that is

used in purging and maintaining positive

pressure of the gas during assembly.

D. Silicone rubber stopper.

E. Stainless-steel flange attached to the

pusher rod for pressing against the

graphite lid during assembly.

F. Graphite lid for the crucible.

G. Slit on the pusher rod. The two halves are sprung out to hold the graphite

thermometer well and lid up while

melting the metal sample.

H. Graphite thermometer well.

I. Borosil icate-gl ass tube.

J. Section of glass tube shrunk to fit the

crucible and lid so that lid can be

easily guided onto the crucible.

K. Graphite crucible.

L. Molten metal sample.

ure 7. Apparatus and method for installing graphite lid and thermometer

well in the graphite crucible containing the molten, metal

freezing-point sample.

26
TO HIGH <t VACUUM *

7)_^TO PURIFIED ^ ARGON SOURCE

A.

8.

C

0EF

6

H.

Silicone elastomer stopper.

Fused quartz tube.

Graphite thermometer well.

Tungsten rod sealed in fused quartz tube,

Graphite lid.

Induction heater coils.

Graphite crucible.

Metal sample.

Figure 8. Apparatus for inserting graphite thermometer well into the

graphite crucible containing the molten metal sample. [The

weighted fused-quartz tube slides the thermometer well through

the hole in the lid into the molten metal. The mass of the

weighted fused-quartz tube was designed to be greater than the

buoyant force of the molten metal.]

27
the filled graphite crucible. (This graphite cell design has been used

with aluminum, cadmium, tin, and zinc samples). Before fitting the lid

snugly into the top of the crucible, the thermometer well was tested to

slide smoothly under its weight through the central hole in the lid.

When the fit was too snug, the well was pushed through the hole several

times to erode some of the graphite to loosen the fit. When the fit of

the thermometer well and lid hole was suitable, they were assembled with

the crucible and placed inside a fused-quartz tube as shown in Figure 8.

[Since the assembly shown in Figure 8 was used also with aluminum, fused

quartz was used instead of borosilicate glass.] A tungsten rod, that

was sealed in a fused-quartz tube and then inserted into the thermometer

well, added enough extra mass so that the well would sink into place

when the metal was melted. With zinc about 0.3 atmosphere pressure of

purified argon was admitted into the fused quartz tube space during the

melting period to reduce the volatilization of the metal. The induction

heating power was gradually increased until melting started as indicated

by the sinking thermometer well. When the thermometer well sank to the

proper position the induction heating power was turned off and the cell was allowed to cool

.

2.4 Assembly of Freezing-Point Cells

The zinc sample, now completely enclosed in the graphite crucible

with thermometer well and lid in place, was slipped into a precisionbore borosilicate glass tube (see Figure 4) for assembly into a zinc

freezing-point cell as shown in Figure 9. A layer or two of Fiberfrax

paper between the bottom and side of the graphite crucible and the

borosilicate glass holder served as a cushion as well as thermal insulation during the freezing experiments. The cell assembly was designed

to seal the stem of the SPRT and the contents of the glass tube

in helium gas to provide an inert atmosphere for the zinc sample and

graphite components and good thermal conductance to the SPRT. The cell was used with the helium-gas pressure slightly above (^ 250 Pa) the

ambient pressure. The two graphite heat shunts (see Figure 6), indicated

as G in Figure 9, were fitted closely to the inner wall of the glass

cell H and the outer wall of the glass thermometer guide tube F. The

shunts provided heat to the thermometer guide tube of the cell which in

turn tempered the stem of the SPRT and increased the effective "immersion"

of the SPRT. The outer section of the glass cell H where the two heat

shunts were placed was ground to fit snugly a metal sleeve, which was in

turn in good thermal contact with the top block of the furnace core so that the temperature of this section could be controlled by the furnace.

(See Section 2.5 for the description of the furnace.)

2.5 Freezing-Point Furnace

The design of the furnace that was employed with the zinc freezingpoint cell is shown schematically in Figure 10. The furnace was designed

to provide a highly uniform and precise temperature control. The furnace

core has a coaxial ly arranged stack of three aluminum cylinders [top

28
A

A. Platinum resistance thermometer.

B. To helium gas supply and pressure gauge.

C. Thermometer stem seal with silicone rubber

D. Silicone rubber stopper.

E. Insulation, washed Fiberfrax.

F. Thermometer guide tube, borosilicate glass

G. Heat shunt, graphite.

H. Borosilicate glass cell.

I. Graphite lid.

J. Graphite thermometer well.

K. Metal sample.

L. Graphite crucible.

M. Insulation, Fiberfrax paper.

Figure 9. Freezing-point cell of zinc, tin, or cadmi urn

29
H— - 35.6 cm H

Figure 10. Schematic of furnace used with zinc- and tin-point cells. [In

the investigation of the reproducibility of the cadmium point,

that is described in this paper, the furnace shown in Fig. 22 was used; however, the furnace shown here can also be used

with cadmium-point cells.]

30
Figure 10. Schematic of furnace used with zinc- and tin-point cells. [In

the investigation of the reproducibility of the cadmium point,

that is described in this paper, the furnace shown in Fig. 22 was used; however, the furnace shown here can also be used

with cadmium-point cells.]

A. Stainless steel tube, 11.4 cm o.d. x 0.76 mm wall.

B. Stainless steel tubes (6); 3.20 mm o.d. x 0.013 mm wall.

C. Insulation, Fi berfrax mats

D. Stainless steel tube.

E. Insulation, Fiberfrax mats and mica sheets.

F. Control heaters for top core block.

G. Top core block.

H. Sleeve (aluminum or Inconel) for metal freezing point

cell

I. Elevation of thermocouple junction (Chromel -P/Alumel )

,

top core block. (See a_)

J. Insulation, Fiberfrax sheets.

K. Brass shell, 4.8 mm thick.

L. Center core block.

M. Insulation, bulk Fiberfrax.

N. Elevation of thermocouple junction (Chromel -P/Alumel )

,

center core block. (See a).

0. Main heaters, held on with Inconel "garters".

P. "Spider" for centering the freezing point cell.

Q. Center core block end plate.

R. Insulation, Fiberfrax sheets.

S. Elevation of thermocouple junction (Chromel -P/Alumel

bottom core block. (See a_)

T. Bottom core block.

U. Control heaters for bottom core block.

V. Insulation, Fiberfrax mats and mica sheets.

W. Stainless steel tube support, 2.5 cm o.d. x 0.51 mm wall

x 11.4 cm long.

X. Heater leads.

Y. Posts for heater leads.

Z. To electric power.

a. Wells (B) for control ihermocoupl es (See I, N, and S).

b. Wells (B) for testing the temperature profile of furnace core,

c. Leads from top core block heaters.

31
(G), center (L), and bottom (T)] which are thermally insulated from each

other with Fiberfrax paper. A Nichrome V wire heater (main heater, 0),

installed inside two-hole oval alumina tubes, extends the full length of

the three blocks. These alumina tubes are closely spaced around the

length of the core and are held tightly against the blocks. The top and

bottom blocks each have an additional heater constructed of Nichrome V

wires (F and U) installed inside two-hole, round alumina tubes and

inserted into holes drilled through the blocks. The entire core of the

furnace, including the three blocks and the heaters, slips into a thinwall stainless steel tube (A). The weight of the core rests on a thinwall stainless steel tube (W) . The space around the core is packed with

Fiberfrax insulation.

The top block (G) was designed to receive either an aluminum or

Inconel sleeve (H), that was bored to fit as closely as possible the

glass freezing-point cell where the heat shunts are located (see Figure

9). [Since the NBS freezing-point cells have uniform outside diameters,

a single sleeve fits all of them. However, the sleeve is removable from

the furnace so that sleeves of other sizes could be used with cells of

other laboratories with slightly different diameters.] After long

continuous use at the zinc point the aluminum sleeve was found to bond

to the aluminum top block; hence, for the last several years split

Inconel sleeves have been employed exclusively with the furnaces.

The temperature of the center core (L) is controlled by means of an "absolute thermocouple" (N) and the main heater (0). The temperatures

of the top (G) and bottom (T) blocks are controlled relative to the

center block temperature by means of differential thermocouples [(I) and

(S), respectively] and heaters [(F) and (U), respectively] in the blocks.

The electric power to each of the heaters is controlled independently

and automatically from the indication of the appropriate thermocouple.

Tests on the uniformity of the furnace core showed that at 235°C the

center core temperature was uniform + 0.02°C [16].

Two furnaces of the above design were constructed for the NBS

Platinum Resistance Thermometer Calibration Laboratory. For operating

convenience, one is being used with zinc-point cells and the other with

tin-point cells; however, they can be used interchangeably. Freezingpoint cells of other metals (e.g., indium, cadmium, lead, or bismuth)

with freezing points below that of aluminum can be used in the furnaces.

Antimony-point cells (freezing point: 630.755°C) should be used with a copper-core furnace similar in design to the aluminum-point furnace

described in Section 4.5 to avoid accidental melting of the aluminum

furnace core by over-heating.

2 . 6 Preparation of Zinc-Point Freeze

Liquid zinc has been found to supercool about 0.02 to 0.06°C;

hence, its freeze can be initiated in the furnace without withdrawing

the zinc-point cell from the furnace. In contrast, with the tin-point

cell where the liquid tin supercools as much as 25°C, the cell is

withdrawn from the furnace to initiate the freeze (see Section 3.5).

32
The following is the procedure usually used in calibrating SPRT's at the

zinc point.

1. ) On the day prior to calibration of SPRT's, the furnace temperature

control is set about 5°C above the zinc melting point and the

sample is allowed to melt overnight. After the sample starts to

melt, it is completely melted within about three hours at this

temperature. Occassional ly, the furnace temperature has been held

as much as 20°C above the melting point to hasten the melting

process. (At the NBS, the zinc and tin freezes are conducted in

separate furnaces. When the cells are not being used, they are stored in the furnaces a few degrees below their respective freezing

points. Whenever it is necessary to remove the cells from the

furnaces, the cells are first cooled slowly in the furnace, by

turning the furnace power off, to avoid possible breakage of the

graphite crucible by too rapid cooling.)

4

2. ) The following morning, after inserting a monitoring SPRT into

the cell, the furnace temperature control is reset to about 5°C

below the freezing point, which results in cooling sufficiently

rapidly to initiate the freeze spontaneously.

3. ) About 5 minutes after the monitoring SPRT indicates that the

recalescence associated with the initiation of the freeze has

occurred, the SPRT is removed and two borosil icate-glass rods are inserted successively into the thermometer well for about 3 to 5 min each to induce an inner freeze immediately next to the thermometer

wel 1

4. ) During this time, the furnace temperature control is reset to

1°C below the freezing point.

5. ) After withdrawing the second borosil icate-glass rod, the cold

monitoring SPRT is inserted into the thermometer well and the

equilibrium temperature is observed after about 45 min. Meanwhile,

a test SPRT is preheated in an auxiliary tube furnace held at about

20°C above the zinc point.

6. ) After completion of the measurements on the monitoring SPRT, it

is removed from the zinc-point cell and the test SPRT is withdrawn

from the preheating furnace and quickly inserted into the cell.

The thermometer temperature, at the time of insertion into the cell

At the NBS, separate monitoring or check SPRT's are used with different

metal fixed points and comparison calibration apparatus. They are used

to test the consistency of the calibration process. Any deviation from

the established results indicates immediately that the measurement

process is not normal.

33
well, should preferably be slightly below that of the cell. (The

SPRT is inserted slightly colder than the cell temperature to avoid

melting and loosening of the solid zinc mantle around the thermometer

well.) When the SPRT is preheated to a temperature close to that

of the cell temperature, temperature equilibrium with the cell is

attained within a few minutes after insertion (see Figure 11).

Preheating of the SPRT also reduces the amount of metal frozen and,

hence, permits more SPRT's to be calibrated in the same freeze.

7. ) The resistance readings of the test SPRT are started about 15

min after inserting it into the zinc-point cell. Meanwhile, a second test SPRT is preheated in the auxiliary furnace.

8. ) After completion of the measurements on the first test SPRT, it

is replaced in the zinc-point cell by the second test SPRT. (Meanwhile,

a third test SPRT is preheated in the auxiliary furnace. Usually

a maximum of six test SPRT's are calibrated in any single zinc

freeze)

9. ) When the final test SPRT is inserted into the zinc-point cell,

the monitoring SPRT is preheated in the auxiliary furnace.

10. ) When the readings on the final test SPRT are completed, it is

replaced in the zinc-point cell by the preheated monitoring SPRT.

This second reading with the monitoring SPRT must not differ from

the first reading by more than 0.5 mK. Usually the difference is

not more than 0.1 or 0.2 mK. If the difference is larger than 0.5

mK, the measurements on some of the test SPRT's that were calibrated

last will be repeated using another zinc freeze. The test SPRT's

on which the measurements are to be repeated are selected on the

basis of the elapsed time since the start of the freeze and the

change in the freezing temperature that is observed with the monitoring

SPRT.

When the zinc-point freeze is prepared as outlined above, freezes

of 14 to 16 hours' duration are obtained. With cells assembled using

SRM-740 zinc standard, the change observed in the freezing point while

the first 50 percent is frozen is less than 0.2 mK. Figure 12 shows the

results of continuous observation of the freezing temperature without

disturbing the SPRT. In this particular freeze, two cold SPRT's were successively inserted into the thermometer well (instead of two cold

glass rods) to induce the inner freeze of solid zinc mantle around the

thermometer well. The freeze, with the furnace temperature at 0.9°C

below the freezing point, lasted about 18 hours with maximum temperature

change of about 1 .7 mK. The freezing curve shows a temperature change

of only 0.5 mK over the initial 75 percent of the freeze. (It is essential,

before using the final assembled cells for calibration of SPRT's, to

obtain first complete freezing curves to assess the purity of the metal

and to assess the quality of the freezing points that can be obtained).

34
i r t r

ZINC POINT

t

UJu

<

IcninUJ

12 MINUTES

mK

r

60 MINUTES

4

TIME

Figure 11. Consecutive measurements of the resistance of an SPRT in a single zinc freeze (SRM-740).

[An a-c bridge was employed in the measurements. Following

each equilibrium resistance observation, the SPRT was completely

withdrawn from the zinc-point cell, exposed to the ambient

temperature for one minute, and reinserted into the cell. In

the actual procedure employed in calibration the SPRT is

preheated close to the zinc point (see text). The data show

that the SPRT comes rapidly to equilibrium even when inserted

into the cell relatively cold. For comparison when the SPRT

was hotter than the cell temperature, see Fig. 14 (tin) and

Fig. 31 (mercury).]

35
O.OOI°C

ZINC POINT FREEZING CURVE

"A" SUPERCOOL

"B" SUCCESSIVE INSERTION OF TWO THERMOMETERS (NO RECORDED VALUES)

FURNACE HELD 0.9°C BELOW ZINC POINT

1 1

28

27

26

25

24

- 23

22

2120

-1-9, - 18 - 17 - 16

-I 15

14

~\ 13 - 12

-I I I 10 0 .05 J .15 .2 2 4 6 8 10 12 14 TIME AFTER START OF FREEZE, hrs

16 18 20

Figure 12. Freezing curve of zinc (SRM-740).

[The head of the SPRT was adapted for coaxial connectors and

an a-c bridge was used. The furnace was controlled at 0.9 K below the zinc point. The mass of zinc sample was 1280 g. In

this measurement, two cold SPRT's were inserted in the cell at

"B" to freeze a coating of zinc around the graphite thermometer

well.]

36
Different monitoring SPRT's are used with zinc and tin point cells.

Measurements at the triple point of water are obtained for every measurement

of the monitoring SPRT's at these metal freezing points. The accumulated

results on the monitoring SPRT's give information on whether the present

calibration process is consistent with the past measurements. At the

NBS, monitoring or check SPRT's are used in both fixed-point calibrations

and comparison calibrations of all SPRT's. Such measurements on the

check SPRT's are part of the nucleus of measurements for measurement

assurance in SPRT calibration at the NBS.

Figure 11 shows the results of consecutive measurements of an SPRT

in the same zinc freeze. Following each equilibrium resistance observation,

the SPRT was completely withdrawn from the zinc-point cell, exposed to

the ambient temperature for one minute, and then reinserted into the

cell. As described earlier, in the actual calibration procedure the

SPRT is preheated close to the zinc point to achieve temperature equilibrium

within the freezing point cell as quickly as possible. The results of

Figure 11 show that the SPRT comes rapidly to equilibrium with the cell even when inserted relatively cold. (See also Figure 14 for equilibrium

behavior when a preheated SPRT was inserted both hotter and colder than

the tin-point temperature).

37
3 . Tin Freezing Point

The state of thermodynamic equilibrium between solid and liquid tin

at 1 standard atmosphere of pressure is assigned the value 231. 9681 °C

(505.1181K), henceforth referred to as the freezing point of tin or,

simply, the tin point; it is the specified alternative to the normal

boiling point of water (steam point, 100°C) which is one of the fixed

points used to define the IPTS-68. Prior to 1966 the steam point was maintained at the NBS for the calibration of SPRT's; however, since 1966

the tin point has been maintained instead. The tin point can be realized

with greater reproducibility and ease than can the steam point.

3 . 1 Tin Samples

Two series of tin freezing-point standards, SRM-42g (lot HPM 6637)

and SRM-741 (lot HPM 6779), are available. 5 SRM-42g tin samples are nominally 99.999 percent pure. They were prepared by el ectrolyzing

(without addition agent) carefully selected commercially refined tin

(99.97 percent pure), melting and homogenizing the electrolyzed product,

and casting into semi-cylindrical bars 5 cm across the flat side and

about 60 cm long.

SRM-741 tin samples are nominally 99.9999 percent pure. They were prepared by zone refining the electrolyzed product by at least twenty

zone passes. The zone refined product was melted and homogenized and

then cast into semi-cylindrical bars similar in size and shape to SRM42g. (At the NBS, only SRM-741 tin is used in the calibration of SPRT's.)

Assay certificates that accompanied the lot HPM 6779 and 6637

samples from the supplier listed the impurities given in Table VIII.

The analysis indicates that the concentration of Pb was significantly

reduced in SRM-741 by zone refining. The assay was based on spectrochemical

analysis of a "master" sample from each lot. Also, samples taken from

the ends of two selected bars each from the two SRM's were analyzed

spectrochemical ly in the Spectrochemical Analysis Section of the NBS and

in the laboratory of the supplier. The results are summarized in Table IX.

The SRM-42g samples were made from tin bars of lot HPM 6637; these

references to the material will be used interchangeably. Similarly, the

SRM-741 samples were made from tin bars of lot HPM 6779; these references

to the material will also be used interchangeably. The NBS-OSRM obtained

from the supplier three lots of tin of about 159 kg each. Two of the

lots, designated by the supplier HPM 6637 and 7039, are nominally 99.999

percent purity; the third lot, designated HPM 6779, is nominally 99.9999

percent pure (NBS Order No. S-45045-67, Contract No. CST-309).

38
Table VIII

Emission Spectrochemical Analysis of the Tin Samples

(From the Assay Certificates of Supplier)

Suppl ier

El ement

SRM-741

3

Lot HPM 6779

(in ppm)

SRM-42g b

Lot HPM 6637

(in ppm)

In 0.1 0.2

Mg <0.1 <0.1

Pb 0.2 1

Si <0.1 0.1

Cu 0.1

a Date: December 23, 1966

b

Date: October 19, 1966

39
Table IX

Emission Spectrochemical Analysis of the Tin Samples3

SRM-741 SRM-42g

(Lot HPM 6779) (Lot HPM 6637)

Specimen Specimen

?ment 1A IB

(in

35A

ppm)

35B 1A IB

(in

35A

ppm)

35E

Ag <1 <1 <10 <1 <1 <1 <1 <1

Al ? ?

Ca ? <1 <1 <1 1 <1 <1

Cu <1 <1 <1 <1 <1

Fe <1 <1 <1 <1 <1 <1 <1

Mg <1 <10 <10 <1 <1 <1 <10 <1

Ni <1 ? ? <1 <1

Pb

Pt ?

Si <10 <10 <10 <10 <10 <10 <10 <10

Analysis of millings of each sample by a general qualitative spectrochemical

method by Elizabeth K. Hubbard, Spectrochemical Analysis Section, National

Bureau of Standards (May 16, 1967).

40 (contd .

)
Table IX

(continuation)

Emission Spectrochemical Analysis of the Tin Samples

SRM-741 SRM-42g

(Lot HPM 6779) (Lot HPM 6637)

Specimen Specimen

El ement 1A IB 35A 35B 1A IB 35A 35B

Analysis by the suppl ier (December 4, 1 967)

Ag <0.1 <0.1 <0.1 <0.1

Ca ND ND ND ND

Cu ND 0.1 0.1 ND

In ND

b

0.1 0.1 0.1 0.5 0.5 0.5 0.5

Mg <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.1

Po ND 0.1 0.1 0.1 1.0 1.0 1.0 0.7

Si 0.1 0.1 0.1 0.1 <0.1 <0.1 <0.1 0.1

b

ND = not detected.

41
The analyses of the two laboratories are in general consistent except

for In and Pb. The analysis by the supplier indicated 0.2 and 1 ppm of

In and Pb, respectively, in samples from lot HPM 6637 and 0.1 and 0.2

ppm of In and Pb, respectively, in samples from lot HPM 6779. The

general qualitative emission spectrochemical analysis method employed at

the time at the NBS did not detect these elements; special methods are probably employed in the laboratory of the supplier to analyze for them.

The results of mass spectrochemical analysis, summarized in Table X,

show that Pb, As, Cu, and S are definitely present in samples from lot

HPM 6779. The analysis on In and Pb is in agreement with the emission

spectrochemical analysis reported by the supplier. The large amount of

sulfur that was detected precludes the possibility that the tin samples

of lot HPM 6779 are 99.9999 percent pure. However, the sulfur is expected

to have insignificant effect on the freezing point. The high value (70

ppm) for silicon is an upper limit; emission spectrochemical analyses

(see Tables VIII and IX) indicate 0.1 ppm or less. The silicon is

probably present as silica and, as such, will not affect the freezing

point.

Table XI summarizes the results of neutron activation analysis of

two samples each from lots HPM 6637 and 6779. The analyses on In, La,

and Cu are consistent with the results of both emission and mass spectrochemical analyses. Arsenic was detected in samples of both lots but was

not determined quantitatively in the analysis.

The bars were received from the supplier individually sealed in

argon-filled polyethylene bags. The bars were identified by numbers and

the two ends were identified by A and B. Although efforts were made to

produce bars of uniform composition within each SRM standard, the RRR

measurements indicate that some segregation of impurities did occur,

i.e., that the bars are not the same in purity nor completely homogeneous

from one end to the other.

Deason et al

.

[8] (at the time with the Cryogenic Properties of

Solids Section of the NBS at Boulder, Colorado) investigated the RRR of

cylindrical samples of about 11 mm diameter and 30 mm length machined

from each end of all of the tin sample bars from the supplier. Before

making the measurements, the specimens were heavily etched Removing

about 0.26 mm from the diameter, washed, annealed at 200°C - 15°C for one hour in a vacuum, and slowly cooled. Wide variations in the RRR

between the paired ends of any single bar and among the bars were reported.

The range of RRR of the thirty-one bars of lot HPM 6637 was from 17,700

to 26,400, while the variations in the RRR between the paired ends of

any single bar ranged from essentially zero to 6900. The range of the

RRR values of the thirty-two bars of lot HPM 6779 was somewhat larger,

from 24,500 to 43,500, while the variations in the RRR between the

paired ends ranged from essentially zero to 13,000. The data indicate

that the impurities that affect the RRR are not uniformly distributed in

both lots of tin samples. To determine whether the RRR values may be

useful for selecting bars that would be suitable for freezing-point

application, samples were selected to represent a wide range of RRR

values from bars on which the RRR values of the two ends were the closest.

42
Table X

Mass Spectrochemical Analysis of the

Tin Samples from SRM-741 (Lot HPM 6779)

Specimen3

Element 22B 26A

(in ppm) (in ppm)

S 28 13

Cu 0.1 0.07

As 0.06 0.04

Pb 0.3 0.5

Sn interfered with the following elements; the values are estimated

upper 1 imits

El ement PPM El ement PPM

C 0.3 V 0.01

N 0.04 Cr 0.1

0 2 Mn 0.05

F 0.6 Fe 0.1

Na 2 Zr 0.06

Mg 0.05 Nb 0.2

Al 0.06 In 0.3

Si 70 Sb 0.5

CI 0.2 Cs 0.2

K 0.4 La 0.1

Ca 2 Au 0.7

There was complete interference with P, Co, and Ni.

(contd .

)

43
Table X (conti nuation)

The following values are the upper 1 imits of elements not detected;

FlU 1 pmpnf CI 11 C 1 1 L PPM r r i i El ement PPM FlL— 1 pmpnt CI II C II Lp PPM r n i

Li 0 01 Ru 0.04 Hnnu 0 03

Bp 0 01 Rh 0.01 L. 1

Ru 0 01 Pd 0.05 Tm1 III 0 03

Np 0 01 Ag 0.03 Yb 0 09

Ar 0 01 Cd 0.05 LuLU 0 03

Sc 0 01 Te 0.05 Hf1 1 1 0 08

1 1 0 01 I 0.02 Ta1 u 0 03

7n 0 01 Xe 0.06 w 0 1 0

0 01 Ba 0.03 Re 0.05

fie 0.02 Ce 0.02 0s 0.08

Se 0 02 Pr 0.02 Ir 0.05

BrL> 1 0 02 Nd 0.07 Pt 0.10

Kr 0 02 Pm 0.02 Hq 0.10

Rhr\U 0U» v|01 Sm 0.08 Te 0.05

Sr 0.01 Eu 0.04 Bi 0.04

Y 0.01 Gd 0.10 U 0.04

Mo 0.05 Tb 0.02

Tc 0.01 Dy 0.09

a The specimens were first etched in hydrochol oric acid solution and then

washed in distilled water prior to sparking. The samples were presparked

to identify and remove any surface contamination. The analysis was performed by C.W. Mueller and P.J. Paulsen, Analytical Chemistry Division,

National Bureau of Standards (December 31, 1968).

44
Table XI

Neutron Activation Analysis of the Tin Samples

SRM-741 SRM-42g

(Lot HPM 6779) (Lot HPM 6637)

Specimen Specimen

Element 26A 22B 1 9A 2B

(in ppm)

In 0.094±0.048a 0.126±0.016a 0.302±0.009a 0.317±0.031 a

La 1 .05 ±0.46xl03 0.62 ±0.51xl0" 31 .53 ±0.35xl0" 30.88 ±0.25xl0'

3

Cu 0.036±0.13 0.057±0.021 0.34 ±0.27 0.102±0.060

As Detected but not determined.

a Figures after the + symbol indicate standard deviation. Analysis by H.L. Rook,

Analytical Chemistry Division, National Bureau of Standards (August 5, 1970).

45
The samples that were selected, the designation of the tin-point cells

that were prepared from the samples, and the corresponding RRR values

are listed in Table XII. Since a sample for freezing-point measurements

required a 20 cm segment of the 60 cm long bar, the "duplicate sample"

from the same bar was from the middle of the bar. The value of RRR of

the second sample was taken to be the mean of the two end values.

The results of the freezing-point measurements (see Figure 13)

appear to be relatively insensitive to the variations of "impurities"

that affect the RRR value. (See Section 3.6 for the description of

freezing-point measurements.) In the region where the RRR values of tin

samples from lots HPM 6637 and 6779 nearly overlap, the freezing points

of the samples from the two lots are distinctly different, indicating

that the "impurities" that affect the RRR values are different from

those that affect the freezing point.

3.2 Graphite Cells for Tin

At the time the tin-point cells were assembled the graphite thermometer well -lid assembly with screw threads shown in Figure 2 was used

with the crucible of Figure 1. The simpler thermometer well-lid assembly

of Figure 3 is expected to be used for the assembly of future tin-point

eel 1 s.

3 . 3 Filling Graphite Cells with Tin

The same equipment arrangement (Figure 5) used to fill graphite

crucible with zinc was used with the tin samples. Since tin at its

melting point has very low vapor pressure, the bell jar was evacuated to

0.01 torr (1 torr = 1 mm Hg, 0°C = 133.322 Pa [37]) or lower pressure by

a mechanical pump that was "trapped" with molecular sieve. (The vapor

pressure of tin at its melting point is estimated to be about 6 x 10- 21 Pa

[28].) The funnel containing the tin sample and the graphite crucible

were heated inductively to a temperature a few degrees below the tin

point while pumping. After about 1 hour, the induction heater power was increased to melt the tin sample into the crucible. It required about

15 minutes to melt the sample completely. The sample was then allowed

to cool nearly to room temperature. The bell jar was filled to atmospheric

pressure with argon gas and the graphite crucible containing the solidified

tin was transferred into the same borosilicate glass tube (shown in

Figure 7) used with zinc for inserting the thermometer well -lid assembly.

The procedure used for inserting the thermometer well -lid assembly was

the same as described for zinc (see Section 2.3).

3.4 Tin Freezing-Point Cells

The tin-point cells were assembled by installing the graphite cells

containing tin samples inside borosilicate glass tubes. The assembly

procedures and the dimensions of the completed cell were the same as those of zinc cells (see Section 2.4 for details). As mentioned earlier,

the cell dimensions have been standardized so that the cells and furnaces

could be interchanged.

46
Table XII

Identification of Tin Samples by Bar Number, Residual Resistivity

Ratios, and Designation of Assembled Tin-Point Cells

PRT-LAB RRR

e Cell

Designation Sample Bar End

(SRM-741 , Lot HPM 6779) A B

C 34B 36,379 40,948

E 29A 36,030 36,060

6 7B 32,993 32,048

H 8B 28,571 27,778

I 23A 41,800 32,487

J 33A 40,504 31,491

K

a 26A 43,456 30,419

L

b 25A 38,642 33,351

M

C 30B 36,436 39,711

N 30

d 36,436 39,711

0 23

d 41,800 32,487

P 33

d 40,504 31,491

Q 26

d 43,456 30,419

(SRM-42g, Lot HPM 6637)

B 13B 19,423 18,520

D 10B 21 ,855 21 ,935

F 25A 25,042 22,757

(LAB STD)

LAB STD (LS) No RRR measurements

a Thermometer well broke after measurements.

^Thermometer well broke before any measurements could be made.

Accidentally overheated to above 500°C before any measurements were made.

d These are second samples from the same bar. The RRR values assigned to these

samples are the mean of the values of the two ends of the bar.

8Measurements by V.A. Deason, A.F. Clark, and R.L. Powell [8]

47
0.2

0.1

0

-0.1

* -0.2

5" - 0.3

-0.4

-0.5

-0.6

-0.7

n 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 r

5B

5

50 5T -

5F

6P

6Qg| 6N

60

1970-1971 COMPARISON OF TIN POINT CELLS

O AVERAGE

X OBSERVATION NOT IN AVERAGE

i i i i i i i i i i i i i i i i i i i i i i i i i i 20 25 30 35

RESISTANCE RATIO (R0°c/R4.2K ) x l0

~3

45

Figure 13. Comparison of tin freezing-point temperatures with

different residual resistance ratios: R(0°C)/R(4.2K)

[The prefixes 5 and 6 indicate samples nominally 99.999 percent

pure (SRM-42g, Lot HPM 6637) and samples nominally 99.9999

percent pure (SRM-746, Lot HPM 6779), respectively. The

alphabetic character identified the cell. The prime (')

identifies a second set of measurements of the same cell.

LAB. STD. is a tin-point cell that has been used at the NBS in

the calibration of SPRT's. Observations from four separate

freezes are shown for each cell. The plateau temperature

selected was the reading 1.5 hours after the initiation of the

freeze; the temperature corresponded to about 25 percent frozen.

The temperatures of the SPRT during the previous hour differed

typically 0.02 or 0.03 mK but not more than 0.1 mK

from the selected temperatures of any measurements. The observed

plateau temperatures are shown relative to the mean value

obtained for the tin-point cells 6C and 6E. Cell 6M was accidentally overheated above 500°C before the measurements.

Only two measurements were obtained for cell 6K.]

48
3 . 5 Freezing-Point Furnace for Tin

For the description of the furnace used for melting and for obtaining

the tin freezing point, see Section 2.5. The furnaces used with the

zinc and tin cells have the same design; however, the procedures used to

obtain their freezes are different.

3.6 Preparation of Tin-Point Freeze

Liquid tin has been found to supercool as much as 25°C. The

degree of supercool depends upon the temperature and length of time the

metal has been allowed to remain melted and upon the rate the liquid

metal is cooled. Impurities (e.g., Fe) seem to reduce the degree of

supercool [5]. If the freeze were initiated by lowering the furnace

temperature with the tin-point cell in the furnace, the furnace block

temperature could not be returned to the freezing-point temperature fast

enough to avoid excessive freezing of the tin. Hence, the tin freeze is

initiated by employing the "outside nucleated" freeze technique described

earlier by McLaren and Murdock [26].

The procedure usually used in preparing the tin-point freeze for

calibrating SPRT's is as follows. (The description follows the procedure

used with SRM-741 tin-point cells; the same procedure could be used with

the less pure SRM-42g tin-point cell.)

1. ) The tin sample is melted overnight in the furnace held at a temperature about 3°C above the melting point. After the

sample starts to melt, it is completely melted within about

five hours at this temperature.

2. ) In the morning, a monitoring SPRT is inserted into the thermometer well and the furnace control is reset down to a temperature 1 °C below the freezing point. (The monitoring SPRT used

with the tin-point cell is different from that used with the

zinc-point eel 1 )

3. ) When the monitoring SPRT indicates that the tin sample has

cooled to near the freezing point, the cell is withdrawn from

the furnace and held out in room temperature where rapid

cooling and the start of freezing occurs (whence the term

"outside nucleated" freeze). When the monitoring SPRT indicates

that recalecence has started, the cell is quickly reinserted

into the furnace.

4. ) The equilibrium temperature resistance of the monitoring SPRT

is usually obtained approximately 45 min after reinserting

the tin-point cell into the furnace. The comparison of measurements with induced inner freeze around the thermometer well with

49
1

I

those measurements prior to inducing the inner freeze has been

found to be within 0.1 mK. [The inner freeze around the

thermometer well is induced by inserting successively two

cold borosil icate-glass rods into the well as described in

Section 2.6], Hence, the SPRT calibration is usually performed

without the inner freeze.

5. ) While the tin-point cell and the monitoring SPRT are equilibrating, a test SPRT is preheated in an auxiliary furnace held

about 20°C above the tin-point.

6. ) When the measurements on the monitoring SPRT are completed,

it is removed from the tin-point cell and the test SPRT is

withdrawn from the preheating furnace and quickly inserted

into the cell so that the thermometer temperature during the

insertion is approximately that of the cell. [In actual

practice, unless the thermometer resistance is continuously

observed, it is not known whether any melting or any additional

freezing occurs upon insertion of the SPRT. In either event

the preheated SPRT reaches temperature equilibrium with the

cell within a few minutes (see Figure 14).]

7. ) The resistance readings of the test SPRT are started 15 min.

after inserting it into the tin-point cell. Meanwhile, a second test SPRT is preheated in the auxiliary furnace.

The preheating and resistance measurement procedure with test

SPRT's is very similar to that used with the zinc-point cell.

As with the zinc-point cell, usually a maximum of six test

SPRT's are calibrated in any single tin freeze.

8. ) When the final test SPRT is inserted into the tin-point cell,

the monitoring SPRT is preheated in the auxiliary furnace.

9. ) After the readings on the final test SPRT are completed, it is

replaced in the tin-point cell by the preheated monitoring

SPRT. As in the measurements of the monitoring SPRT used with

the zinc-point cell, this second reading of the monitoring

SPRT used with the tin-point cell must not differ from the

first reading by more than 0.5 mK. Usually the difference is

not more than 0.1 or 0.2 mK. If the difference is larger than

0.5 mK, the measurements on some of the test SPRT's that were calibrated last will be repeated next day using another tin

freeze. The test SPRT's on which the measurements are to be

repeated are selected on the basis of the elapsed time since

the start of the freeze and the difference in the freezing

temperatures that is observed with the monitoring SPRT.

When the tin-point freeze is prepared as described above, freezes

of 12 to 14 hours' duration are obtained. The duration of the freeze

depends upon the amount to which the tin sample supercooled before the

freeze was initiated (see Section 3.6). With cells containing SRM-741

50
i I r

TIN POINT

t

UJo

P

COCO

UJ

6 MINUTES

I mK

60 MINUTES

I I I TIME

Figure 14. Consecutive measurements of the resistance of an SPRT in a single tin freeze (SRM-741).

[An a-c bridge was employed in the measurements. Following

each equilibruim resistance observation, the SPRT was completely

withdrawn from the tin-point cell, preheated close to the tin

point in an auxiliary furnace, and reinserted into the cell.

The data show that the preheated SPRT comes to equilibrium in

a very short time. In two of the cases shown, the temperature

of the SPRT when inserted into the cell was slightly hotter than

the tin point. For comparison when the SPRT was not preheated

close to the cell temperature, see Fig. 11].

51
tin standards, after the initial 5 percent of the freeze has occurred,

the temperature change during the following 50 percent of the freeze is

less than 0.2 mK. Figure 15 shows the results of continuous observation

of the freezing temperature. The SPRT remained continuously in the cell

from the very beginning of the freezing process. No inner freeze of

solid tin was induced by inserting cold glass rods into the tin-point

cell. The freeze, with the furnace temperature controlled 1 °C below the

freezing point, lasted about 15 hours with less than 1.4 mK total change

in temperature. The freezing curve shows a temperature change of only

0.5 mK while over the initial 75 percent of the freeze similar to the

SRM-740 zinc standard.

Figure 14 shows the results of consecutive measurements of an SPRT

in the same tin freeze. Following each equilibrium resistance observation,

the SPRT was completely withdrawn from the tin-point cell, preheated in an auxiliary furnace maintained at a temperature 20°C above the tin

point, and reinserted into the cell. The results show that the preheated

SPRT attains temperature equilibrium in a very short time. In two of

the cases shown, the temperature of the SPRT when inserted into the cell was slightly above the tin-point temperature. (See Figure 11 where the

SPRT was inserted into the zinc-point cell relatively cold.)

52
Figure 15. The freezing-curve of tin (SRM 741). [The head of the SPRT

was adapted for coaxial connectors and an a-c bridge was used.

The furnace was controlled at 0.9 K below the tin point.

The mass of tin sample was 1300 g.]

53
I

4. Aluminum Freezing Point

The freezing point of aluminum, a secondary reference point with an assigned value of 660.46°C (933.61 K) on the IPTS-68, has been used for

many years in calibrating thermocouples [32]. In recent experimental

investigations [13,24] the aluminum point has been shown to be highly

reproducible (within ±0.5 mK) . Moreover, the aluminum point has also

been suggested as one of the defining fixed points of the future international SPRT temperature scale [14].

The freezing point of antimony (630.755°C), another secondary

reference point, is close to the freezing point of aluminum and has also

been used for many years in calibrating thermocouples. However, the

aluminum point has many advantages over the antimony point. Aluminum is

much more abundant than antimony and, because of the greater technological importance of aluminum, it is available in greater amounts at

high purity and at lower cost. At the freezing point, the vapor pressure of aluminum is about 2.5 X 10-6 p a ; on the other hand, that of

antimony is about 26 Pa [28]. Also, aluminum is less toxic than antimony.

The thermal diffusivity of liquid and solid aluminum at its freezing

point was estimated to be many times greater than the corresponding

values of liquid and solid antimony [2,15,17,30]. Liquid aluminum

supercools 1 or 2°C while liquid antimony can supercool more than 20°C

[24]. Hence, the realization of the freezing point of aluminum is

simpler than that of antimony.

4 . 1 Aluminum Samples

Chemical analyses of aluminum bars as originally received from the

supplier showed them to be of high purity; however, due to their poor

general appearance they were indeed not suitable for distribution as SRM

aluminum of high purity. Graphite particles were embedded in the aluminum

bars of one lot, while numerous voids were scattered through the bars of

another lot. The bars were subsequently recast to remove the above

imperfections. However, before recasting, freezing-point investigations

were made on samples from the original bars to test their suitability as freezing-point standards. The procedures that are described in this

paper for assembling the freezing-point cells and for carrying out the

freezing-point measurements were those followed on the original samples.

The procedures are equally applicable to the recast samples. At this

writing, freezing-point cells have been assembled using the recast

aluminum samples; however, freezing-point measurements are yet to be

made on them.

The original aluminum samples were received from the supplier in

two lots in the form of cylindrical bars about 91 cm long. The bars of

lot EM 1558 were 3.2 cm diameter, while the bars of lot EM 2571 were 3.8 cm diameter. Both lots had been purified by zone refining techniques

and homogenized by melting together the various zone refined units.

Bars of lot EM 1558 were made by casting in graphite molds, while those

of lot EM 2571 were made by casting in alumina molds. The bars of lot

54
EM 1558 contained specks of graphite; the bars of lot EM 2571 contained

a number of voids. The information supplied with lot EM 2571 suggests

that the voids were caused by hydrogen that was dissolved in the liquid

aluminum which on solidification expelled the hydrogen.

The chemical analyses given in the assay certificates that accompanied the samples from the supplier are summarized in Table XIII. The

assay is based on emission spectrochemical analysis. Except for Fe, the

analyses of the supplier for the two lots of aluminum are about the

same. The mass spectrometric analysis of lots EM 2571 obtained at the

NBS, summarized also in Table XIII, shows that Fe and Zn are present in

relatively high concentrations. The emission spectrochemical analysis

apparently also did not detect the 0.2 ppm of Na that was detected in

the aluminum by the mass spectrometric analysis. The limits of detection

by the mass spectrometric analysis of some of the elements are listed in

Table XIII.

If the worst case is considered, the analysis given in Table XIII

indicates that the aluminum of lots EM 2571 could be about 99.9993

percent pure. The available analysis on lot EM 1558 indicates the

material to be 99.9998 percent pure; however, the analysis on lot EM 1558

is not as extensive as that on lot EM 2571.

M.B. Kasen of the Cryogenics Division of the NBS at Boulder, Colorado,

made RRR measurements of samples taken from the ends, middle, between

middle and end, and outer surfaces of the aluminum bars of lot EM 2571

[20]. Of the total of 45 specimens (taken from 17 bars) that were examined, only two specimens were not within the RRR values of 5000 - 250. The RRR values of the two exceptions were low (4670), these specimens

being taken from the outer surface at the ends of a bar. They also had

high surface porosity. The RRR measurements indicate that the aluminum

sample bars of lot EM 2571 are fairly uniform. The "ultra-purity aluminum"

samples (SRM-RM-1R and SRM-RM-1C, see sample certificate Appendix G)

have the RRR values of about 16,200 or 16,600 [21], which suggests that

the aluminum of lot EM 2571 could be of lower purity.

For freezing-point investigations the presence of graphite particles

or voids in the original sample bars have very little influence on the

results. Reference to freezing-point measurements or results in this

paper are on specimens taken from the original aluminum bars of lots

EM 1558 and EM 2571

The solubility of hydrogen at 1 atm pressure in aluminum at the freezing

point is about 0.5 to 0.8 cm3 per 100 g of the liquid and about 0.04 to

0.05 cnr per 100 g of the solid [2]. The solubility of hydrogen in

solid aluminum is apparently less at the lower temperatures, the reported

solubilities at 400 °C being about 0.003 to 0.004 cm3 per 100 g [2].

(The above volumes of hydrogen are in terms of the gas at 1 atm pressure

and 20°C).

55
Table XIII

Chemical Analyses of the Aluminum Samples

Lot Lot

EM 1558 EM 2571

Element Supplier Supplier NBS

(in ppm)

ci (in ppm) be(in ppm)

Ca 0.2 0.2

Fe 0.5 0.1 1.4

Mg <0.1 0.1 0.2

Mn 0.1 0.2 <0.3C

Si 0.3 0.3

Cr 0.2 <0.04

Cu 0.2 0.1 <0.1

Ni ^0.2

Ti <0.05

V ^0.04

Na 0.2

Ga ^0.06

Zn 5

a Based on spectrochemical analysis (August 30, 1971).

^Based on spectrochemical analysis (June 30, 1972).

c Mass spectrometric analysis by P.J. Paulsen, Analytical Mass Spectrometry

Section, National Bureau of Standards (September 7, 1972).

^The values preceded by the symbol < indicate the upper limit of the

elements not detected.

56
The aluminum sample bars from lots EM 1558 and EM 2571 have subsequently been recast, in presence of chlorine, in fused quartz molds into

thirty cylindrical bars of 3.8 cm diameter and about 46 cm length, the

lot weighing about 50 kg total. The bars are now free of graphite

particles and large voids. J.G. Hust (Thermophysical Properties Division,

National Bureau of Standards) determined the RRR of the recast aluminum

by the eddy current decay method in three different specimen sizes

(approximately 50 cm 3

, 0.6 cm 3 and 0.06 cm

3

) taken from five bars [19].

The RRR values of seven 50 cm 3 gSpecimens taken from the five bars were investigated first; the 0.6 cm specimens were then prepared by sawing

and then machining the 50 cm 3 specimens. The 0.06 cm 3 specimens were prepared by swaging the~0.6 cm 3 specimens to 0.15 cm diameter rods. The

RRR values of the 50 cm specimens "as received" averaged 10,300 with a range of t 7 percent about the average. When annealed by heating in air

at 400°C for 30 min., the RRR values increased to an average of 11,400

with a range of t 9 percent. These values are more than twice the value

of 5000 obtained on the aluminum samples before recasting. (The specimens

used for the RRR measurements before recasting were either 3 or 4 mm

diameter, while the 50 cm3 specimens were 3.6 cm diameter. The average

density of the recast aluminum was found to be 2.683 t 0.005 g/cm 3 as compared to 2.699 g/cm3 for aluminum that is free of defects. No information is available on the density of the aluminum samples before recasting.) The average RRR value of the nine specimens of 0.6 cm3 volume

(4.1 mm diameter rods) was found to be 9230 with a range of t 12 percent.

The average RRR value of the six specimens of 0.006 cm* volume (1.5 mm diameter rods) was found to be 6760 with a range of i 15 percent. The

size effect corrections for these specimens of different sizes would

increase the RRR values by 1 , 7, and 15 percent, respectively. But, even with these corrections, the two smaller specimens give RRR values

that are smaller than the largest specimen. The reduction in the RRR

values is attributed to possible contamination during the process of

reducing the specimen size. These data show that the recasting process

increased the RRR values of the original aluminum samples. However, the

RRR values of the recast samples are still not as high as the ultrapurity aluminum samples, SRM-RM-1R and SRM-RM-1C (see Appendix G), which

have RRR values of about 16,000.

4.2 Graphite Cells for Containing Aluminum

Figures 16 and 17 show the dimensions of the graphite crucible,

lid, and thermometer well that were employed to contain about 358 g of

aluminum sample. Because of the relatively high chemical reactivity of

molten aluminum the filled graphite cell was completely enclosed in a fused-quartz envelope. Figures 18, 19, and 20 show three types of

assembled aluminum-point cells. The relatively large graphite thermometer

well accommodates the fused-quartz thermometer well into which the SPRT

is inserted during measurement.

4.3 Filling Graphite Cells with Aluminum

The aluminum samples of lot EM 1558, which were 3.2 cm diameter and

16.7 cm long, were inserted directly into the crucible and the lid

attached snugly at the top of the crucible. The samples of lot EM 2571,

57
h-43.7-

I h~33.3—

235.0

244.5

DIMENSIONS

IN mm

I

1

I —21.8 RADIUS

CRUCIBLE

Figure 16. Graphite crucible used to contain aluminum samples in freezingpoint measurements.

58
-43.7

1510

2 19.0

33.3—

LID

DIMENSIONS

IN mm

WELL

Graphite lid and thermometer well (slide-through type) for the

crucible of Fig. 16. [The thermometer well diameter is larger

than those of Fig. 2 and Fig. 3 so that a glass tube (fused

quartz) could be accommodated inside.]

59
5.1 cm

Figure 18. Aluminum-point cell sealed in fused-quartz envelope

(Type A eel 1 )

60
Figure 18. Aluminum-point cell sealed in fused-quartz envelope

(Type A cell )

A. Thermometer guide tube (Inconel,

7.94 mm o.d. x 0.13 mm wall) with

an adapter at the top for attaching

a handle for lifting or lowering the

cell into the furnace well.

B. Magnesia (MgO) block insulation.

C. Heat shunts (Inconel flange).

D. Fiberfrax insulation.

E. Fused-quartz outer envelope

(48 mm o.d. x 1.5 mm wall).

F. Graphite lid.

G. Graphite thermometer well.

H. Fused-quartz thermometer well

(precision bore (7.94 mmi.d.)

and continuous with the outer

envel ope)

I. Fused-quartz fiber, woven tape

for cushioning.

J. Graphite crucible.

K. Aluminum sample.

L. Inconel case.

M. Fused-quartz fiber pad for cushioning

the thermometer.

N. Fiberfrax paper liner.

61
— 51 cm-*

Figure 19. Al umi num-p

casing (Ty

oint cell in fuscd-quartz envelope with Inconel

pe B eel 1 )

62
Figure 19. Aluminum-point cell in fused-quartz envelope with Inconel

casing (Type B eel 1 )

A. Connection to high vacuum, purified

argon gas source, and manometer.

B. Fused quartz-to-Kovar graded seal

C. Fused-quartz connecting tube

(7.4 mm o.d. x 1 mm wall), outer

surface matte finished to minimize

radiation piping.

D. Thermometer guide tube (Inconel,

7 . 94 mm o.d. x 0.13 mm wa 11 )

E. Heat shunts (Inconel disks).

F. Insulation, cut from Fiberfrax blanket.

G. Fused-quartz outer envelope (48 o.d. x 1.5 mm wa 1 1 )

H. Graphite lid.

I. Graphite thermometer well.

J. Fused-quartz thermometer well

(precision bore (7.94 mm i.d.) and

continuous with the outer envelope).

K. Fused-quartz fiber, woven tape for

cushioning

L. Aluminum sample.

M. Graphite crucible.

N. Fused-quartz fiber pad for cushioning

the thermometer.

0. Fiberfrax paper liner.

63
Figure 20. Aluminum-point cell with fused-quartz casing (Type C cell).

64
Figure 20. Aluminum-point cell with fused-quartz casing (Type C cell).

A. Connection to high vacuum, purified

argon gas source, and manometer.

B. Fused quartz-to-Kovar graded seal.

C. Insulation, fused-quartz wool.

D. Fused-quartz thermometer well

(precision bore 7.94 mm i.d. and

continuous with the outer fused-quartz

envelope), outer surface matte

finished to minimize radiation piping.

E. Heat shunts, graphite disks.

F. Graphite lid.

G. Graphite thermometer well.

H. Graphite crucible.

I. Fused-quartz fiber, woven tape for

cushioning.

J. Aluminum sample.

K. Fused-quartz fiber pad for cushioning

the thermometer.

65
which were 3.8 cm diameter and 11.4 cm long, could not be slipped into

the crucible and, hence, were melted into the crucible. The same arrangement that was used for melting zinc into the crucible was employed (see

Figure 5), except that the melting occurred at a higher temperature and,

similar to the procedure with tin, that the system was continuously

pumped to a high vacuum (0.01 torr or less).

The arrangement shown in Figure 8 for zinc was employed to insert

the graphite thermometer well into the aluminum sample in the crucible.

With aluminum, the system was continuously pumped at high vacuum while

gradually increasing stepwise the induction heating power. The pumping

system was trapped with liquid nitrogen. When the temperature reached

approximately 600°C (a slight red glow in the graphite crucible), the

heating power was maintained constant for about one hour at a pressure

of about 0.01 torr. The heating power was then gradually increased

until the aluminum sample began to melt and the weighted thermometer

well began to sink into the crucible. After the sample began to melt,

the thermometer well sank into place within about thirty minutes. The

heating power was turned off and the sample was allowed to cool.

Purified argon was admitted into the system and the filled graphite cell

assembly was removed ready for assembly into an aluminum freezing-point

eel 1

.

4.4 Aluminum Freezing-Point Cells

Since molten aluminum is highly reactive, the filled graphite cell was completely enclosed in a fused-quartz envelope to protect the aluminum

sample. Three types of fused-quartz envelopes were employed (see Figures

18, 19, and 20). The procedures used to assemble the cells in the

fused-quartz envelopes were as follows. (Henceforth, for convenience

these cells will be referred to as types A, B, and C, respectively.)

The fused-quartz envelope was made first to fit the graphite cell. The

central thermometer well was made of precision bore fused quartz (8 mm

I.D. X 10 mm O.D.). The outer tube was sawed apart at the top. The

thermometer well was then sand-blasted on the outer surface starting

from about 3 cm from the tip to nearly the top of the well to obtain a matte finish to reduce light (heat) piping [27] from the aluminum-point

cell. (Any energy loss through light piping causes the SPRT readings to

be low.) The graphite crucible was then inserted into the outer fusedquartz tube using woven fused-quartz fiber tape as a cushion between the

crucible and the outer tube. Before the outer tube was resealed at the

top, either the fused-quartz thermometer well or the outer tube was shortened so that when assembled the bottom of the fused quartz thermometer well would be about 3 mm above the bottom of the graphite thermometer well. (When the aluminum is melted the buoyant force of liquid

aluminum will cause the bottom of the graphite thermometer well to rest

against the bottom of the fused-quartz thermometer well. Hence, during

operation of the cell the thermal resistance of the original 3 mm gap

will no longer exist.) The above allowance for the graphite thermometer

well to rise 3 mm was made to compensate for any unknown differences in

the thermal expansions of the graphite and fused quartz that were used

to fabricate the aluminum-point cell assembly. Also, when the aluminum

66
sample solidifies and contracts on cooling, the 3 mm allowance will

permit the sample to rest on the bottom of the crucible and not "hang"

on the thermometer well which could cause breakage of the well when the

cell is tilted.

Recently, while assembling new type A cells, the bottom of the fusedquartz envelope ruptured on two occasions. Although yet untried, the

breakage may possibly be avoided by increasing the gap between the

bottom of the graphite thermometer well and the fused-quartz well to

4 mm and by restricting the rise of the graphite well to 2 mm. It is

felt that, under certain cooling or heating conditions and certain

surface conditions of the graphite in contact with the sample, the

graphite crucible and its solid aluminum contents had become confined

between the fused-quartz well and envelope. Hence, to avoid strains

that would rupture the envelope, a gap should be maintained between

the bottom of the graphite well and the fused-quartz well.

Before sealing, the completely sealed type A cell, shown in Figure

18, had the same configuration as^that of type B cell of Figure 19.

After pumping at high vacuum (10~ torr; see Figure 21 for general

arrangement of pumping and gas handling system) for three days with the

cell at 670 to 675°C, the cell was filled with purified argon gas and

maintained at slightly above atmospheric pressure until the cell cooled

to the ambient temperature. The cell was then immersed in water at a known temperature. After the cell reached an equilibrium temperature,

the argon pressure in the cell was adjusted so that its pressure would

be 1 atm when heated to the aluminum point; the cell was then sealed at

the location shown in Figure 18. These type A cells were employed

during the preliminary freezing-point experiments. Although the effect

of the pressure on the freezing point is relatively small, the temperature

uncertainty due to the inability to determine the actual pressure inside

the sealed cell was disturbing; therefore, these cells were soon converted

to type. B configuration shown in Figure 19. Except for the sealed top

of the cell, type C aluminum-point cell is very similar to the zinc- and

tin-point cells (see Figure 9.)

4.5 Freezing-Point Furnace for Aluminum

The design of the furnace that has been employed with the aluminumpoint cells is shown schematically in Figure 22. The furnace is similar

in design to those used with the zinc- and tin-point cells (see Figure

10). (The furnace can be used at temperatures below the aluminum point

and possibly up to 800°C.) However, there are a number of basic differences in the design. The furnace consists of a central Inconel alloy

well (C) for the freezing-point cell. An Inconel disk (5) was "heliarc"

welded to seal the bottom end of the Inconel well. An Inconel flange

(B) was heliarc welded to the top end of the Inconel well. This Inconel

well assembly and the furnace shell (Q) form a vacuum tight enclosure

which protects the stack of four cylindrical coaxial blocks of copper

[top (H), end (J), center (0), and bottom (U)] from oxidation. The

space surrounding the copper blocks was packed with Fiberfrax insulation.

The space was evacuated and purged with argon gas several times and

finally filled to T-atm pressure with argon gas.

67
A

V

B

TO MECHANICAL

VACUUM PUMP

V

Figure 21. Schematic arrangement of the high vacuum, argon-gas purification

and handling, and pressure-measurement system for aluminumpoint measurements.

68
Figure 21. Schematic arrangement of the high vacuum, argon-gas purification

and handling, and pressure-measurement system for aluminumpoint measurements.

[High-purity argon gas (99.999 percent pure) is first trapped

at the oxygen boiling point to remove condensables and reacted

first with copper metal then with copper oxide at a furnace

temperature of 700°C. The argon gas is again trapped at the

oxygen point before use. The purification process is expected

to remove any hydrocarbon, oxygen, hydrogen, carbon dioxide,

or water.]

A. High-purity argon gas supply cylinder.

B. Cryogenic traps, liquid oxygen.

C. Inconnel tube (packed with copper metal and with copper

oxide) in furnace.

D. Cryogenic traps, liquid nitrogen.

E

Vacuum gauge, cold cathode type.

F. Vacuum gauge, thermocouple type.

G. Oil diffusion pump.

H. Dial type manometer, lowest subdivision of scale : 1 mm Hg

(torr)

J. Flexible loop of tubing (Inconel, 3.18 mm o.d. x 0.20 mm

wall) for connecting to the aluminum-point cell (type

B or C).

V. Valve, bellows type.

69
39.4 cm

A BC

35.6 cm

Figure 22. Schematic of furnace used with aluminum-point cells.

70
Figure 22. Schematic of furnace used with aluminum-point cells.

A. Hermetic insulated posts for electric leads.

B. Inconel flange welded to tube C.

C. Inconel tube furnace core.

D. Brass cover plate.

E. Electric leads (8) to control heaters.

F. Inconel tubes (8, 3.97 mm o.d. x 0.20 mm wall) with thermocouples

for temperature control or for monitoring the furnace temperature

G. Heater for the top core block.

H. Top core block.

I. Copper cylinder for holding the main heater M tightly

against the copper blocks H, J, 0, and U.

J. End core block.

K. Heater for the end core block.

L. Axial ly centered copper cylinder for receiving the furnace

core.

M. Nickel -chromium alloy wire for suspending and centering

copper cyl inder L. N. Main heater.

0. Center core block.

P. Coil for circulating cooling water.

Q. Stainless-steel furnace shell.

R. Inconel "spider" for centering the quartz glass cell (type

C cell) shown in Fig. 20.

S. Inconel disk welded to tube C.

T. End plate for the center core block.

U. Bottom core block.

V. Heater for the bottom core block.

W. Fused-quartz ring for supporting the main heater N.

X. Fiberfrax insulation.

71
I

The main heater (N) extends the whole length of the four copper

blocks. The heater was constructed from "U" shaped loops of heater wire

and oval-shaped twin-bore alumina tubes. These elements were assembled

tightly around the four copper blocks and the "U" shaped loops were heliarc welded in series to form a cylindrical heater. The control

heaters for the top, end, and bottom copper blocks were constructed in a manner similar to the main heater from "U" shaped loops of heater wire

and round twin-bore alumina tubes. These elements were inserted into

circularly arranged holes (see G, K, and V of Figure 22) and heliarc

welded in series to form the control heaters. (For more details of the

construction, see reference [13]).

The operation of the furnace is very similar to that used for the

zinc-and tin-point furnaces. A type K thermocouple (F), with its junction

located at the middle of the center block, is used in conjunction with

the main heater (N) to control the temperature of the center block at a desired temperature. The temperatures of the top, end, and bottom

blocks are controlled relative to that of the center block so that the

furnace well temperature is uniform. Automatic temperature controls are

used.

4.6 Preparation of Aluminum-Point Freeze

Liquid aluminum has been found to supercool 0.4 to 1.5°C, usually

about 0.4 to 0.6°C. Hence, the freeze can be initiated in the furnace

without the outside nucleation method used for tin (see Section 3.5).

The method for preparing the alurninum-point freeze was the same for the

three types of cells that were assembled (see Section 4.4). The completely

sealed type A cell, in which the argon gas was sealed to be 1 atm at the

aluminum point, did not require control of the argon gas pressure, but

cell types B and C required means for controlling and measuring the

argon gas pressure at 1 atm with appropriate accuracy. The effect of

pressure on the freezing point of aluminum is 7.1 mK/atm or 9.3 uK/torr

[13]. During operation the pressure is controlled to within ± 0.1

torr.

Except for operating at a higher temperature, the aluminum freeze

is prepared by a procedure very similar to that used with zinc.

1. ) With type B and C cells, if the argon gas pressure had

been below ambient during storage, the cell is evacuated to remove air, water vapor, or other gases that may have leaked

into the cell. (Figure 21 shows schematically the system that

is used for pumping, for filling with argon gas, and for

measuring the gas pressure.)

2. ) The furnace control is set 5°C above the aluminum point

to melt the sample overnight. During this time, the evacuation

of the cell is continued.

72
3. ) Next morning a monitoring SPRT is inserted into the cell

and purified argon gas is introduced to pressurize the freezing

process at 1 atm. [The monitoring SPRT and other SPRT's for

measurement at the aluminum point were specially constructed

[12, 40]. The insulation supports for the platinum thermometer

resistor and leads were either fused quartz or sapphire and

the thermometer sheath is fused quartz.] For the aluminumpoint cell, high-purity grade argon (99.995 percent) is first

cooled to the boiling point of oxygen to remove moisture and

other condensable impurities (see Figure 21). The gas is then

passed through a tube containing, successively, copper metal

and cupric oxide held at 700°C. The copper metal should

remove any oxygen and the cupric oxide should react with any

hydrogen, hydrocarbon, or other organic impurities. To remove

any carbon dioxide or water that was formed the gas is again

cooled to the boiling oxygen temperature before being introduced

into the aluminum-point cell.

4. ) To start the freeze, the furnace temperature control is

reset to about 2°C below the freezing point.

5. ) After the monitoring SPRT indicates that recalescence has

occurred, the SPRT is removed and two fused-quartz rods are inserted successively into the thermometer well for about 5 min each to induce an inner freeze immediately next to the

thermometer wel 1

6. ) During this time, the furnace temperature control is

reset to 1°C below the aluminum freezing point.

7. ) After withdrawing the second fused-quartz rod, the monitoring

SPRT is inserted stepwise into the thermometer well. Since

the SPRT will cool considerably between the time it is withdrawn

from any auxiliary preheat furnace and inserted into the

aluminum-point cell, the SPRT is preheated instead in the

section of the thermometer guide tube which is maintained

close to the furnace temperature. The SPRT is inserted initially

to a location where its tip is about 3 cm above the graphite

crucible lid. After about 10 min the monitoring SPRT is

inserted 5 cm and after another 5 min another 5 cm and so on until the tip of the SPRT is at the bottom of the thermometer

well. About 15 to 20 min are required after completion of the

insertion to be certain that the monitoring SPRT has attained

temperature equilibrium.

8. ) After completion of the reading, the monitoring SPRT is

withdrawn first from the cell to a location where the thermometer

tip is about 3 cm above the graphite crucible lid. After

about 5 min the SPRT is withdrawn 5 cm and after another 5 min

another 5 cm and so on until the SPRT is completely out of the

cell. The SPRT is then inserted into an auxiliary annealing

furnace held at 480 °C. [In platinum resistance thermometry

it is standard practice to determine the SPRT resistance at

73
the unknown temperature and then at the triple point of water

in order to determine the resistance ratio, W(t) = R(t)/R(0°C).

The aluminum point is high enough in temperature so that, if

the SPRT is withdrawn quickly out of the cell, lattice defects

will be quenched into the platinum resistor. By removing the

SPRT in steps, as described above, the quenched in defects are expected to be less. Before resistance measurements at the

triple point of water is made, the SPRT is annealed at 450 to

480°C for about 30 min. The withdrawing of the SPRT from a furnace at 480°C and cooling at ambient temperature show very

little effects (i.e., <2 parts in 10?) of quenched- in lattice

defects [1].]

9. ) After removing the monitoring SPRT from the aluminumpoint cell, the test SPRT's are each inserted, the resistance

readings obtained, and then removed from the cell and annealed

SPRT. Measurements of as many as seven test SPRT's have been

made with a single aluminum freeze.

10. ) After the measurements on the last test SPRT are com- pleted, measurements are then made on the monitoring SPRT.

[At present, SPRT's are not regularly calibrated at the

aluminum point; however, in test calibrations, this second

reading with the monitoring SPRT has not differed from the

first reading by more than 0.2 or 0.3 mK.]

When the aluminum-point freeze is prepared as outlined above,

freezes of about 10 to 12 hours' duration are obtained. Freezes of

longer duration have been obtained by controlling the furnace temperature

closer to the freezing-point temperature. Figure 23 shows freezing

curves that were obtained with five different cells containing the

aluminum samples described in Sections 4.1 and 4.4. Although results of

consecutive insertion and reading of an SPRT have not been obtained as of this writing, the results of Figure 23 indicate that consecutive

readings obtained over several hours should not differ by more than

about 0.1 mK.

74
TIME

Figure 23. Typical freezing curves of five aluminum samples recorded

starting approximately fifteen minutes after the insertion of

an SPRT in the eel 1 . [Measurements were obtained with an a-c bridge. The curves

are displaced both vertically and horizontally; they are

intended to show only the relative temperatures within the

individual freezing curves].

75
5 . Cadmium Freezing Point

The freezing point of cadmium, a secondary reference point, is

assigned the value 321.108°C (594.258K) on the IPTS-68 [36]. The metal

behaves in many respects similar to zinc. Like zinc, the vapor pressure

of cadmium at its melting point is relatively high (15 Pa [28]). Since

the metal and its compounds are toxic, cadmium should be handled with

caution.

5.1 Cadmium Sample

The SRM-746 cadmium is normally issued as a vapor-pressure standard

in the form of rods 6.4 mm diameter and 64-mm length. Enough rods were obtained from the NBS-OSRM to assemble two freezing-point cells for test

measurements. On the basis of chemical analyses and RRR measurements,

the material is considered to have a purity greater than 99.999 percent

(see SRM-746 certificate, Appendix H.)'7

A selected lot of cadmium was used as the starting material in the

preparation of the SRM-746 samples. The starting material was vacuum distilled twice followed by at least twenty zone refining passes. The zone refined units were homogenized into one lot (approximately 45.4 kg)

and cast into 14 billets of 4.8-cm diameter and 30.5 cm length. The

billets were then extruded into 6.4-mm diameter rods using a tungsten

carbide die. The rods were cut into 45.7-cm lengths, etched, washed,

and dried. The pieces were packaged in individual plastic tubes and

identified with respect to the billet and the position in the succession

of pieces that were extruded from the billet (e.g., piece E-4 is the

fourth piece extruded from billet E). Except for two billets, 29 or 30

pieces were obtained from each billet. The supplier furnished emission

spectrochemical analysis on samples taken from the starting 45.4 kg lot

of zone refined cadmium before casting into billets and samples taken

from each of the 14 billets. The analysis is summarized in Table XIV.

Table XV shows the results of mass spectrometric analysis of specimens

from pieces A-l and G-12. These results are generally consistent with

the emission spectrometric analysis given in Table XIV. The relatively

high Cr impurity found by the mass spectrometric method was not reported

in the emission spectrometric analysis.

V.A. Deason and R.L. Powell (at the time with the Cryogenic Properties

of Solids Section of the NBS at Boulder, Colorado) investigated the RRR

of the 6.4-mm diameter specimens from all of the billets and found the

values to range from 3,370 to 24,000. They estimated the purity from

the results to be from 99,9991 to 99.99987 percent [7, 9, 10].

The NBS-OSRM obtained from the supplier 45.4 kg of high-purity cadmium

in the form of rods 6.4-mm diameter by 45.7-cm length (384 pieces),

under NBS Order No. S-457534-67. The material is identified by the

supplier as Lot No. HPM 7541, Contract No. EMS 1426.

76
Table XIV

Emission Spectrochemical Analysis of the Cadmium

Samples (From the Assay Certificate of Supplier,

Lot HPM 7541, November 25, 1967

Impurity Elements, in ppm

CuMifeSiAaPbAlCa

Starting Material <0.1 0.1 ND

a 0.1 0.1 ND

a NDa ND

a

Billet

nA 0. 1 0. 1 ND 0 . L 0. 1 m n ND ND m n

nD 0 . 1 0 . L n o U . L 0 . b <0 . 1 m n ND ND ND

c 0.1 0 .0 .

0 . 5 <0.ND ND ND

D 0 . 1 0 . 1 01 0 . b 0. 1 ND ND ND

E 0 . 2 0.0 .

0 .0.ND ND ND

F 0.1 0.2 0.1 0.3 0.1 ND ND ND

G ND 0.1 ND 0.3 0.3 ND 0.2 0.1

H ND 0.1 ND 0.2 0.1 ND 0.2 ND

I ND <0.1 0.2 0.3 <0.1 0.3 ND ND

J ND <0.1 0.1 0.3 <0.1 0.3 0.2 ND

K ND <0.1 ND 0.3 0.1 0.5 ND ND

M <0.1 <0.1 ND 0.2 <0.1 0.5 ND 0.1

N 0.1 <0.1 0.1 0.3 <0.1 ND ND 0.1

0 0.1 0.1 0.3 0.3 0.1 ND ND 0.1

a ND = not detected. No information is available on the limits of

detection of the elements.

77
Table XV

Mass Spectrometric Analysis of Cadmium Specimens from SRM-746 (Lot HPM 7541)'

Specimen

A-l G-12

Element (in ppm) (in ppm)

Li 0.004 0.005

C

d

0.1 0.1

0

d

2 1

Na

d

2 3

Mg 0.1 0.07

CI 0.2 0.1

K

d

4 3

Ca

d

0.6 0.6

V 0.04 0.07

Cr 0.4 0.4

Cu 0.07 0.05

Zn 0.1 0.07

As

6

0.1 0.1

Rb

e 0.1 0.1

Pb 0.8 0.4

(contd)

78
Table XV (continuation)

Cadmium interfered with the following elements;

the values are estimated upper limits:

El ement PPM El ement PPM

B 0.004 Ni 0.05

N 0.06 In 0.6

F 2 Sb 0.2

Ne 0.01 Te 0.9

Al 5 I 0.4

Si 3 Cs 0.2

P 0.04 Ta 9

S 0.2 Pt 0.3

Ar 0.6 Au 10

Sc 0.4 Th 2

Ti 0.2

79
1

Table XV (continuation)

The following values are the upper limits of elements not detected:

El ement PPM El ement PPM

Be 0.001 Sm 0.2

Co 0.02 Eu 0.08

Ga 0.02 Gd 0.09

Ge 0.04 Tb 0.02

Se 0.03 Dy 0.08

Br 0.03 Ho 0.02

Kr 0.02 Er 0.07

Sr 0.01 Tm 0.02

Y 0.02 Yb 0.08

Zr 0.02 Lu 0.03

Nb 0.02 Hf 0.07

Mo 0.04 W 0.09

Tc 0.01 Re 0.04

Ru 0.04 0s 0.07

Rh 0.01 Ir 0.05

Pd 0.08 Hg 0.1

Ag 0.1 Tl 0.05

Sn 0.7 Bi 0.03

Xe 0.06 Po 0.04

Ba 0.5 Ra 0.04

La 0.04 Ac 0.04

Ce 0.04 Pa 0.04

Pr 0.04 U 0.04

Md 0.1 Pu 0.04

Pm 0.04

i was complete interference with Mn and Fe.

80
Table XV (continuation)

The analysis was performed by C.W. Mueller and P.J. Paulsen,

Analytical Chemistry Division, National Bureau of Standards (March

25, 1970).

^The specimens were cut with a saw, filed with a new unused file

and then etched in 10 percent solution of nitric acid to remove surface contamination. The samples were pre-sparked before analysis

to further lower surface contamination.

c Due to lack of a comparison standard, all results are based on sensitivity factors. Therefore, the results can be expected to be

in error by a factor of 3 to 10.

^These elements were found in all samples run by spark source mass spectrography; the values given should be considered as upper

limits since they may represent instrument contamination and other

factors

g Results of possible memory from previous analyses; the values

given should be considered as upper limits.

81
5.2 Graphite Cells for Cadmium

The SRM-746 cadmium samples were placed in two graphite cells of

different thermometer well dimensions. One cell had the thermometer

well-lid dimensions given in Figure 3; the other cell had larger thermometer-well dimensions, 11.3 mm I. D. X 14.5 mm O.D., to accommodate calibrations of special thermometers in holders. The former cell was filled

with about 1670 g of Cd and the latter cell with about 1400 g.

5.3 Filling Graphite Cells with Cadmium

The procedure for filling graphite crucibles with cadmium samples

was similar to that used with zinc samples (see Section 2.3). The vapor

pressure (about 15 Pa [28]) of cadmium at its melting point is high

enough to require argon gas to inhibit vaporization. Since the cadmium

sample rods were 6.4-mm diameter, some rods were packed directly into

the crucible and the remainder were placed in the funnel (see Figure 5).

After pumping at about 200°C for an hour, the bell jar was filled with

argon gas to about 0.3 atm pressure and the rods in both the crucible

and the funnel were quickly melted (in about fifteen minutes). (A

cadmium metal mirror formed at the top of the bell jar during this

process. In one case a thin brown ring also formed around the bell jar near the top of the crucible.) The filled crucible was allowed to cool

nearly to room temperature and the bell jar was filled to atmospheric

pressure with argon gas. The installation of the thermometer wells in

both filled crucibles involved sliding the well through the central hole

in the lid as shown in Figure 8. However, instead of "weighting" the

thermometer well into the melted cadmium, the "push rod" technique was

used. The filled crucible with attached lid was inserted into the

special borosilicate glass holder of Figure 7 and the thermometer well was positioned on the push rod and inserted part way into the hole in

the lid. After thoroughly purging the system with purified argon, the

cadmium was melted, and the thermometer well was then inserted fully

through the lid (see the procedure used with zinc in Section 2.3).

5.4 Cadmium Freezing-Point Cells

The cadmium-point cells were assembled by installing the graphite

cells containing the cadmium samples inside borosilicate glass holders

(see Figure 4). The assembly procedures and the dimensions of the

completed cells were similar to those of zinc cells (see Section 2.4 for

details). However, the SPRT guide tube diameters (see Figure 9) were different for the two cells; they were selected to match the inside

diameters of the respective graphite thermometer wells which were different (see Section 5.2).

5.5 Freezing-Point Furnace for Cadmium

Since cadmium melts below the melting point of aluminum, the aluminum

core furnaces used for realizing the zinc point and the tin point (see

Figure 10 and Section 2.4) could be used with the cadmium-point cell.

However, since the NBS aluminum core furnaces are used regularly for

calibrating SPRT's at the tin point and the zinc point, the testing of

the cadmium-point cells was actually done in the furnace used for realizing

the aluminum point (see Figure 22 and Section 4.5).

82
5.6 Preparation of Cadmium-Point Freeze

Liquid cadmium has been found to behave very similar to liquid zinc

and to supercool not more than 0.5 K. Hence, the freeze can be initiated

by lowering the furnace temperature a few degrees below the freezing

point and, when the freezing starts, re-adjusting the furnace temperature

to 1°C below the freezing point to prolong the freezing time. The

effect of pressure on the freezing point of cadmium is about 50 percent

larger than that of zinc (see Table III). The effect is large enough to

require the pressure over the freezing metal to be controlled instead of

being allowed to follow any change in the ambient pressure. The effect

of pressure on the freezing temperature of 6.2 mK/atm corresponds to

8.2 yK/torr. In the Washington, D.C. area the barometric pressure

almost never averages 760 torr and during a storm can change 20 torr or more. A change of 20 torr corresponds to 0.16 mK change which can be

readily detected on the SPRT. The effect of pressure and the problem of

toxicity can be minimized by sealing the cell in borosilicate glass (see

Figure 18). A manostat, similar in design to that described by Brombacher,

et al. [3], was used to control the helium-gas pressure over the cadmium

cell to about +0.1 torr. The pressure was controlled through the gas

inlet tube B of the freezing-point cell (see Figure 9).

The cadmium freeze is usually prepared as follows:

1. ) The helium-gas pressure over the cadmium metal in the cell is

set to control at 1 atm.

2. ) The furnace temperature control is set 5°C above the cadmium

point to melt the sample overnight.

3. ) Next morning an SPRT is inserted into the cell to monitor the

freezing process.

4. ) To start the freeze, the furnace temperature control is set to

control 5°C below the freezing point.

5. ) When the SPRT indicates that recalescence has occurred, the

SPRT is removed and two borosilicate glass rods are inserted

successively into the thermometer well for about 5 min each to

induce an inner freeze immediately next to the thermometer

wel 1

6. ) During this time the furnace temperature control is reset to

control 1°C below the cadmium freezing point.

7. ) After withdrawing the second glass rod, the monitoring SPRT is

inserted into the cadmium-point cell. Figure 24 shows the

results of monitoring a complete freeze; the temperature

changed about 0.4 mK over most of the freezing range. Over

the first 75 percent of the freeze the temperature change was about 0.2 mK.

83
Figure 24. The freezing curve of cadmium (SRM-746)

[The head of the SPRT was adapted for coaxial connectors and

an a-c bridge was used. The furnace was controlled to be 1 K below the cadmium point. The mass of cadmium was 1668g].

84
The cadmium point has not as yet been employed in routine measurements

for checking the calibration of SPRT's. However, the results of repeated

measurements with an SPRT showed the test measurements to be highly

reproducible (see Figure 25). The total change in the observed resistances

during seven hours since initiation of the freeze is shown to be about

0.2 mK.

85
<

go

CO

uu

DC

CADMIUM POINT

12 MINUTES

1 mK

x

4 5 6

HOURS SINCE INITIATION OF FREEZE

Figure 25. Consecutive measurements of the resistance of an SPRT in a single cadmium freeze (SRM-746).

[An a-c bridge was employed in the measurements. Following

each equilibruim resistance measurement, the SPRT was completely

withdrawn from the cadmium-point cell and then quickly

reinserted into the cell. The data show the somewhat cooled

SPRT to come to equilibrium rapidly.]

86
6. Mercury Triple Point

The state of thermodynamic equilibrium between the solid and liquid

mercury at 1 standard atmosphere of pressure is assigned the value

-38.836°C (234.314 K) on the IPTS-68 [36]. The freezing-point measurements

at 1-atm of pressure with mercury require a closed dry-gas handling

system or other procedures to avoid condensation of moisture in the

sample; however, the moisture condensation problem could be avoided by

using sealed cells of mercury and making "triple-point" measurements.

When the mercury cells are sealed, accidental contamination by metal

impurities is also avoided. The triple point of mercury will have the

value -38.841 °C (234.309 K) when the above value for the freezing point

at 1-atm pressure is adjusted for the effect of pressure on the freezing

point [36] (see also Table III).

This section describes the apparatus and procedures that were used

to assemble mercury triple-point cells. Equipment and techniques for

realizing the triple point of mercury by freezing and by melting techniques

are also described. In the freezing experiment, the principal difference

between mercury and the other metals discussed earlier (zinc, tin,

aluminum, and cadmium) is that the freezing curve for mercury is observed

at its triple-point pressure while with others the freezing curves are observed at 1-atm of pressure. (Henceforth, unless specifically indicated,

any reference to freezing points or freezing-point measurements with

mercury are at "triple-point conditions" with no external pressure other

than the vapor pressure of mercury. Similar triple-point conditions

apply when reference is made to melting-point measurements with mercury.)

6.1 Mercury Samples

Mercury samples, designated SRM-743, are available in 0.68 kg units

sealed under argon in soft-glass ampoules (see Appendix I). Mercury can readily be made purer than 99.9999 percent by nitric acid washings

followed by triple distillation, the first and second distillations

being conducted with small amounts of air bubbling through the boiling

mercury and the third distillation being conducted at high vacuum. Potassium or sodium hydroxide washings may precede the nitric acid

washings to remove such metals as tin, zinc, and lead. It was found,

using radioactive zinc, copper, and cobalt, that these elements can be

removed by nitric acid washings to levels below 1 in 10 11

, 10 10

, and

10 10

, respectively, in each case the limiting sensitivity of the analytical

method that was used [18]. The distillation in presence of air oxidizes

the base metals dissolved in the mercury; the low-temperature vacuum distillation minimizes the volatilization of the noble metals and leaves

these metals behind.

87
Mercury purified by procedures as outlined above was received from

the supplier in specially reinforced polyethylene bottles with leakproof screw caps containing about 0.68 kg each. The assay certificate

that accompanied the sample is summarized in Table XVI. On the basis of

the analysis and assuming that other elements are not present, the total

impurity content is less than about 10 parts in 10 9

. However, small

amounts of foreign material that appeared to be possibly polyethylene

powder or other organic solids were found to be floating on the mercury

surface in each of the polyethylene bottles. The material was returned

to the supplier to be re-packaged in soft-glass ampoules under argon

atmosphere. This re-packaged mercury is designated SRM-743.

3

To obtain information on the reproducibility of the triple point of

mercury, samples from two other sources were investigated along with the

SRM-743 mercury. The other two mercury samples were purified by procedures

very similar to those outlined above for SRM-743. One of the samples is

the NBS density standard mercury which had been prepared over twenty

years ago. The other mercury sample was prepared recently in the "Pressure

Section" of the NBS. Comparison of their triple points showed that the

values are well within +0.1 mK of the average.

This high purity mercury was originally received from the supplier in

two lots of about 45 kg each in the polyethylene bottles containing

about 0.68 kg each 65 bottles under NBS Order No. S-l 98495-71 and 67

bottles under NBS Order No. S-208195-72) . The supplier identified the

two lots HP 10,009 and HP 10,010 as being the same under lot No. EM 2279

and furnished one assay certificate. Both lots were returned to the

supplier to be re-packaged in flame sealed soft-glass ampoules. After

processing, 132 ampoules containing 0.68 kg each were obtained.

88
Table XVI

Emission Spectrochemical Analysis of the

Mercury Sample (SRM 743)

(From the Assay Certificate of Supplier)

Element Sample

Lot EM 2279

(in ppm)

Al <0.001

Ca <0. 001

Cu <0.001

Fe <0.001

Pb <0.001

Mg <0.001

Si <0.005

Ag <0.001

'Date: January 24, 1972

89
6.2 Borosil icate-Glass Cells for Mercury

Soft glass is recommended over borosil icate glass for storing

mercury [18]. However, for mercury triple-point cells, soft glass is

less resistant to rapid temperature changes and, therefore, more susceptible to breakage than borosil icate-glass cells. To test whether borosil icate glass causes significant contamination of the high-purity

mercury, mercury cells of borosil icate glass were made and the triple

points were observed by the progressive freezing method as employed with

zinc, tin, aluminum, and cadmium. The tests showed that borosil icate

glass over a period of three years has not caused noticeable change in

the freezing temperatures to indicate contamination.

Figure 26 shows schematically the borosil icate-glass cell for

mercury triple-point measurements. The two entrance tubes into the cell were required for thorough steam cleaning of the cell before filling

with mercury. After cleaning, the shorter entrance tube was sealed and

the cell was filled through the larger entrance tube (see Section 6.3).

Approximately 2.3 kg of mercury were used per cell. When the SPRT was inserted in the cell, the height from the mid-point of the SPRT coil to

the upper mercury surface, i.e., the "SPRT immersion", was 15.5 cm.

To investigate the possible application of stainless steel for

mercury triple-point cells, two type 304 stainless-steel cells with

dimensions very similar to those of the borosil icate-glass cells were

also constructed. Since we have as yet no "long term" information on stainless-steel mercury cells, this paper will deal principally with

borosil icate-glass mercury cells. However, some preliminary results

with stainless-steel cells will be shown.

6.3 Filling Borosilicate Glass Cells with Mercury

Figure 27 shows schematically the arrangement used to fill the

mercury cells by distillation from a reservoir. To avoid changing the

chemical composition of the original mercury source, the required amount

of sample was weighed into the reservoir and all of the sample was distilled into the cell.

The procedure used for filling mercury cell was as follows. The

borosil icate-glass distilling apparatus was first treated with reagentgrade nitric acid to remove any metallic impurities. After thoroughly

rinsing with distilled water, the apparatus was then steam-cleaned for

about eight hours. The filling tube at the top of the reservoir was sealed with a Teflon coupling and the distilling apparatus was mounted

with a steam-cleaned mercury cell as shown in Figure 27. The system was freed of moisture by pumping and warming. After thorough drying, dried

nitrogen was admitted and the reservoir was quickly filled with a requisite

amount of mercury and the filling tube to the reservoir flame sealed.

(The filling tube was made long enough to permit a number of sealing and

opening by breaking the tube.) The mercury cell was then heated to

about 400°C, by means of a small tube furnace placed around the cell,

and the system was pumped at high vacuum overnight. In the morning the

tube furnace was removed, cooling water was flowed through the condenser,

90
|^I4»!<14»-|

Mercury Freezing Point Cell

Borosilicate Glass

Figure 26. Borosil icate-glass cell used to contain mercury samples for

triple-point measurements.

[Two inlet tubes into the cell were essential for thorough

cleaning. After cleaning, the shorter 7 mm tube was flame

sealed.]

91
1

I

I

KJ

Arrangement for Filling Mercury Cell

Figure 27 Arrangement used for filling mercury cells by vacuum distillation.

[The borosil icate-glass cell was baked at 400°C under high vacuum for about 16 hours; all of the mercury sample in the

reservoir was then vacuum distilled into the cell; when filled,

the cell was flame sealed under vacuum.]

92
and the power to the heating mantle around the reservoir was switched

on. The system was continuously pumped at high vacuum during the distillation

process. When all of the mercury was distilled into the cell, the cell was flame sealed under high vacuum. (With the stainless-steel cell, the

filling tube was pinched flat near the top of the cell and then severed

and sealed at the flat by arc welding under protective argon atmosphere.)

6.4 Realization of Mercury Triple Point by a Freezing Method

A suitable temperature controlled bath that could be operated

unattended overnight was not immediately available for mercury freezing

experiments. A vacuum-tight stainless-steel jacket (see Figure 28) was designed to control the heat transfer between the mercury cell and the

Dry Ice-ethyl alcohol bath (henceforth to be referred to as the cooling

or freezing bath, see Figure 29) in which the assembly was immersed.

The heat transfer rate was controlled by adjusting the gas (air) pressure

in the space between the cell and the jacket. Freeze durations of 14

hours or longer have been obtained when the space was continuously

pumped at high vacuum.

The mercury freeze is usually prepared as follows.

1. ) The mercury cell is first installed inside the stainless-steel

jacket (refer to Figure 28). The copper foil (0.1 mm thick)

cylinder (I) plus the paper tissue cylinder (H) help to guide the

cell onto the centering stand (M). (The inside diameter of the

copper foil cylinder is slightly larger than the cell so that the

cylinder would not be in contact with the cell. Since the mercury

supercools somewhat, it is felt that the overall heat capacity

should not be increased unnecessarily. Otherwise, when the mercury

recovers from the "supercool" and begins to freeze, the extra heat

capacity can freeze too much mercury. The thermometer well extension (B) is sealed at (A). A vacuum valve is attached to the

holder before connecting to a vacuum system.

2. ) With air in the jacket space, the assembly is immersed in the

cooling bath inside one of the aluminum tubes shown in Figure 29.

The insides of the aluminum tubes are free of solid Dry Ice to

permit ready insertion of the cell assembly. The thermometer well

is filled with ethyl alcohol and a monitoring SPRT is inserted into

the wel 1

3. ) When the SPRT indicates that the mercury has cooled to a temperature near the freezing point, the jacket is evacuated.

Depending upon when the evacuation is started and is complete

enough to reduce the cooling rate to a small value, the sample

temperature could become cold enough for the mercury to start

freezing or it could become supercooled to a temperature where

spontansous freezing would not occur. In borosil icate-glass cells,

the mercury supercooled about 6°C; in the two type 304 stainlesssteel cells, the mercury supercooled only 0.1 to 0.3°C.

4. ) Whether the mercury has started to freeze or is supercooled,

the monitoring SPRT is removed from the cell and inserted into an auxiliary glass tube of ethyl alcohol immersed in the cooling

93
VALVE AND

VACUUM

A = "O" ring tube seal B = Thermometer well C = Ethyl alcohol in well D = Indium gasket seal E = Insulation, rolled paper tissue F = Stainless steel jacket

G = Tubular connection for cleaning and filling H = Insulation, tissue paper

rolled around (I) for centering

I = Copper foil cylinder

J = Borosilicate glass cell K = Mercury

L = Thermometer cushion

(fused quartz wool)

M = Stand for mercury cell N = Insulation (Aluminum

silicate wool)

Figure 28. Mercury triple-point cell assembly.

[Cooling is started with air in the stainless-steel jacket

space. When the mercury cell is adequately cooled, the space

is evacuated to reduce the cooling rate. In melting experiments,

the space is continuously evacuated.]

94
r* 32 H

I Diameter I

I

I

Bath for Mercury Freezing Points

Figure 29. Dry Ice - ethyl alcohol bath used in freezing experiments with

mercury.

[Two mercury triple-point cell assemblies can be accommodated. The

space inside the aluminum tubes is free of solid Dry Ice. Large

pieces of Dry Ice surround the space around the tubes. The "flange"

of the stainless-steel jacket rests on top of the aluminum tube

during cooling.]

95
mixture. [It was felt that ethyl alcohol containing dissolved

carbon dioxide may exhibit heat effects as the concentration of

carbon dioxide in solution changed; hence, every effort was made to

prevent any ethyl alcohol containing carbon dioxide from entering

the thermometer well.]

5. ) A closed-end, thin-wall stainless-steel tube containing ethyl

alcohol that was precooled in the above auxiliary glass tube (see

step 4) is inserted into the thermometer well of the mercury cell.

[Henceforth, the auxiliary glass tube containing ethyl alcohol that

is immersed in the freezing bath and used to precool the stainlesssteel tube or the SPRT to the temperature of Dry Ice will be referred

to as the precool ing bath.]

6. ) Three glass rods which had been precooled in liquid nitrogen

are successively inserted into the stainless-steel tube to induce

the mercury to freeze around the thermometer well. Each of the

rods is allowed to remain in the stainless-steel tube from 3 to 5 min before it is removed and the next rod inserted. [The auxiliary

stainless-steel freezing tube is used inside thermometer well to

avoid introducing ice particles into the well from the frostcovered, liquid-nitrogen-cooled glass rods. Any ice that is

introduced into the stainless-steel tube will be removed with the

tube. If the auxiliary stainless-steel freezing tube is not used,

the ice that is introduced into the thermometer well will slowly

dissolve in the ethyl alcohol. The heat of solution of the process

will be detected by the SPRT and will interfere with the mercury

triple-point measurements. On the other hand, whenever the auxiliary

stainless-steel tube or the SPRT is removed from the precool ing

bath and inserted into the thermometer well, some moisture from the

air is expected to dissolve during the transit period in the ethyl

alcohol that adheres to the stainless-steel tube or the SPRT sheath.

However, in this case the water will be already dissolved in the

ethyl alcohol; the heat effect, if detectable at all, should arise

from the mixing of adhering water-ethyl alcohol solution with the

ethyl alcohol that is in the thermometer well. This possible heat

effect has not been distinguishable under the conditions of the

experiments that have been employed.]

Figure 30 shows the results of continuous freezing of mercury

as observed with an SPRT that is left undisturbed in the cell

during the whole freezing process. Results from six cells are

shown, two cells each of the three mercury sample source. The curves are shown displaced, in temperature, so that the

shape of each freezing curve can be fully seen. Those curves with an initial higher value ("hump") are considered to be the

result of heating effects associated with the dissolution of

ice in the ethyl alcohol in the thermometer well. The ice was introduced when the frost-covered, liquid-nitrogen-cooled

rods were inserted directly into the well. Those curves stainless-steel freezing tube was used with the liquid-nitrogenwere cooled rods to freeze mercury onto the thermometer well.

96
0 2 4 6 8 10 12 14 16 18

HOURS

Figure 30. Freezing curves of six mercury cells.

[An a-c bridge was used in the measurements. Cells 1 and 2:

density standard mercury; cells 3 and 4: SRM-743 mercury;

and cells 5 and 6: Pressure Section mercury. Freezing curves were recorded approximately fifteen minutes after insertion of

the SPRT in the cell. The small initial anomaly (about 0.02

mK) is attributed to the heat of solution in ethyl alcohol of

small ice particles that were introduced into the thermometer

well when a liquid nitrogen cooled rod was inserted directly

into the thermometer well to induce an "inner freeze" around

the thermometer well (see text). The anomaly was absent, when

no ice was introduced. The curves are displaced vertically

to show the relative temperature of individual freezing curves.

The numbers identifies the cells; the prime indicates

a second freeze of the cell.]

97
I

7. ) After removing the third rod and the auxiliary stainless-steel

freezing tube, the monitoring SPRT is removed from the precooling

bath and is inserted into the thermometer well. With the induced

inner freeze around the thermometer well and when the SPRT is

precooled to the Dry- Ice temperature, the SPRT reaches temperature

equilibrium within about 4 min (see Figure 31).

8. ) After the monitoring SPRT is read, other SPRT's, after being

precooled in the precooling bath, may be successively inserted into

the mercury cell, for calibration. After a group of SPRT's (perhaps

six) is calibrated, the triple point is checked with the monitoring

SPRT; the second reading must be within + 0.1 mK of the reading

obtained before the test SPRT's were calibrated.

Figure 31 shows consecutive measurements of SPRT resistance in the same mercury freeze (or melt). After each equilibrium resistance observation,

the SPRT was withdrawn from the mercury cell and placed into the precooling

bath. After about two to five minutes, the SPRT was reinserted into the

mercury cell. Measurement sets B and C show the SPRT warming to the

equilibrium temperature. Measurement set A shows the readings obtained

with a stainless-steel cell during a melting experiment. In this case,

the SPRT was removed from the cell momentarily, then reinserted, so that

the SPRT cools to the equilibrium temperature.

6.5 Realization of Mercury Triple Point by a Melting Method

Since the mercury samples are extremely pure the triple points

obtained by freezing and by melting should yield nearly the same values.

The impurity segregation during freezing should be relatively small, so that the temperatures that are observed during melting are expected to

be quite close to those obtained during the freezing experiments. With

less pure samples, the relatively larger impurity segregation will cause

the temperatures during melting, particularly during the initial period,

to be lower than those obtained in the freezing experiments. When the

triple point was observed by melting immediately after observing the

triple point of the mercury sample by slow freezing, where impurity

segregation is expected to be relatively higher, the values differed at

most by about 0.2 mK (see Figure 32, curves BS/F and BS/M). However, if

the mercury had previously been frozen as rapidly as possible to minimize

impurity segregation, the values of the triple point by melting would be even closer to those obtained during freezing experiment.

In preparation for the melting-point experiments, the mercury is

frozen as rapidly as possible by either of two procedures. In the first

procedure the cell assembly is immersed in the freezing bath. A monitoring

SPRT is inserted in the thermometer well of the cell. With 1-atm pressure

of air in the jacket space, the mercury is usually completely frozen

within 20 to 25 min after the start of freezing. When the freezing is

complete, the jacket is immediately evacuated and the cell assembly

is transferred to a plastic-foam container (see Figure 33) for the

melting experiments. To prolong the melting process the jacket is

continuously pumped during the -experiment. Under such conditions, the

melting time has been about 8 to 10 hours.

98
MERCURY TRIPLE POINT

A ^ V—— Vs ^ ^ ^- 0.1 mK

,|. ^

MELTING HG T

a.'

Mi

Q. I HI

1 H B ^ Y ^

j>

^

01mK

P FREEZING HG

<_J

HIcc> , /

0.1 mK

t FREEZING HG

20MIN

TIME

Figure 31. Consecutive measurements of the resistance of an SPRT in

two mercury freezes (measurement sets B and C) and in the same mercury melt (measurement set A) [SRM-743].

[An a-c bridge was employed in the measurements. In the case of the mercury freezing experiments, following each equilibrium

resistance observation, the SPRT was withdrawn from the cell

and inserted into the "precooling bath" at the Dry-Ice temperature.

The SPRT was then reinserted into the mercury cell. The curves show that the SPRT warmed to the cell temperature. In the case of the mercury melting experiments, following the

equilibrium resistance observation, the SPRT was withdrawn

completely out of the cell and then quickly reinserted into

the cell. In this case, the curves show that the SPRT cooled

to the cell temperature.]

99
HOURS SINCE INITIATION OF FREEZE QR MELT

Figure 32. Freezing (F) and melting (M) curves obtained with mercury

samples in borosil icate-glass (BS) and stainless-steel (SS)

eel 1 s

.

[An a-c bridge was used in the measurements. The melting curve

(BS/M) was obtained immediately following the slow freezing-curve

(BS/F) measurements. The melting curve (SS/M) was obtained after

freezing the mercury with a heat pipe in the thermometer well.]

100
Plastic Foam Container

for Mercury Melting Points

Figure 33. Plastic foam container used in melting experiments with mercury.

[The stainless-steel jacket containing the mercury ceil is

inserted into the well of the container. The jacket space is

continuously evacuated to minimize the heat transfer rate.]

101
In the second freezing procedure a heat pipe similar to that described

by Evans and Sweger [11] is used inserted in the thermometer well of the

mercury cell. As during measurements of the cell temperature, ethyl

alcohol is used in the well to enhance heat transfer. The cell assembly

is inserted into the plastic-foam container and the jacket space is

continuously evacuated during the freezing process. The fluid in the

heat pipe is di f 1 uoro-dichl oromethane (CF2CI2) and the coolant is Dry

Ice - ethyl alcohol mixture. The mercury is usually completely frozen

within about 15 to 20 min after the start of freezing. To monitor the

freezing process, a thermocouple (type E) is attached to the mercury

cell and brought out through a vacuum seal at the top of the stainlesssteel jacket. When the thermocouple voltage indicates that the mercury

sample is completely frozen, the heat-pipe cooler is removed from the

thermometer wel 1

.

For both freezing procedures, the same melting technique is used to

observe the triple point. The stainless-steel jacket space is pumped at

high vacuum to prolong the melting time. After the frozen mercury

reaches a steady-state temperature as observed with the monitoring SPRT,

the number of warming rods that must be inserted into the thermometer

well to just start melting is estimated from the temperature of the

frozen mercury and the melting point. This estimate is based on previous

melting experiments using the thermocouple indication as guide, where

warming rods were successively inserted until the thermocouple indicated

that the melting point was reached. (An industrial PRT, which is more

sensitive, could be attached to the mercury cell instead of a thermocouple.

A heater could also be attached to the cell for heating to the melting

point.) After sufficient warming, a thin film of melted mercury is

formed around the thermometer well placing the liquid-solid interface

close to the SPRT during measurements. The triple point is first observed

with the monitoring SPRT. This is followed successively with test

SPRT's that have been precooled in the precooling bath. Figure 32 shows

freezing and melting curves obtained with borosil icate-glass and stainlesssteel mercury triple-point cells. The melting curve (BS/M) with the

borosil icate-glass cell was obtained immediately after the slow-freezing

experiments (BS/F). The melting curve (SS/M) with the stainless-steel

cell was obtained immediately after freezing the mercury with a heat

pipe inserted in the thermometer well. Figure 31 shows successive

observations with the same SPRT to demonstrate the repeatability of the

realization of mercury triple point during melting (see measurement set A).

102
7. Summary

The paper has described equipment and procedures that have been

developed at the NBS to realize the freezing (or melting) points of

SRM's of zinc, tin, aluminum, cadmium, and mercury. The reproducibilities

of the freezing points of the purer SRM's of these metals are shown to

be about ± 0.1 mK or better. The less pure SRM's can be used by similar

techniques to obtain freezing points; however, the temperatures that are observed will have greater variation because of the effect of impurities

(see Appendices for the certified freezing-point values of the less pure

metal SRM's).

Techniques for the realization of the metal fixed points are being

improved continuously. Other metals (e.g., indium, lead, bismuth, etc.) are expected to be tested in the near future.

103
8. References

[I ] Berry, R.J., Platinum resistance thermometry in the range 630-

900°C, Metrologia 2, No. 2, 80-90 (Apr. 1966).

[2] Brandt, J.L., Properties of pure aluminum, in Aluminum. Vol. I. Properties, Physical Metallurgy, and Phase Diagrams, Van Horn,

K.R., editor, 1-30 (American Society for Metals, Metals Park, Ohio,

1 967).

[3] Brombacher, W.G., Johnson, D.P., and Cross., J.L., Mercury Barometers

and Manometers, National Bureau of Standards (U.S.), Monograph 8,

59 pages (May 1960).

[4] Crovini, L., Bedford, R.E., and Moser, A., Extended list of secondary

reference points, Metrologia 1_3, 1 97-206 (1 977).

[5] Connolly, J.J., and McAllan, J.V., The tin-iron eutectic, Acta Met.

_23, 1 209-1 214 (1 975).

[6] Cutkosky, R.D., An a-c resistance thermometer bridge, J. Res. Nat.

Bur. Stand. (U.S.) 74C (Engr. and Instr.), Nos. 1 and 2, 15-18

(Jan. -June 1970).

[7] Deason, V.A., Electronic characterization of 1/4" cadmium rods,

private communication to R.E. Michael is (0SRM), September 20,

1 968.

[8] Deason, V.A., Clark, A.F., and Powell, R.L., Electronic impurity

characterization of tin samples, private communication to R.E.

Michael is (0SRM), November 22, 1 968.

[9] Deason, V.A. and Powell, R.L., Electronic characterization of 1/4"

cadmium rods, private communication to R.E. Michaelis (0SRM),

July 22 , 1 968.

[10] Deason, V.A., and Powell, R.L., Electronic characterization of 1/4"

cadmium rods, private communication to R.E. Michaelis (0SRM),

August 20, 1968.

[II] Evans, J. P., and Sweger, D.M., Immersion cooler for freezing ice

mantles on tri pi e-point-of-water cells, Rev. Sci. Instr. 40, No.

2 , 376-377 (1 969).

[12] Evans, J. P., and Wood, S.D., An intercompari son of high temperature

platinum resistance thermometers and standard thermocouples,

Metrologia 7 (No. 3), 108-130 (July 1971).

[13] Furukawa, G.T., Investigation of freezing temperatures of National

Bureau of Standards aluminum standards, J. Res. Nat. Bur. Stand.

(U.S.) 78A (Phys. and Chem.), No. 4, 477-495 (1974).

[14] Furukawa, G.T., Bigge, W.R., Riddle, J.L., and Rei 11 y, M.L., The

freezing point of aluminum as a temperature standard, in

Temperature Measurement 1975 , 389-397, edited by Billing, B.F., and

Quinn, T.J., Inst. Phys. Conf. Ser. No. 26 (The Institute of Physics,

Bristol and London, 1975).

104
[15] Furukawa, G.T., Douglas, T.B., and Pearlman, N., Heat Capacities,

in American Institute of Physics Handbook, pp. 4-105 to 4-118,

third edition (McGraw-Hill Book Company, New York, N.Y., 1972).

[16] Furukawa, G.T., Riddle, J.L., and Bigge, W.R., Investigation of

freezing temperatures of National Bureau of Standards tin standards

in Temperature, Its Measurements and Control in Science and Industry , Vol. 4, Part 1, 247-263 (Instrument Society of America, Pittsburgh,

Pa., 1 972).

[17] Gmelins Handuch der Anorganischen Chemie, Achte Auflage, Antimon,

System-Number 18, Teil B., Lieferung 1 (Verlag Chemie, Berlin, 1943).

[18] Gordon, C.L., and Wichers, E., Purification of mercury and its

physical properties, Ann. NY Acad. Sci. 65, 369-387 (1957).

[19] Hust, J.G., Informal report on RRR investigation of recast C0MINC0

aluminum, private communication to R.E. Michaelis (OSRM), May 15,

1 978).

[20] Kasen, M.B., Residual resistivity ratio of special C0MINC0 59

aluminum, private communication to R.E. Michaelis (OSRM), August 15,

1 972.

[21] Kasen, M.B., and Powell, R.L., Report of Investigation, UltraPurity Aluminum: RM-1R and RM-1C, private communication, October

27, 1970

[22] Kusters, N.L., MacMartin, M.P., and Berry, R.J., Resistance thermometry

with the direct current comparator, in Temperature, Its Measurement and

Control in Science and Industry , Vol . 4, Part 2, 1477-1485 (Instrument

Society of America, Pittsburgh, 1972).

[23] Mangum, B.W., and Thornton, D.D., Determination of the triple-point

temperature of gallium, Metrologia 1_5, 201 -21 5 (1 979).

[24] McAllan, J.V., and Ammar, M.M., Comparison of the freezing points

of aluminium and antimony, in Temperature, Its Measurement and

Control in Science and Industry , Vol. 4, Part 1, 273-285 (Instrument

Society of America, Pittsburgh, Pa., 1972).

[25] McLaren, E.H., The freezing points of high purity metals as precision

temperature standards, in American Institute of Physics, Temperature

,

Its Measurement and Control in Science and Industry , Vol , 3 , Part

1, 185-198 (Reinhold Publishing Corp., New York, N.Y., 1 962).

[26] McLaren, E.H., and Murdock, E.G., The freezing points of high

purity metals as precision temperature standards. V., Thermal

analysis on 10 samples of tin with purities greater than 99.99+

percent. Can. J. Phys. 3J3, 100-118 (1 960).

[27] McLaren, E.H., and Murdock, E.G., Radiation effects in precision

resistance thermometry. I. Radiation losses in transparent thermometer

sheaths, Can. J. Phys. 44, 2631-2652 (1966).

105
Nesmeyanov, A.N., Vapor Pressure of the Elements (Academic Press,

Inc. , New York, N.Y. , 1 963).

Powell, R.L., Report on the analysis of zinc SRM's, private communication

to R.E. Michael is (OSRM), May 31 , 1966.

Powell, R.L., and Childs, G.E., Thermal conductivity, in American

Institute of Physics Handbook, pp. 4-142 to 4-162, third edition

(McGraw-Hill Book Company, New York, N.Y., 1972).

Riddle, J.L., Furukawa, G.T., and Plumb, H.H., Platinum Resistance

Thermometry, Nat. Bur. Stand. (U.S.) Monograph 126, 129 pages

(April 1 973).

Roeser, W.F., and Lonberger, S.T., Methods of Testing Thermocouples

and Thermocouple Materials, Nat. Bur. Stand. (U.S.), Circ. 590, 21

pages (February 6, 1958).

Stimson, H.F., Precision resistance thermometry and fixed points,

in American Institute of Physics, Temperature , Its Measurements and

Control in Science and Industry , Vol. 2, 141-168 (Reinhold Publishing

Corp., New York, N.Y., 1955).

Stimson, H.F., International Practical Temperature Scale of 1948.

Text revision of 1960, J. Res. Nat. Bur. Stand. (U.S.) 65A (Phys.

and Chem.), No. 3, 139-145 (May-June 1961).

Stull, D.R., and Prophet, H., JANAF Thermochemical Tables, Second

Edition, National Standard Reference Data Series - National Bureau

of Standards (U.S.), 37, 1141 pages (June 1971).

The International Practical Temperature Scale of 1968. Amended Edition of

1 975, Metrologia 1_2, 7-1 7 (1 976).

United States Department of Commerce, National Bureau of Standards

Handbook 102, ASTM Metric Practice Guide, 53 pages (March 1967).

United States Department of Commerce, National Bureau of Standards,

Special Publication 481, The Gallium Melting-Point Standard, Editors:

Mangum, B.W., and Thornton, D.D., 35 pages (June 1977).

United States Department of Commerce, National Bureau of Standards,

Special Publication 260, NBS Standard Reference Materials Catalog,

1 979-1 980 Edition, 107 pages (April 1 979).

Wood, Sharrill D., An investigation of the stability of and insulation

leakage in some high temperature resistance thermometers: An

interim report, Nat. Bur. Stand. (U.S.), Tech. Note 764, 31 pages

(May 1 973).

106
9. Appendices

Certificates and Analyses of Standard Reference Materials and

Research Materials of Zinc, Tin, Aluminum, Cadmium, and Mercury.

107
U. 8. Depaxtmtirt'of Commerce

Maurice H Stans 9[W6»ny APPENDIX A

National B4rel£ oD SUndardi

L. M BrartWwe. Director

(Ecritficate

STANDARD REFERENCE MATERIAL 740

Zinc

Freezing Point on the

International Practical Temperature Scale (1968)

419.58 °C

The temperature given above is the value assigned to the freezing point of pure zinc as one of the

defining fixed points on the International Practical Temperature Scale of 1968. The fixed point is realized as the plateau temperature (or liquidus point) on the freezing curve of slowly frozen

high-purity zinc.

The zinc for this standard is of exceptional purity with the total of all other elements that affect

the freezing point less than one part per million. Based on samples tested, the temperature range of

melting for the bulk material is not expected to exceed 0.001 degree. Plateau temperatures for

samples of this material are not expected to differ from one another by more than about 0.0002

degree, and by not more than 0.001 degree from the assigned temperature.

Further information on temperature scales and metal freezing points may be found in f 1] "The

International Practical Temperature Scale of 1968." Metrologia. Vol. 5, p. 35 (April 1969). and [2J

McLaren, E. H., "The Freezing Points of High-Purity Metals and Precision Temperature Standards'"

Temperature, its Measurement and Control in Science and Industry, Vol. 3, Part 1, Rcinhold

Pubhshing Corp., New York, N. Y. (1962).

The zinc metal for the preparation of this standard reference material was obtained from

Cominco American Incorporated of Spokane. Washington. Evaluation of puritv and homogeneity

were performed in the NBS Institute for Materials Research by Robert Powell of the Cryogenic

Properties of Solids Section and by Robert Alvarez and Paul Paulsen of the Speetrochemical

Analysis Section. Temperature studies were performed in the NBS Institute for Basic Standards by

John P. Evans of the Temperature Section.

Washington, D. C. 20234 J. Paul Cali, Acting Chief

February 19, 1970 Office of Standard Reference Materials

(over)

108
APPENDIX A (contd)

SUPPLEMENTARY INFORMATION

PREPARATION, TESTING, AND HANDLING OF HIGH-PURITY

ZINC FREEZING POINT MATERIAL

PREPARATION: The zinc was prepared to obtain material of the highest possible purity and

homogeneity.

1. The starting material was a carefully selected lot of electrolytic special high-grade zinc (99.99+).

2. Processes included vacuum distillation, zone refining, and homogcnization.

3. Analytical control was maintained throughout the entire preparation.

4. The final bars (about two feet long) were individually sealed in argon filled polyethylene bags.

TESTING: Selected samples were taken for testing which were representative of the entire lot.

1. Testing initially was performed by optical emission and spark source mass spectroscopic

procedures and by resistivity ratio measurements. As a result of this testing the material

tentatively was accepted both from the homogeneity and composition standpoints.

2. Finally thermal analysis of selected samples was performed to determine the suitability of the

material for the freezing point standard. Based on the samples tested, the temperature range of

melting for the bulk material is not expected to exceed 0.001 degree, and by not more than

0.001 degree from the assigned temperature of 419.58 °C.

HANDLING: Any handling procedures on ultra high-purity material are apt to introduce

contamination. The procedures which follow were established to minimize contamination

particularly with respect to the elements that could affect the freezing point measurements.

1. The zinc bars were cut dry with a carbide tipped cutter (to form 350g bar sections) in such a way that only the carbide tip touched the zinc metal. At no time were the zinc bars touched by

hand (polyethylene gloves were used), and at no time were the bars in contact with am part ot the milling machine other than through the polyethylene cover.

2. Following cutting, the samples were acid cleaned in high-purity dilute nitric acid, rinsed with

distilled water, and then air dried.

3. Individual samples were bagged in polyethylene and sealed.

SHOULD ANY CUTTING OF THE SAMPLES BE REQUIRED OR SHOULD ANY

CONTAMINATION BE SUSPECTED, IT IS RECOMMENDED THAT THE APPROPRIATE

HANDLING PROCEDURES DESCRIBED ABOVE BE EMPLOYED JUST PRIOR TO USE.

109
S. Depurtment of Commerce

Frederick B. Dent

Secretary APPENDIX B

National Bureau of Standards

Richard W. Roberta, Director

Rational ^Puram nf Standard*

Certificate

Standard Reference Material 43h

Zinc

Freezing Point on the International

Practical Temperature Scale (1968)

419.5 °C

The freezing point of SRM 43h, 419.5 °C, is the value assigned based on the relative chemical

purity with respect to all former SRM 43 standards and with respect to the highest purity zinc

freezing point standard, SRM 740. It is estimated that the freezing point will not deviate from the

assigned temperature by more than ±0.1 °C.

Full details of the precautions that should be observed in freezing point determinations are given

in National Bureau of Standards Circular 590, and in Metrologia No. 3, 108-130 (Julv 1971). The

International Practical Temperature Scale of 1968 is described in Metrologia .5, No. 2, 35-44 (April

1969).

The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference Materials

by R. E. Michaelis.

Washington, D. C. 20234 J. Paul Cali. Chief

August 15, 1973 Office of Standard Reference Materials

110
U. S. Department of Commerce

C. R-.Smith

APPENDIX C

Certificate of &nal>fite

Standard Reference Material 683

Zinc Metal'

This standard of zinc metal is issued as a special research material to further both

chemical and physical methods of characterization. Two other zinc metal standards are also

available: SUM 682, High-Purity Zinc, and SUM 782, Intermediate Purity Zinc. All three

standards were prepared from the same starting material.

Recommended Range of Values

Value Reported 3 Method of

Element2 (ppm by wt.) (ppm by wt.) Analysis 4

Lead 11.1 (9.6 - 11.3) SSMS-ID, POL

Copper 5.9 (5.3 - 6.1) SSMS-ID, POL

Iron 2.2 (1.7 - 3.1) POL.SPPH

Silver 1.3 (1.0 - 1.4) SSMS-ID, NAA

Cadmium LI (1.0 - 1.2) SSMS-ID, POL

Thallium (0.17 - 0.18) SSMS-ID

Tin (0.02) (0.013 - 0.023) SSMS-ID

1. Hie zinc is in the form of semicircular bar segments about 2 1/4 inches in diameter, 1 inch deep at nrid-diameter, and

3/4 inch long. ^ j ; \

2. In the course of analysis by neutron activation, additional elements were sought. The following elements were not

detected and are reported with an estimated upper limit of concentration in parts per million by weight: As «0.002) <V

Ga «0.002K ~ C

in «r>r02) \'y

Potassium was not detected by either flame emission spectroscopy or by neutron activation at the 0.2 ppm level.

Mn «0.2)

Mo «0.02>

Rh «0.3>

Sc <(0.003>

V «0.005>

W «0.0001)

Aluminum, antimony, and sodium were detected by several techniques. The results were variable, but in no case are these elements present to a greater concentration than 3 ppm. Gold appears to be < 0.02 ppm.

3. The range of values reported is the extreme variation of the individual results reported by the methods of analysis used. The recommended value is based on considerations of the estimated systematic bias of each of the methods employed. From 7 to 13 individual determinations were made for each element certified.

4. SSMS-ID — Spark-Source Mass Spectrometry — Isotonic Dilution (R. Alvarez and P. Paulsen) POL - Polarography (E. J. Maienthal) SPPH - Spectrophotometry (E. R. Deardorff) NAA — Neutron Activation Analysis ( B. A. Thompson and D. A. Becker)

5. Values in parentheses are not certified since only one method of analysis was used, but are provided for additional information on the composition.

Washington, D. C. 20234 W. Wayne Mcinke, Chief

July 9, 1968 Office of Standard Reference Materials

(over)

111
APPENDIX C (contd)

This standard has been established to provide a homogeneous reference material for the

analysis of pure zinc and analogous metaJs. It should also serve a useful function for the

physicist and materials engineer involved in the preparation and characterization of

phosphors and other solid-state compounds, where a knowledge of the purity of the starting

material is important. The materia! was prepared by Cominco American. Inc. from a special

lot of high-grade electrolytic zinc which was homogenized and cast in the form of

semicircular bars. Each bar was etched, dried, and sealed in a polyethylene pouch to minimize contamination.

Extensive homogeneity testing was performed by NBS Washington and Boulder and the

material was found to be satisfactory for the elements certified. The samples selected for

testing were carefully chosen to represent the extreme variations that might be expected as a

result of the preparation procedures. However, practical limitations precluded the testing of

the number of samples from each bar that would have been required to guarantee absolute

limits of homogeneity. Therefore, some inhomogeneity in the untested material is possible

but not probable. The testing was performed using combinations of the following methods:

optical emission and spark-source mass spectrographic analysis, polarographic analysis, flame

emission and atomic absorption analysis, neutron activation analysis, and electrical measurements for residual resistivity ratios.

Although the spark-source mass spectrographic measurements indicated homogeneity

with respect to microsamples, it is recommended that samples as large as possible be

utilized, preferably representative of the full cross-section.

The technical and support aspects involved in the preparation, certification, and issuance

of this Standard Reference Material were coordinated through the Office of Standard

Reference Materials by R. E. Michaelis.

CAUTION

Before use, it is recommended that possible surface contamination be removed by placing

the sample in dilute high-purity nitric acid for about one minute, followed by rinsing in

distilled water.

112
U. S. Department of Commerce

Peter G. Poterson

Secretary

APPENDIX D

^National ^uram of Standards

Certificate

Standard Reference Material 42g

Tin

Secondary Freezing Point Standard

231.967 °C

(International Practical Temperature Scale - 1968)

The freezing point of SRM 42g, 231.967 °C, is the value determined by comparison with

SRM 741, Tin Primary Freezing Point Standard (International Practical Temperature Scale— 1968).

The freezing point ofSRM 741 is estimated to be 231.9681 °C ± 0.0007 °C. [The IPTS-68 assigned

freezing point of pure tin is 231.9681 °C]

The observed freezing points (with 25 percent in the solid phase) of three selected samples of

SRM 42g, each of which weighed 1300 g, agreed within ± 0.1 mK and averaged 0.5 mK lower than

the average value of eleven samples of SRM 741. (For full details of the preparation and inter- comparison of the freezing points of the tin specimens, see Proceedings of the 5th Symposium on

Temperature, June 1971.)

In the freezing-point experiments, the furnace surrounding the sample cell was maintained at 0.9 K below the tin point (where the total freezing time, tvpicallv about 14 hours, was dependent on the degree of super-cooling). During these experiments, the freezing points generally did not

decrease by more than 0.3 mK over a period of seven hours. The standard deviation of the freezing

temperatures of each sample was ± 0.05 mK from freeze to freeze (with about 25 percent in the

solid phase).

Technical measurements at NBS leading to certification were performed by G. T. Furukawa,

J. L. Riddle, and W. R. Bigge of the NBS Heat Division.

The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference Materials

by R. E. Michaelis.

Washington, D.C. 20234

July 18, 1972

J. Paul Cali, Chief

Office of Standard Reference Materials

113
U. S. Department »f Commerce APPENDIX E

Frederick B. Dent

Secretary

National Bureau of Standards

Richard W. Roberta, Director ^National ^nxmn of Standards

Certificate

Standard Reference Material 741

Tin

Primary Freezing Point Standard

231.9681 °C

(International Practical Temperature Scale -1968)

The temperature given above is the value assigned to the freezing point of pure tin as one of the

fixed points on the International Practical Temperature Scale of 1968 [1]. Based on chemical

analyses and freezing point experiments, the freezing point of SRM 741 is estimated to be:

231.9681 ± 0.0007 °C

SRM 741 is high-purity tin, with a total of all other elements that would affect the freezing

point to be about 1 ppm. The freezing points (with 25 percent in the solid phase) of eleven selected specimens of SRM 741, each of which weighed 1300 g, agreed within ± 0.1 mK. (For full details of

the preparation and comparison of freezing points of tin specimens, see Proceedings of the 5th

Symposium on Temperature, June 1971.)

In the freezing-point experiments, the furnace surrounding the sample-cell was maintained at 0.9 K below the tin point (where the total freezing time, typically about 14 hours, was dependent on the degree of super-cooling). During these experiments, the freezing points generally did not

decrease by more than 0.1 mK over a period of seven hours. The standard deviation of the freezing

temperatures of each specimen was ± 0.05 mK from freeze to freeze (with about 25 percent in the

solid phase).

The tin metal for the preparation of SRM 741 was obtained from Cominco American, Inc., Spokane, Washington.

Temperature studies were performed by G. T. Furukawa, J. L. Riddle, and W. R. Bigge of the

NBS Heat Division.

Evaluations of purity and homogeneity were performed in the NBS Cryogenics Division

(Boulder, Colo.) by V.A. Deason, A. F. Clark, and R. L. Powell; and in the NBS Analytical

Chemistry Division by C. W. Mueller, P. J. Paulsen, H. L. Rook, and P. D. LaFleur.

The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference Materials

by R. E. Michaelis.

Washington, D.C. 20234

July 18, 1972

(over)

114

J. Paul Cali, Chief

Office of Standard Reference Materials
APPENDIX E (contd)

For further information of temperature scales and metal freezing points, see:

[1] The International Practical Temperature Scale of 1968, Metrologia 5_, 35 (April 1969).

[2] McLaren, E. H., The Freezing Point of High-Purity Metals and Precision Temperature

Standards. Temperature, its Measurements and Control in Science and Industry, Vol. 3, Part 1, Rheinhold Publishing Corp., New York, N.Y. (1962).

SUPPLEMENTARY INFORMATION

Preparation, Testing and Handling of

High-Purity Tin Freezing Point Material

PREPARATION: The tin was prepared to obtain material of the highest possible purity and homogeneity. The starting material was a carefully selected lot of refined tin (99.97). Purification processes included electrolysis, zone refining, homogenization, and degasification. Close analytical control was maintained throughout the entire preparation. The final bars (about two feet long) were

individually sealed in argon-filled polyethylene bags.

TESTING: Selected samples, representative of the entire lot, were tested. The tests were initially

performed by optical emission and spark source mass spectroscopic procedures and by residual

resistivity ratio measurements. As a result of these tests, the material was tentatively accepted both

from the homogeneity and composition standpoints. Finallv, freezing point studies of selected

specimens were performed to determine the suitability of the material. Based on the specimens

tested, the freezing point of the lot was estimated to be within ±0.7 mK of that of 100 percent pure

tin. HANDLING: Any handling procedures on high-purity materials are apt to introduce contamination.

The procedures described below were followed to minimize contamination particularly with respect

to the elements that could affect the freezing point measurements.

1. The tin bars were cut dry with a carbide tipped cutter in such a way that only the carbide tip

touched the tin metal. At no time were the tin bars touched by hand (polyethylene gloves were

used), and at no time were the bars in contact with any part of the milling machine other than

through the polyethylene cover.

2. The resulting specimens were acid cleaned by immersion in 40% HC1 for one minute, and then in

40% HC1 plus 10% HN03 for one minute. After acid cleaning, the bars were rinsed in distilled water, then in alcohol, and finally air dried.

3. Individual specimens were sealed in polyethylene bags.

SHOULD ANY CUTTING OF SRM 741 BE REQUIRED OR SHOULD ANY SURFACE CONTAMINATION BE SUSPECTED, IT IS RECOMMENDED THAT THE HANDLING AND CLEANING

PROCEDURES DESCRIBED ABOVE BE EMPLOYED JUST PRIOR TO USE.

115
U. S. Department i.f Commerce APFENDIX I

7

Frederick B. Dent

Secretary

National Bureau of Standards

Richard W. Roberta, Director

Rational ^uram of j^tandards

Certificate

Standard Reference Material 44f

Aluminum

Freezing Point on the International

Practical Temperature Scale (1968)

660.3 °C

The freezing point of SRM 44f, 660.3 °C, is the value determined on representative samples of the

lot- It is estimated that the uncertainty in the NBS certified value of the freezing point does not

exceed ± 0.2 °C. (The estimated uncertainty includes the imprecision and possible systematic

errors of the measurement.)

The material for this standard was obtained from Cominco American, Inc., Spokane, Washington,

and is a selected lot of high-purity aluminum containing less than the total of 5 ppm of impurities

that would affect the freezing point. The freezing point of SRM 44f may not necessarily be the same as the freezing point of pure aluminum.

Full details of the precautions that should be observed in freezing point determinations are given in National Bureau of Standards Circular 590, and in Metrologia 7, No. 3, 108-130 (July 1971). The

International Practical Temperature Scale of 1968 is described in Metrologia 5, No. 2, 35-44 (April

1969).

Technical measurements at NBS leading to certification were performed by G. T. Furukawa and J. L. Riddle of the NBS Heat Division.

The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference Materials

by R. E. Michaclis.

Washington, D. C. 20234

April 5, 1973

116

J. Paul Cali, Chief

Office of Standard Reference Materials
APPENDIX G

Report of ^ntaattgattott

ULTRA-PURITY ALUMINUM: RM-1R & RM-1C

M. B. Kasen and R. L. Powell

Ultra-Purity Aluminum is available as a Research Material (RM)

in the form of round polycrystalline rods (RM-1R) approximately

4.17 mm (0.164 in) in diameter and 2.54 cm (1 in long) and as

single crystal cubes (RM-1C) approximately 1 cm (0.4 in) per

side.

Applications

The polycrystalline rods provide specimens for research on the

mechanical and physical properties of extremely pure aluminum;

for example , in determining resistivity as a function of strain

at cryogenic temperatures to facilitate design of cryogenic

magnets or superconductor stabilizing elements.

The single' crystal material is intended for use in studies

of solid state phenomena for which both extreme purity and

knowledge of crystallographic orientation are required; e.g.,

in studies of electron spin resonance, De Haas-Van Alphen effect

cyclotron resonance, etc., and in studies relating to the Fermi

surface and the transport properties of aluminum. Both the

polycrystalline rods and the single crystals are useful in

studies of point and line defects and in studies of the effect

of such defects on the mechanical and physical properties of

metals. The materials are also well suited for use in studies

of grain boundary phenomena, e.g., boundary structure,

migration rates and segregation phenomena.

Material Preparation

The materials were produced from an approximately 12 mm

thick slab saw cut from one side of an ingot of aluminum

specially zone-refined by Cominco American, Inc. A large

single crystal constituting about 2/3 of the slab was reserved for production of the cubes. The remaining material

was rolled to sheets M).19 inch thick for use in rod

production.

Rod Production

The rods were formed by swaging at room temperature. The

swaging stock was prepared by cutting square sections from

the sheet and filing them to an octagonal cross section.

117
APPENDIX G (contd)

The octagonal sections were cleaned before the first swaging

pass through a 0.176 inch diameter die, cleaned after the

first pass and then swaged through a 0.167 inch die for the

final pass after which they were given a final cleaning

treatment.

The cleaning reagent was a solution consisting of reagent

grade phosphoric acid (15 parts) , sulfuric acid (5 parts)

and nitric acid (1 part) (all by volume) . The initial cleaning

was for a total immersion time of 15 minutes in the reagent

held at a temperature of 19 0 to 200 °C, The treatment consisted

of two separate 7 1/2 minute immersions. A 5 minute immersion

was used after the first swaging pass and a 2 minute immersion

was used after the final pass. Each immersion was in a fresh

batch of cleaning reagent. Each cleaning operation was followed

by a careful rinse in distilled water.

The swaged and cleaned rods were cut to length, supported in

a quartz boat in an air furnace, and annealed at 450° ± 5 °C

for 1 hour and then cooled to room temperature at a rate of

approximately 3 °C per minute.

Following a 10 minute cleaning in the mixed acids, the cubes

were supported in a quartz boat in an air furnace, annealed

10 to 15 minutes at 575 ± 5 °C, and cooled to room temperature

at approximately 3 °C per minute. The latter treatment removed

the damaged surface layer remaining from the grinding operation.

The cubes were given a final cleaning for 20 minutes in the

mixed acids.

Material Characterization

Rods

The rod material has been characterized by a measured residual

resistivity at 4K and by a calculated residual resistivity

ratio, p237K/p4K, at the NBS laboratories in Boulder, Colorado.

Residual resistivity was determined by the eddy current decay

technique in which the residual resistivity of a cylindrical

rod is given by the formula I^l

p = 21. 7R2

. 10" 9 ohm-cm

T where R is the specimen radius in cm and t is the time constant

of the decay curve in seconds. From these data, the residual

resistivity ratio was calculated, assuming p273K = 2.44 X 10~ 6

ohm-cm.

The size effect correction (pSE) subtracted from the measured

residual resistivity was determined as a function of the specimen diameter, d, by the relationship

118
APPENDIX G (contd)

pSE « (pjl)°° [3]

d

in which

(pfc)°» 0.7 X 10" 15 ohm m

2

. [41

TABLE I

imen Dia. T (4K) p4K X 10 y ohm-cm p273K/p4K

No, mm msec measured Corrected Calculated Corrected

1 4. 1763 600 0.1587 0. 1409 15 ,470 17,310

2 4.1707 570 0. 1655 0. 1487 14 ,740 16 ,410

3 4, 1(569 545 0. 1728 0. 1560 14 ,120 15,640

4 4.1669 573 0. 1644 0. 1476 14,840 16,530

5 4.1709 563 0. 1676 0. 1508 14,650 16,100

6 4.1730 563 0. 1677 0 , 1510 14 ,5 40 16 ,160

7 4.1575 550 0. 1704 0. 1536 14 ,320 15,890

8 4.1600 540 0. 1738 0. 1570 14 ,040 15,540

9 4.1661 550 0. 1712 0. 1544 14 ,250 15,810

10 4.1704 565 0. 1670 0.1502 14,610 16 ,240

Corrected Corrected

4K X 10 9 ohm-cm 273K/4K

Mean Values 0.151 " 16,160

Standard Deviation 0.0047 485

95% Confidence Interval Upper 0.1544 Upper 16,507

on Mean [6] Lower 0.1476 Lower 15,813

The inaccuracy of a single resistivity determination is estimated

to be <±3%. Material variability is slightly larger than

the inaccuracy of the experimental technique.

The estimated equivalent solute in solid solution based upon

the corrected residual resitivity is 0.25 pom atomic. f 7 J

Cubes

The residual resistivi ty and the residual resistivity ratio

of the cubes also was determined by the eddy current method.

The resistivity of a specimen of rectangular cross section

is given by

p = 1.27 a

2 b

2 10" 9 ohm-cm

T a

2 + b 2

in which a and b refer to the cross sectional dimensions in

cm. The above formula assumes a rod of infinite length; consequently, nine cubes were stacked in a vertical array in order

119
APPENDIX G (contd)

to simulate the required geometry. The resulting time constant

was therefore the average of the group. The following results

were obtained:

p4K 0.1473 X 10"9 ohm-cm

p273K/p4K = 16,570.

This result is in agreement with the data obtained from the

rod samples. (corrected for size effect).

Analysis of Laue diffraction patterns from two adjacent sides

of one of the cubes revealed the normal to the cube faces to have

the following crystallographic orientation:

This approximate orientation is expected to be valid for all cubes

References

1. C. P. Bean, R. W. De Blois and L. B. Nesbitt, J. Appl. Phvs.

3j0 , 1976 (1969) . 2. T. Broom, Proc. Phys. Soc. 65B , 871 (1952).

3. P. Cotti , unpublished work.

4. P. Cotti, E. M. Fryer, and J. L. Olsen, Helv. Phys. Acta.

37, 585 (1964)

5. Experimental Statistics, NBS Handbook 91, (1963), par. 1-6.

6. Ibid, par. 2-1.4.1.

7. Assuming average specific contribution to resistivity per ppm

atomic to be 0.6 X 10~ 9 ohm-cm (cf. V. D. Arp, M. B. Kasen

,

and R. P. Reed, Tech. Rept. AFAPL-TR-6 8-87 , Wright-Patterson

AFB, Ohio (Feb. 1969), p. 48.

Purchasers and users of these research materials are encouraged

to report findings and data to:

Research Materials

Office of Standard Reference Materials

(OSRM)

National Bureau of Standards

Washington, D. C. 202 34.

When a sufficient body of such information has been received,

an updated Report of Investigation will be issued by the

OSRM and sent to all purchasers of these materials.

120
U. S. Department' of Commerce APPENDIX H

Maurice II. Stans SecreUr*

National Bpn»&ll ofj Standards

L. H. BranranafOraJr: Director (Eertifttat* of JVttalgBts

Standard Reference Material 746

Cadmium-Vapor Pressure

R.C. Paule and J. IVlandel

Vapor Pressure as a Function of Temperature (350-594 K)

(1/T X 103

) T(K,IPTS-68) P(atm)a (K 1

, IPTS-68) Log P(atm)a

350 2.49 X 10 12.857 -10.603

400 2.97 X lO" 2.500 - 8.528

450 1.20X 10-7 2.222 - 6.922

500 2.31 X 10-6 2.000 - 5.637

550 2.56 X 10- 5 1.818 - 4.592

594 (M. P.) 1.51 X 10- 4 1.684 - 3.820

a l atmosphere = 101,325 newtons* meter2

This SRM is available in the form of rod 6.4 mm (0.25 in) in diameter and 64 mm (2.5 in) long.

The material has been determined to be homogeneous and of 99.999+ percent purity. 1 The material

for this standard was supplied by Cominco American, Inc.. Spokane, Washington. A specially selected lot of cadmium was further purified by distillation and zone refining, homogenized into a

single lot, and processed into the final rod size.

The above vapor pressure-temperature values for solid cadmium are a composite resulting from a weighted averaging of over 200 vapor pressure-temperature measurments made by six laboratories*

experienced in such measurements. The composite temperature-pressure results were obtained by

using a pooled third law heat of sublimation 2 at 298 K (26660 cal/mol) and back-calculating

through the third law equation:

AH°ub2 98 - T

r- a

/_Gt-H|98\_ RIn p(atm

)j

(i)

For the convenience of the user, the above tabic also includes calculated reciprocal temperatures

and log pressures. The free energy functions for use in the equation are listed. 3

\ The results from the six experienced laboratories have also been used to estimate statistically the uncertainties of vapor pressure measurement. The thermodynamic aspects considered in the data

analysis, as well as a description of two statistical tests for use by a laboratory wishing to evaluate

its results, are presented on the following pages.

The overall coordination and evaluation of data leading to certification of SRM 746 was

performed by R. C. Paule and J. IVlandel.

The technical and support aspects involved in the preparation, certification, and issuance ol this Standard Reference Material were coordinated through the Office of Standard Reference MateriaJs

by R. E. Michaelis.

Washington, D. C. 20234 J. Paul Cali, Acting Chief

August 11, 1970

^overj Office of Standard Reference Materials

121
APPENDIX H (contd)

Thermodynamics and Statistics

The individual third law heal for each dala point was calculated hy use of equation (I). Each

laboratory's data have been recalculated at IN I IS using a single set. of free energy functions3 and

identical calculation procedures. All temperatures have been converted to the 1968 International

Practical Temperature Scale (IPTS-68).4

Eighteen curves from the above mentioned laboratories were also used to obtain a pooled second

law heat of sublimation. 5 The second law heat for each vapor pressure-temperature curve was

obtained by least-squares fitting of the A and B constants in the following equation:

A / Gt - H298 \_ R | n p (atm) = A + — (2)

The slope, B, is a second law heat of sublimation at 298 K. The above calculational procedure is similar to the sigma method, and does not require the specification of a mean effective

temperature.6 - 7 The procedure is very convenient when the calculations, including the interpolation

of free energy functions, are made by computer.

The results from the six laboratories were used to obtain a statistical estimate of the within- and

the between-laboratory uncertainties exhibited by a typicaJ in-control laboratory. It should be

realized that the following uncertainty limits are not absolute or fixed with respect to time (and

progress), but rather represent current average levels of uncertainty of measurement. The data from

the six laboratories represent a broad cross-section of measurement techniques and include Knudsen

(weight loss), torque Knudsen. and mass spectrometric methods. Temperatures were measured using

several types of thermocouples. The average temperature range of the curves was 120 K and the

average number of points was 13.

A laboratory wishing to evaluate its apparatus and techniques mav use SRM 746 to measure a

In P vs. 1/T curve using about the same temperature range and number of data points as given

above, and may then use the two statistical tests listed below. The monomer vapor species should be

used in the calculations. The following Knudsen cell materials have been used satisfactorilv: impervious alumina, berylia. dense graphite, iron, and iridium. Experience has shown that the orifice area should be kept smaller than 1 X 102 cm2 to avoid problems of a possible non-unit evaporation

coefficient. Should SRM 746 develop a slight oxide coating after extensive exposure to air, the

coating may be removed by briefly rinsing the sample in 1-2 N I1N0 3

, followed bv several distilled water rinses. After making the vapor pressure-temperature measurements, the following statistical tests should be performed.

Test I. Evaluation of the Value of the Slope

The least-squares fit of data for a single temperature-pressure curve using equation (2) should

give a B value (AH°U i>298) which agrees approximate!v 95 percent of the time with the value

26,660 cal/mol (1 1 1,550 J/moK 8 >) within the following limits:

Test:

I 26,660 - AII° ub2 9o| < J 140 cal/mol

If a laboratory prefers to least-squares fit the equation

R In P = A' + 11' T

to obtain AJ if , and then adjust to AII998 using literature A(llf - 1

1 2 9 tt ) values, the above slope

limits should still be approximately correct. 122
APPENDIX H (contd)

Test II. Evaluati on of the Absolute V;ilucs of the Vapor Pressures

The average third law AH° lb2 98 ^or a sing'1

' curve. calculated as an average of individual point

AH298 values using equation (I) should agree approximately 95 percent of the lime with 26,660

cal/mol (111,550 J/moK u )) within the limits described below:

Test: |26,660- AH°ub2 9 8 | < 420 cal/mol

A further description of the evaluation of the variances for SUM 746 and their use in applications involving two or more temperature-pressure curves is given in NBS Spec. Publ. 260-21.

*List of Participating Laboratories

Bureau of Mines, A. Landsberg

Douglas Advances Research Laboratories, D. L. Hildenbrand

Gulf General Atomic, Inc., H. G. Staley

Los Alamos Scientific Laboratory, G C. Herrick and R. C. Feber

Marquette University, T. C. Ehlert

National Bureau of Standards, E. R. Plante and A. B. Sessoms

Sandia Corporation, D. A. Northrop

Footnotes

1 The following methods were used in the homogeneity testing and purity evaluations:

Linear Electron Accelerator (G.J. Lutz)

Optical Emission Spectroscopy (Cominco American, Inc., Spokane, Washington - J. G.

Frettingham)

Residual Resistivity Ratio, R273k/R4k(V. A. Deason and R. L. Powell)

Spark-Source Mass Spectrometry (C. W. Mueller and P. J. Paulsen).

2 The third law AHfub298 = 26.660 ± 150 cal/mol (111,550 ± 630 J/mol). The ± uncertainty

value represents two standard error limits of the pooled value. These limits tacitly assume the

error in the free energy functions is negligible relative to the error in the heat of sublimation. This

is believed to be the case.

Condensed phaseb Gas phase

Temperature Gt - H298

T

Gf - H998

T

K,(IPTS-68) cal'mol" 1 -deg- 1 cal'mol" 1 'deg-1

298.15 12.38 40.065

350 12.51 40.164

400 12.63 40.260

450 12.87 40.444

500 13.10 40.628

550 13.36 40.820

594 (M.P.) 13.61 41.011

b Converted to IPTS-684 using data of llullgren, R., Orr, R. L., and Kellcy, K. K., loose-leaf supplement to Selected Values of Thermodynamic Properties of Metals and Alloys (Sept.

1966). Conversions were made to IPTS-68 using equations given by Douglas, T. B., J. Res. NBS, 73A, 451-69 (1969).

4 The International Practical Temperature Scale of 1968, Metrologia 5, 35-49 (1969).

123
APPENDIX H (contd)

5 The second law All°ub298 = 26,610 ± 380 cal/mol ( I I 1,340 ± 1600 J/raol). The ± uncertainty

represents two standard error limits of the pooled value. The pooled second law and third law

heats2 are in excellent agreement. The pooled third law heal, however, is helieved to he more

accurate and has therefore heen used in the calculation of vapor pressures and as the "best

value" in the statistical tests.

6 Horton, W. S., J. Res. NBS, 70A, 533-9 (1966).

7 Cubicciotti, D., J. Phys. Chem., 70, 2410-3 ( 1966).

8 1 calorie = 4.1840 joules.

124
U.S. Department of Commerce APPENDIX I Elliot L. Richardson.

Secretary

National Bureau of Standards

Ernest Ambler, Acting DirectorRational ffintmn of Standards

(tteritfttaie

Standard Reference Material 743

Mercury

Triple Point on the

International Practical Temperature Scale (1968)

-38.841 °C

This Standard Reference Material is recommended for calibrating temperature measuring devices.

The temperature given above is based on the experimental comparison of slowly freezing mercury

(14 hour freeze) in four sealed cells which were free of any gas. Two of these cells were prepared by

combining seven ampoules of SRM 743 and two contained the high purity NBS Density Standard

Mercury. The triple point temperatures of all four cells were found to agree within 0.1 mK.

The absolute value of the triple point temperature given above is based on measurements with one of the NBS

Density Standard mercury cells and five platinum resistance thermometers calibrated in terms of the IPTS-68.

Twelve observations were made with the thermometers in two freezes. The standard deviation of these observations is 0. 1 mK.

The calculated effect of pressure on the freezing point of mercury is +5.4 mK per atm. Although

temperature measurements are more precise and convenient with sealed, gas-free mercury cells, for

those who prefer to employ cells open to the ambient pressure the freezing point at 1 atm pressure

is estimated to be —38.836 °C. Since the fixed point is below ambient temperature, precautions

should be taken to avoid condensation of moisture on the mercury sample.

Technical measurements at NBS leading to certification were performed by G. T. Furukawa, J. L. Riddle, and W. R. Bigge of the NBS Heat Division.

The technical and support aspects involved in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference Materials

by R. E. Michaelis and R. K. Kirby.

Washington, D.C. 20234 J. Paul Cali, Chief

April 22, 1976 Office of Standard Reference Materials

(over)

125
APPENDIX I (contd)

SUPPLEMENTARY INFORMATION

The freezing point of mercury is one of the secondary reference points for the IPTS-68. The

experimentally realized value given in this certificate is the same as that adopted by the Comite'

International des Poids et Mesures. For further information of/ temperature scales and metal freezing

points, see The International Practical Temperature Scale of 1968, Metrologia 5, 35 (April 1969)

and McLaren, E. H., The Freezing Point of High-Purity Metals and Precision Temperature

Standards, Temperature, its Measurements and Control in Science and Industry, Vol. 3, Part 1,

Rheinhold Publishing Corp., New York, N. Y. (1962).

The mercury for this standard is of exceptional purity with the total of other elements estimated to be less than 20 parts per billion (ng/g) by emission spectrograph^ analysis. This mercury was

obtained from Cominco American, Inc., Spokane, Washington. SRM 743 is packaged in argon-filled

soft-glass ampoules and weighs about 680 g (50 ml).

126
NBS-114A IREV. 2-8C) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publ ication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBS SP 260-77 August 1982

4. TITLE AND SUBTITLE

Standard Reference Materials:

Application of Some Metal SRM's as The rmome trie Fixed Points

5. AUTHOR(S)

George T. Furukawa, John L. Riddle, William R. Bigge, and Earl R. Pfeiffer

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, Z)P}

Same as item 6

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 82-600578

~2 Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a significant

bibliography or literature survey, mention it here)

Equipment and procedures are described for the realization of liquid-solid phase

equilibrium states of some pure metals as thermometric fixed points for platinum

resistance thermometry. The design and techniques for assembling fixed-point cells

using Standard Reference Materials (SRM) 740 (zinc), 43h (zinc), 42g (tin), 741 (tin),

44f (aluminum), 746 (cadmium), and 743 (mercury) are given and the results of the

measurements using the purer of these metal SRM's are analyzed and evaluated. The

reproducibility of temperature measurements with these metal SRM's is shown to be

about ± 0.1 mK.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Aluminum point; cadmium point; check thermometers; freezing point; melting point; mer- cury point; phase equilibrium; standard platinum resistance thermometer (SPRT);

thermometric fixed point; tin point; triple point; zinc point.

13. AVAILABILITY

[X] Unlimited

| | For Official Distribution. Do Not Release to NTIS

[Xn Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

140

15. Price

$6.00

USCOMM-DC 6043-P80 ftU.S. GOVERNMENT PRINTING OFFICE: 1 982—360* 997/2224



NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research of the National Bureau of Standards reports NBS research and develop- ment in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry,

engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement

methodology and the basic technology underlying standardization. Also included from time to time are survey articles on topics

closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete

citations to all recent Bureau publications in both NBS and non- NBS media. Issued six times a year. Annual subscription: domestic

$18; foreign $22.50. Single copy, $4.25 domestic; $5.35 foreign.

NONPERIODICALS

Monographs—Major contributions to the technical literature on

various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with in- terested industries, professional organizations, and regulatory

bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special publications

appropriate to this grouping such as wall charts, pocket cards, and

bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and

studies of special interest to physicists, engineers, chemists,

biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NBS under

the authorilv of the National Standard Data Act (Public Law

90-396).

NOTE: The principal publication outlet for the foregoing data is the Journal of Physical and Chemical Reference Data (JPCRD)

published quarterly for NBS by the American Chemical Society

(ACS) and the American Institute of Physics (AIP). Subscriptions,

reprints, and supplements available from ACS, I 155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research results,

test methods, and performance criteria related to the structural and

environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards— Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's technological marketplace. Order the above NBS publications from: Superintendent of Documents, Government Printing Office. Washington, DC 20402. Order the following NBS publications—FIPS and NBSIRs—from

the National Technical Information Services. Springfield. VA 22161

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended.

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717(38 FR 12315, dated May II, 1973)and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. DC 20234 POSTAGE AND FEES PAID U S DEPARTMENT OF COMMERCE

COM-215

OFFICIAL BUSINESS

Penalty for Private Use. $300

THIRD CLASS