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NIST PUBLICATIONS

NIST SPECIAL PUBLICATION 260-124

U.S. DEPARTMENT OF COMMERCE/Technology Administration

National Institute of Standards and Technology

Standard Reference Materials:

Standard Reference Material 1744:

Aluminum Freezing-Point Standard

100

157

! 1260-124

395

Gregory F. Strouse
Zhe National Institute of Standards and Technology was established in 1988 by Congress to "assist industry

in the development of technology . . . needed to improve product quality, to modernize manufacturing processes,

to ensure product reliability . . . and to facilitate rapid commercialization ... of products based on new scientific discoveries." NIST, originally founded as the National Bureau of Standards in 1901, works to strengthen U.S. industry's competitiveness; advance science and engineering; and improve public health, safety, and the environment. One of the agency's basic functions is to develop, maintain, and retain custody of the national standards of measurement, and

provide the means and methods for comparing standards used in science, engineering, manufacturing, commerce,

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'At Boulder, CO 80303.

2 Some elements at Boulder, CO 80303.
NIST Special Publication 260-124

Standard Reference Materials:

Standard Reference Material 1744:

Aluminum Freezing-Point Standard

Gregory F. Strouse

Process Measurements Division Chemical Science and Technology Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

U.S. DEPARTMENT OF COMMERCE, Ronald H. Brown, Secretary

TECHNOLOGY ADMINISTRATION, Mary L. Good, Under Secretary for Technology NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Arati Prabhakar, Director Issued May 1995
National Institute of Standards and Technology Special Publication 260-124

Natl. Inst. Stand. Technol. Spec. Publ. 260-124, 39 pages (May 1995)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1995

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
Foreword

Since its inauguration in 1901, the National Institute of Standards and Technology (NIST),

formerly the National Bureau of Standards (NBS), has issued nearly 2000 different Standard Samples or Standard Reference Materials* (SRMs). Many of these have been renewed several

times; others have been replaced or discontinued as technology changed. Today, over 1000 SRMs

are available, together with a large number of scientific publications related to the fundamental and applied characteristics of these materials. Each material is certified for chemical composition, chemical properties, or its physical or mechanical characteristics. Each SRM is provided with a

Certificate or a Certificate of Analysis that contains the essential data concerning its properties or

characteristics. The SRMs currently available cover a wide range of chemical, physical, and

mechanical properties, and a corresponding wide range of measurement interests in practically all aspects of fundamental and applied science. These SRMs constitute a unique and invaluable means of transferring to the user accurate data obtained at NIST, and provide essential tools that can be used to improve accuracy in practically all areas where measurements are performed.

In addition to SRMs, the National Institute of Standards and Technology issues a variety of Reference Materials (RMs) which are sold, but not certified by NIST. They meet the ISO

Guide 30-1981 (E) definition for RMs, and many meet the definition for CRMs. The

documentation issued with these materials is either a: (1) "Report of Investigation," the sole authority being the author of the report. RMs are intended to further scientific or technical research on particular materials. The principal consideration in issuing RMs is to provide a homogeneous material so that investigators in different laboratories are assured that they are investigating the same material. (2) "Certificate," issued by the certifying agency (other than NIST), e.g., other national laboratories, other government agencies, other standardizing bodies,

or other non-profit organizations. When deemed to be in the public interest and when alternate means of national distribution do not exist, NIST acts as the distributor for such materials.

Further information on the reference materials available from NIST may be obtained from

the Standard Reference Materials Program, National Institute of Standards and Technology.

Information on other NIST services may be obtained from Technology Services, National Institute of Standards and Technology, Gaithersburg, MD 20899.

In addition to these types of materials, NIST provides a number of additional services. They include: Calibration and Related Measurement Services, National Standard Reference Data

system, Accreditation of Testing Laboratories, National Center for Standards and Certification Information, Weights and Measures Program and Proficiency Sample Programs.

iii
Preface

Standard Reference Materials* (SRMs) as defined by the National Institute of Standards and Technology (NIST) are well-characterized 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. SRMs 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 is carried out through the mechanism and use of SRMs. 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 NIST in arriving at the

certified values of the SRMs produced. The NIST Special Publication 260 Series is a series of papers reserved for this purpose. The 260 Series is dedicated to the dissemination of information on different phases of the preparation, measurement, certification, and use of NIST SRMs. 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 greatest care and accuracy. These papers also should provide sufficient additional information so SRMs can be utilized in new applications in diverse fields not foreseen at the time the SRM was originally issued.

Inquiries concerning the technical content of this paper should be directed to the author(s). Other questions concerned with the availability, delivery, price, and so forth, will receive prompt attention from:

Standard Reference Materials Program

Bldg. 202, Rm. 204

National Institute of Standards and Technology

Gaithersburg, MD 20899

Telephone: (301) 975-6776 FAX: (301) 948-3730

Thomas E. Gills, Chief

Standard Reference Materials Program

iv
OTHER NIST PUBLICATIONS IN THIS SERIES

Trahey, N.M., ed., NIST Standard Reference

Materials Catalog 1992-93, NIST Spec. Publ. 260

(February 1992). SN003-003-03 146-1*

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

Iron Spectro-chemical Standards, NBS Misc.

Publ. 260-1 (June 1964). COM74-11061**

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

Char-acterization of an NBS Spectrometric Low-Alloy Steel Standard, NBS Misc. Publ. 260-3 (October 1964). COM74-11060**

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

Information, 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 (February 1965). PB168068**

Shultz, J.I., Standard Reference Materials: Methods

for the Chemical Analysis of White Cast Iron

Standards, NBS Misc. Publ. 260-6 (July 1965). COM74-11068**

Bell, R.K., Standard Reference Materials: Methods

for the Chemical Analysis of NBS Copper-Base

Spectro-chemical 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). COM74-11066**

Anspach, S.C., Cavallo, L.M., Garfinkel, S.B., et

al., Standard Reference Materials: Half Lives of

Mate-rials Used in the Preparation of Standard Reference Materials of Nineteen Radioactive Nuclides Jssued by the National Bureau of

Standards, NBS Misc. Publ. 260-9 (November

1965) . COM74-11065**

Yakowitz, H., Vieth, D.L., Heinrich, K.F.J. , et al., Standard Reference Materials: Homogeneity

Charac-terization of NBS Spectrometric Standards

II: Cartridge Brass and Low-Alloy Steel, NBS

Misc. Publ. 260-10 (December 1965). COM74-11064**

Napolitano, A., and Hawkins, E.G., Standard Reference Materials: Viscosity of Standard

Lead-Silica Glass, NBS Misc. Publ. 260-11** (November 1966). Yakowitz, H., Vieth, D.L., and Michaelis, R.E.,

Standard Reference Materials: Homogeneity

Charac-terization of NBS Spectrometric Standards

III: White Cast Iron and Stainless ^Steel Powder

Compact, NBS Misc. Publ. 260-12 (September

1966) . Spijkerman, J.J., Snediker, D.K., Ruegg, F.C., et

al., Standard Reference Materials: Mossbauer

Spectroscopy Standard for the Chemical Shift of Iron Compounds, NBS Misc. Publ. 260-13**

(July 1967). Menis, O., and Sterling, J.T., Standard Reference

Materials: Determination of Oxygen in Ferrous

Mate-rials (SRMs 1090, 1091, 1092), NBS Misc.

Publ. 260-14** (September 1966).

Passaglia, E. and Shouse, P.J., Standard Reference

Materials: Recommended Method of Use of

Standard Ligh-Sensitive Paper for Calibrating Carbon Arcs Used in Testing Testiles for Colorfastness to Light, NBS Spec. Publ. 260-15

(July 1967). Superseded by SP 260-41.

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

Charac-terization of NBS Spectrometric Standards

IV: Pre-paration and Microprobe Characterization of W-20% Mo Alloy Fabricated by Powder

Metallurgical Methods, NBS Spec. Publ. 260-16

(January 1969). COM74-11062**

Catanzaro, E.J., Champion, C.E., Garner, E.L., et

al., Standard Reference Materials: Boric Acid;

Isotopic, and Assay Standard Reference

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

1970). PB189457**

Geller, S.B., Mantek, P. A., and Cleveland, N.G.,

Calibration of NBS Secondary Standards Magnetic Tape Computer Amplitude Reference Amplitude Measurement "Process A", NBS Spec. Publ. 260-18 (November 1969). Superseded by

SP 260-29.

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

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressure of Gold

(Certification of SRM 745). NBS Spec. Publ. 260-19 (January 1970). PB190071**

260-20: Unassigned

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). COM74-11359**

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

Charac-terization of Fe-3 Si Alloy, NBS Spec.

Publ. 260-22 (February 1971). COM74-11357**

Napolitano, A., and Hawkins, E.G., Standard Reference Materials: Viscosity of a Standard

Borosilicate Glass, NBS Spec. Publ. 260-23 (December 1970). COM7 1-00157**

Sappenfield, K.M., Marinenko, 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., et al., Standard Reference Materials: A Standard Reference Material Containing Nominally Four

Percent Austenite, NBS Spec. Publ. 260-25

(February 1971). COM74-11356**

Martin, J.F., Standard Reference Materials: NBS-U.S. Steel Corp. Joint Program for Determining Oxygen and Nitrogen in Steel, NBS

Spec. Publ. 260-26 (February 1971). 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.,

et al., Standard Reference Materials: Preparation and Evaluation of SRMs 481 and 482 Gold-Silver and Gold-Copper Alloys for Microanalysis, NBS

Spec. Publ. 260-28 (August 1971). COM71-50365**

Geller, S.B., Standard Reference Materials:

Calibration 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** Supersedes Measurement System in SP 260-18.

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 Accu-racy Spectrophotometry, NBS

Spec. Publ. 260-32 (December 1973). COM74-50018**

vi
Wagner, H.L., Standard Reference Materials: Compar-ison of Original and Supplemental SRM

705, Narrow Molecular Weight Distribution Polystyrene, NBS Spec. Publ. 260-33 (May

1972). COM72-50526**

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

Material: Thermoelectric Voltage of Silver-28 Atomic Percent Gold Thermocouple Wire, SRM

733, Verses Common Thermocouple Materials (Between Liquid Helium and Ice Fixed Points), NBS Spec. Publ. 260-34 (April 1972). COM72-

50371**

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

Materials: Thermal Conductivity of Austenitic

Stainless Steel, SRM 735 from 5 to 280 K, NBS

Spec. Publ. 260-35 (April 1972). COM72-50368**

Cali, J.P., Mandel, J., Moore, L.J., et al., Standard

Reference Materials: A Reference Method for the Determination of Calcium in Serum NBS SRM

915, NBS Spec. Publ. 260-36 (May 1972). COM72-50527**

Shultz, J.I., Bell, R.K., Rains, T.C., et al., Standard Reference Materials: Methods of Analysis of NBS Clay Standards, NBS Spec. Publ. 260-37

(June 1972). COM72-50692**

Richard, J.C., and Hsia, J.J., Standard Reference

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., et al., 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

Differential Thermal Analysis Temperature

Standards Through a Cooperative Study (SRMs

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 Light-Sensitive 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., et al., Standard Reference Materials: Development of NBS SRM 1579 Powdered Lead-Based Paint, NBS Spec. Publ. 260-45 (March 1973). COM73-50226**

Hust, J.G., and Giarratano, P.J., Standard Reference

Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials: Austenitic Stainless Steel, SRMs 735 and 798, from 4 to 1200 K, NBS Spec. Publ. 260-46 (March 1975). COM75-10339**

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 Reference

Materials: Description and Use of Precision Thermo-meters for the Clinical Laboratory, SRM

933 and SRM 934, NBS Spec. Publ. 260-48 (May 1974). Superseded by NIST Spec. Publ. 260-113. COM74-50533**

vii
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 Reference

Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials: Electrolytic Iron, SRMs 734 and 797 from 4 to 1000 K, NBS Spec. Publ. 260-50 (June 1975). COM75-10698**

Mavrodineanu, R., and Baldwin, J.R., Standard Reference Materials: Glass Filters As a SRM for Spectrophotometry-Selection, Preparation,

Certification, and Use-SRM 930 NBS Spec. Publ. 260-51 (November 1975). COM75-10339**

Hust, J.G., and Giarratano, P.J., Standard Reference

Materials: Thermal Conductivity and Electrical Resistivity SRMs 730 and 799, from 4 to 3000 K, NBS Spec. Publ. 260-52 (September 1975). COM75-11193**

Durst, R.A., Standard Reference Materials: Standard-ization of pH Measurements, NBS Spec. Publ. 260-53 (December 1978). Superseded by SP 260-53 Rev. 1988 Edition. PB88217427**

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). PB272168**

Ditmars, D.A., Cezairliyan, A., Ishihara, S., et al., Standard Reference Materials: Enthalpy and Heat

Capacity; Molybdenum SRM 781, from 273 to 2800 K, NBS Spec. Publ. 260-55 (September

1977) . PB272127**

Powell, R.L., Sparks, L.L., and Hust, J.G., Standard Reference Materials: Standard Thermocouple Material, Pt-67: SRM 1967, NBS

Spec. Publ. 260-56 (February 1978). PB277172**

Cali, J. P., and Plebanski, T., Standard Reference

Materials: Guide to United States Reference

Materials, NBS Spec. Publ. 260-57 (February

1978) . PB277173**

Barnes, J.D., and Martin, G.M., Standard Reference

Materials: Polyester Film for Oxygen Gas Transmission Measurements SRM 1470, NBS Spec.

Publ. 260-58 (June 1979). PB297098**

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

Materials: Electron Paramagnetic Resonance

Intensity Standard: SRM 2601; Description and

Use, NBS Spec. Publ. 260-59 (August 1978). PB292097**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al., Standard Reference Materials: A Reference Method for the Determination of Sodium in Serum, NBS Spec. Publ. 260-60 (August 1978). PB286944**

Verdier, P.H., and Wagner, H.L., Standard Reference Materials: The Characterization of

Linear Poly-ethylene (SRMs 1482, 1483, 1484), NBS Spec. Publ. 260-61 (December 1978). PB289899**

Soulen, R.J., and Dove, R.B., Standard Reference

Materials: Temperature Reference Standard for Use Below 0.5 K (SRM 768), NBS Spec. Publ. 260-62 (April 1979). PB294245**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al., Standard Reference Materials: A Reference Method for the Determination of Potassium in Serum, NBS Spec. Publ. 260-63 (May 1979). PB297207**

Velapoldi, R.A., and Mielenz, K.D., Standard

Reference Materials: A Fluorescence SRM

Quinine Sulfate Dihydrate (SRM 936), NBS

Spec. Publ. 260-64 (January 1980). PB80132046**

Marinenko, R.B., Heinrich, K.F.J. , and Ruegg,

F.C., Standard Reference Materials: Micro-Homogeneity Studies of NBS SRM, NBS

Research Materials, and Other Related Samples,

NBS Spec. Publ. 260-65 (September 1979). PB300461**

viii
Venable, W.H., Jr., and Eckerle, K.L., Standard Reference Materials: Didymium Glass Filters for Calibrating the Wavelength Scale of Spectro- photometers (SRMs 2009, 2010, 2013, 2014). NBS Spec. Publ. 260-66 (October 1979). PB80104961**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al., Standard Reference Materials: A Reference Method for the Determination of Chloride in Serum, NBS Spec. Publ. 260-67 (November

1979). PB80110117**

Mavrodineanu, R., and Baldwin, J.R., Standard Reference Materials: Metal-On-Quartz Filters as a SRM for Spectrophotometry SRM 2031, NBS

Spec. Publ. 260-68 (April 1980). PB80197486**

Velapoldi, R.A., Paule, R.C., Schaffer, R., et al., Standard Reference Materials: A Reference Method for the Determination of Lithium in Serum, NBS Spec. Publ. 260-69 (July 1980). PB80209117**

Marinenko, R.B., Biancaniello, F., Boyer, P. A., et

al., Standard Reference Materials: Preparation

and Char-acterization of an

Iron-Chromium-Nickel Alloy for Microanalysis:

SRM 479a, NBS Spec. Publ. 260-70 (May 1981). SN003-003-02328-1*

Seward, R.W., and Mavrodineanu, R., Standard Reference Materials: Summary of the Clinical Laboratory Standards Issued by the National Bureau of Standards, NBS Spec. Publ. 260-71 (November 1981). PB82135161**

Reeder, D.J., Coxon, B., Enagonio, D., et al., Standard Reference Materials: SRM 900, Antiepilepsy Drug Level Assay Standard, NBS Spec.

Publ. 260-72 (June 1981). PB81220758

Interrante, C.G., and Hicho, G.E., Standard Reference Materials: A Standard Reference

Material Containing Nominally Fifteen Percent

Austenite (SRM 486), NBS Spec. Publ. 260-73

(January 1982). PB82215559**

Marinenko, R.B., Standard Reference Materials: Pre-paration and Characterization of K-411 and K-412 Mineral Glasses for Microanalysis: SRM

470, NBS Spec. Publ. 260-74 (April 1982). PB82221300**

Weidner, V.R., and Hsia, J.J., Standard Reference

Materials,: Preparation and Calibration of First Surface Aluminum Mirror Specular Reflectance

Standards (SRM 2003a), NBS Spec. Publ. 260-75 (May 1982). PB82221367**

Hicho, G.E., and Eaton, E.E., Standard Reference

Materials: A Standard Reference Material Containing Nominally Five Percent Austenite (SRM 485a), NBS Spec. Publ. 260-76 (August

1982). PB83115568**

Furukawa, G.T., Riddle, J.L., Bigge, W.G., et al., Standard Reference Materials: Application of Some Metal SRMs as Thermometric Fixed

Points, NBS Spec. Publ. 260-77 (August 1982). PB83 117325**

Hicho, G.E., and Eaton, E.E., Standard Reference

Materials: Standard Reference Material Containing Nominally Thirty Percent Austenite (SRM 487), NBS Spec. Publ. 260-78 (September

1982). PB83 115576**

Richmond, J.C., Hsia, J.J., Weidner, V.R., et al., Standard Reference Materials: Second Surface Mirror Standards of Specular Spectral Reflectance (SRMs 2023, 2024, 2025), NBS Spec. Publ. 260-79 (October 1982). PB84203447**

Schaffer, R., Mandel, J., Sun, T., et al., Standard

Reference Materials: Evaluation by an ID/MS

Method of the AACC Reference Method for Serum Glucose, NBS Spec. Publ. 260-80

(October 1982). PB84216894**

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

Reference Materials: Accuracy in Analytical

Spectrophoto-metry, NBS Spec. Publ. 260-81

(April 1983). PB83214536**

Weidner, V.R., Standard Reference Materials: White

Opal Glass Diffuse Spectral Reflectance Standards for the Visible Spectrum (SRMs 2015

and 2016), NBS Spec. Publ. 260-82 (April 1983). PB83220723**

ix
Bowers, G.N., Jr., Alvarez, R., Cali, J. P., et al., Standard Reference Materials: The Measurement

of the Catalytic (Activity) Concentration of Seven Enzymes in NBS Human Serum (SRM 909), NBS

Spec. Publ. 260-83 (June 1983). PB83239509**

Gills, T.E., Seward, R.W., Collins, R.J., et al., Standard Reference Materials: Sampling,

Materials Handling, Processing, and Packaging of NBS Sulfur in Coal SRMs 2682, 2683, 2684, and

2685, NBS Spec. Publ. 260-84 (August 1983). PB84109552**

Swyt, D.A., Standard Reference Materials: A Look

at Techniques for the Dimensional Calibration of

Standard Microscopic Particles, NBS Spec. Publ. 260-85 (September 1983). PB841 12648**

Hicho, G.E., and Eaton, E.E., Standard Reference

Materials: A SRM Containing Two and One-Half

Percent Austenite, SRM 488, NBS Spec. Publ. 260-86 (December 1983). PB84143296**

Mangum, B.W., Standard Reference Materials: SRM

1969: Rubidium Triple-Point - A Temperature

Reference Standard Near 39.30° C, NBS Spec.

Publ. 260-87 (December 1983). PB84149996**

Gladney, E.S., Burns, C.E., Perrin, D.R., et al., Standard Reference Materials: 1982 Compilation

of Elemental Concentration Data for NBS

Biological, Geological, and Environmental

Standard Reference Materials, NBS Spec. Publ. 260-88 (March 1984). PB84218338**

Hust, J.G., Standard Reference Materials: A Fine- Grained, Isotropic Graphite for Use as NBS

Thermophysical Property RMs from 5 to 2500 K, NBS Spec. Publ. 260-89 (September 1984). PB85 112886**

Hust, J.G., and Lankford, A.B., Standard Reference

Materials: Update of Thermal Conductivity and

Electrical Resistivity of Electrolytic Iron, Tungsten, and Stainless Steel, NBS Spec. Publ. 260-90 (September 1984). PB851 15814**

Goodrich, L.F., Vecchia, D.F., Pittman, E.S., et

al., Standard Reference Materials: Critical Current Measurements on an NbTi

Superconducting Wire SRM, NBS Spec. Publ. 260-91 (September 1984). PB85 118594**

Carpenter, B.S., Standard Reference Materials:

Calibrated Glass Standards for Fission Track Use

(Supplement to NBS Spec. Publ. 260-49), NBS

Spec. Publ. 260-92 (September 1984). PB85113025**

Ehrstein, J.R., Standard Reference Materials: Preparation and Certification of SRM for Calibration of Spreading Resistance Probes, NBS

Spec. Publ. 260-93 (January 1985). PB85 177921**

Gills, T.E., Koch, W.F., Stolz, J.W., et al., Standard Reference Materials: Methods and

Procedures Used at the National Bureau of

Standards to Certify Sulfur in Coal SRMs for Sulfur Content, Calorific Value, Ash Content, NBS Spec. Publ. 260-94 (December 1984). PB85 165900**

Mulholland, G.W., Hartman, A.W., Hembree,

G.G., et al., Standard Reference Materials: Development of a 1mm Diameter Particle Size Standard, SRM 1690, NBS Spec. Publ. 260-95 (May 1985). PB861 13693**

Carpenter, B.S., Gramlich, J.W., Greenberg, R.R.,

et al., Standard Reference Materials: Uranium235 Isotopic Abundance Standard Reference

Materials for Gamma Spectrometry Measurements, NBS Spec. Publ. 260-96

(September 1986). PB87108544**

Mavrodineanu, R., and Gills, T.E., Standard

Reference Materials: Summary of the Coal, Ore,

Mineral, Rock, and Refactory Standards Issued by the National Bureau of Standards, NBS Spec.

Publ. 260-97 (September 1985). PB861 10830**

Hust, J.G., Standard Reference Materials: Glass Fiberboard SRM for Thermal Resistance, NBS

Spec. Publ. 260-98 (August 1985). SN003-003-02674-3*

Callanan, J.E., Sullivan, S.A., and Vecchia, D.F.,

Standard Reference Materials: Feasibility Study

for the Development of Standards Using

Differential Scanning Calorimetry, NBS Spec.

Publ. 260-99 (August 1985). PB86106747**

x
Taylor, J.K., Trahey, N.M., ed., Standard

Reference Materials: Handbook for SRM Users, NBS Spec. Publ. 260-100 (February 1993). PB93183796**

Mangum, B.W., Standard Reference Materials: SRM

1970, Succinonitrile Triple-Point Standard: A

Temperature Reference Standard Near 58.08° C, NBS Spec. Publ. 260-101 (March 1986). PB86197100**

Weidner, V.R., Mavrodineanu, R., Mielenz, K.D.,

et al., Standard Reference Materials: Holmium

Oxide Solution Wavelength Standard from 240 to 640 nm - SRM 2034, NBS Spec. Publ. 260-102

(July 1986). PB86245727**

Hust, J.G., Standard Reference Materials: Glass

Fiberblanket SRM for Thermal Resistance, NBS

Spec. Publ. 260-103 (September 1985). PB86109949**

Mavrodineanu, R., and Alvarez, R., Standard

Reference Materials: Summary of the Biological and Botanical Standards Issued by the National Bureau of Standards, NBS Spec. Publ. 260-104 (November 1985). PB86155561**

Mavrodineanu, R., and Rasberry, S.D., Standard

Reference Materials: Summary of the Environmental Research, Analysis, and Control

Standards Issued by the National Bureau of

Standards, NBS Spec. Publ. 260-105 (March

1986). PB86204005**

Koch, W.F., ed., Standard Reference Materials: Methods and Procedures Used at the National Bureau of Standards to Prepare, Analyze, and

Certify SRM 2694, Simulated Rainwater, and Recommendations for Use, NBS Spec. Publ. 260-106 (July 1986). PB86247483**

Hartman, A.W., and McKenzie, R.L., Standard

Reference Materials: SRM 1965, Microsphere

Slide (10 /xra Polystyrene Spheres), NBS Spec.

Publ. 260-107 (November 1988). PB89153704**

Mavrodineanu, R., and Gills, T.E., Standard

Reference Materials: Summary of Gas Cylinder and Permeation Tube Standard Reference

Materials Issued by the National Bureau of

Standards, NBS Spec. Publ. 260-108 (May 1987). PB87209953**

Candela, G.A., Chandler-Horowitz, D., Novotny, D.B., et al., Standard Reference Materials: Preparation and Certification of an

Ellipsometrically Derived Thickness and

Refractive Index Standard of a Silicon Dioxide Film (SRM 2530), NIST Spec. Publ. 260-109

(October 1988). PB89133573**

Kirby, R.K., and Kanare, H.M., Standard Reference

Materials: Portland Cement Chemical Composition Standards (Blending, Packaging, and

Testing), NBS Spec. Publ. 260-110 (February

1988). PB88193347**

Gladney, E.S., O'Malley, B.T., Roelandts, I., et al., Standard Reference Materials: Compilation of Elemental Concentration Data for NBS Clinical, Biological, Geological, and Environmental Standard Reference Materials, NBS Spec. Publ. 260-111 (November 1987). PB88156708**

Marinenko, R.B., Blackburn, D.H., and Bodkin,

J.B., Standard Reference Materials: Glasses for Micro-analysis: SRMs 1871-1875, NIST Spec.

Publ. 260-112 (February 1990). PB90215807**

Mangum, B.W., and Wise, J. A., Standard Reference

Materials: Description and Use of a Precision Thermometer for the Clinical Laboratory, SRM

934, NIST Spec. Publ. 260-113 (June 1990). PB90257643**

Vezzetti, C.F., Varner, R.N., and Potzick, J.E., Standard Reference Materials: Bright-Chromium

Linewidth Standard, SRM 476, for Calibration of Optical Microscope Linewidth Measuring

Systems, NIST Spec. Publ. 260-114 (January 1991). PB91167163**

Williamson, M.P., Willman, N.E., and Grubb,

D.S., Standard Reference Materials: Calibration of NIST SRM 3201 for 0.5 in. (12.65 mm) Serial Serpentine Magnetic Tape Cartridge, NIST Spec. Publ. 260-115 (February 1991). PB91 187542** Mavrodineanu, R., Burke, R.W., Baldwin, J.R., et

al., Standard Reference Materials: Glass Filters as a Standard Reference Material for Spectrophotometry -Selection, Preparation, Certification and Use of SRM 930 and SRM

1930, NIST Spec. Publ. 260-116 (March 1994). PB94-188844/AS**
Vezzetti, C.F., Varner, R.N., and Potzick, J.E., Standard Reference Materials: Anti- reflecting-Chromium Linewidth Standard, SRM

475, for Calibration of Optical Microscope

Linewidth Measuring Systems, NIST Spec. Publ. 260-117 (January 1992). PB92-149798**

Williamson, M.P., Standard Reference Materials: Calibration of NIST Standard Reference Material 3202 for 18-Track, Parallel, and 36-Track,

Parallel Serpentine, 12.65 mm (0.5 in), 1491 cpmm (37871 cpi), Magnetic Tape Cartridge,

NIST Spec. Publ. 260-118 (July 1992). PB92- 226281**

Vezzetti, C.F., Varner, R.N., and Potzick, Standard Reference Materials: Antireflecting-Chromium Linewidth Standard, SRM 473, for Calibration of Optical Microscope Linewidth Measuring

System, NIST Spec. Publ. 260-119 (September

1992). Caskey, G.W., Philips, S.D., Borchardt., et al., Standard Reference Materials: A Users' Guide to NIST SRM 2084: CMM Probe Performance

Standard, NIST Spec. Publ. 260-120 (1994). Rennex, B.G., Standard Reference Materials:

Certification of a Standard Reference Material for the Determination of Interstitial Oxygen

Concentration in Semiconductor Silicon by

Infrared Spectrophotometry, NIST Spec. Publ. 260-121 (1994) PB95-125076/AS.

*Send order with remittance to: Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20102. Remittance from foreign

countries should include an additional one fourth of the purchase price for postage. **May be ordered from: National Technical Information Services (NTIS), Springfield, VA

22161. For information phone (703-487-4650)

To Place an Order with PB# phone (800-553-

6847)

xii
TABLE OF CONTENTS

PAGE

1. Introduction 2

2. Procedure 2

2.1 SRM 1744 Sample 2

2.2 Aluminum Fixed-Point Cell 2

2.3 Assembling of the Fixed-Point Cell 4

3. Experimental Evaluation and Analysis of Results 6

3.1 Measurement Procedure 6

3.2 Analysis of Freezing Curves 6

3 .3 Analysis of Melting Curves 7

3.4 Direct Comparison of Fixed-Point Cells 14

3.5 Application 14

4. Conclusions 16

5. References 17

6. Appendix A 18

xiii
LIST OF FIGURES

FIGURE NO. PAGE

1 . Schematic of an aluminum fixed-point cell assembly 3

2. Freezing curves for aluminum fixed-point cell Al 94-1 8

3. Freezing curves for aluminum fixed-point cell Al 94-2 9

4. Freezing curves for aluminum fixed-point cell Al 94-3 10

5. Melting curves for aluminum fixed-point cell Al 94-1 11

6. Melting curves for aluminum fixed-point cell Al 94-2 12

7. Melting curves for aluminum fixed-point cell Al 94-3 13

8. Direct freezing plateau comparison results of Al 94-1, Al 94-2 and

Al 94-3 with Al 78-1 (laboratory standard) 15

xiv
APPENDIX

PAGE

Certificate for SRM 1744: Aluminum Freezing-Point Standard 18

xv

Standard Reference Material 1744: Aluminum Freezing-Point Standard

Gregory F. Strouse

National Institute of Standards and Technology

Process Measurements Division

Gaithersburg, MD 20899

Abstract The freezing point of aluminum (660.323 °C) is a defining fixed point of the International Temperature Scale of 1990 (ITS-90). Realization of this freezing point is performed using a fixed- point cell containing high-purity (>99.9999% pure) aluminum. The aluminum constituting Standard Reference Material 1744 (SRM 1744) has been evaluated, and certified as suitable for use in the

realization ofthe freezing-point temperature of aluminum for the ITS-90. Based on results obtained with three fixed-point cells containing random samples of SRM 1744, the plateau temperature of a freezing curve of the SRM 1 744 aluminum is expected to differ by not more than 1 m°C from the ITS-90 assigned temperature. In this document, the methods used for the construction of the aluminum freezing-point cells and for the evaluation of SRM 1 744 are described.

Disclaimer

Certain commercial equipment, instruments or materials are identified in this paper in order to adequately specify the experimental procedure. Such identification does not imply recommendation

or endorsement by NIST, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Acknowledgment

The author wishes to thank the Standard Reference Materials Program for their support in the development ofthis new SRM and B. W. Mangum and G.T. Furukawa for their helpful discussions.

1
1. Introduction One ofthe freezing points required to realize the International Temperature Scale of 1990 (ITS-90) from 420 °C to 962 °C is that of aluminum (660.323 °C) [1]. This freezing point is realized by using

a thermometric fixed-point cell containing high-purity (> 99. 9999% pure) aluminum metal. Such a fixed-point cell is used for the ITS-90 calibration of standard platinum resistance thermometers (SPRTs) from -189 °C to 660.323 °C and for high-temperature SPRTs (HTSPRTS) from 0 °C to 961.78 °C. To provide evaluated and certified material for this purpose, we have developed a Standard Reference Material (SRM), the Aluminum Freezing-Point Standard (SRM 1744). The certification of SRM 1744 was performed by evaluating three fixed-point cells (designated Al 94-1, Al 94-2 and Al 94-3) containing random samples of the high-purity (> 99. 9999% pure) metal

constituting SRM 1744. The certification included the evaluation of freezing and melting curves and

the direct comparison with the laboratory aluminum freezing-point standard (Al 78-1) in the NIST

Platinum Resistance Thermometry (PRT) Laboratory. Using the three fixed-point cells, these methods of evaluation were used to confirm the purity and the freezing-point temperature of SRM 1744 relative to the aluminum (99.9999% pure) used in Al 78-1. The construction of the aluminum fixed-point cells used to certify SRM 1744 are described and the results from the

certification of SRM 1744 are given.

2. Procedure

2.1 SRM 1744 Sample The high-purity (s: 99.9999% pure) aluminum metal used for SRM 1744 was purchased by the NIST

Standard Reference Materials Program from Johnson Matthey Company of Spokane, Washington.

A 20-kg lot (M2075) of high-purity aluminum metal, in millimeter size button "shot" form

(nominally 0.7 cm o.d. by 0.3 cm thick) constitutes SRM 1 744. Johnson Matthey Company placed

the randomized metal in plastic bottles sealed in an argon atmosphere, each containing 200 g of

aluminum. The purification of SRM 1744 was performed by the Johnson Matthey Company. The

emission spectrographic assay provided by Johnson Matthey Company with the aluminum shows the

total impurity level to be 0.4 ug/g (0.4 parts per million), resulting from 0.3 ug/g of Ca and 0. 1 ug/g of Si.

2.2 Aluminum Fixed-Point Cell

Three thermometric fixed-point cells (Al 94-1, Al 94-2 and Al 94-3) were constructed to certify the aluminum metal for use as an ITS-90 freezing-point standard. Each cell contained 356 g of the high- purity metal from randomly selected bottles of lot M2075. As shown in figure 1 (schematic not drawn to scale), the aluminum (J) was contained within a high-purity graphite crucible (K) with a

2
A

Figure 1 . A schematic (not to scale) of an aluminum fixed-point cell assembly showing: (A) a

25.5 Q SPRT; (B) a matte-finished silica-glass pumping tube; (C) thermal insulation (washed Fiberfrax); (D) a matte-finished, silica-glass thermometer guide tube; (E) twelve Inconel radiation shields; (F) thirteen silica-glass spacers; (G) a silica-glass envelope with a matte-finished, silica-glass re-entrant well; (H) a graphite cap for the

graphite crucible; (I) a graphite re-entrant well; (J) aluminum metal SRM 1744; (K)

a graphite crucible; (L) silica-glass tape for cushioning; and (M) an Inconel protecting

tube.

3
high-purity graphite cap (H) and a high-purity graphite re-entrant well (I). To prevent possible breakage of the cell, the graphite re-entrant well can float freely up and down (about 0.3 cm) to accommodate shrinking of the metal during freezing and some of the buoyant force of the metal experienced during melting of the metal. The graphite assembly was placed inside a silica-glass envelope (G), with a silica-glass re-entrant well (G) inserted into the graphite well. Axially located

in the annular space between the graphite crucible and the silica-glass envelope was a piece of silica- glass tape (L) (46 cm long, 2.5 cm wide and 0.03 cm thick) for cushioning the graphite assembly. The silica-glass re-entrant well had a matte finish to prevent "light piping." The silica-glass envelope

also had an attached silica-glass pumping tube (B) (0.6 cm o.d.) that allowed the cell to be evacuated and filled with purified argon during use. This silica-glass pumping tube had also a matte finish to prevent light piping. The fixed-point cell was inserted into an Inconel protecting tube (M) that was 61 cm long by

5 cm o.d., with a 0.08 cm wall thickness (see fig. 1). Axially located in the annular space between

the silica-glass envelope and the Inconel protecting tube and between the silica-glass envelope and

the graphite crucible was a piece of silica-glass tape (each 46 cm long, 2.5 cm wide and 0.03 cm

thick) for cushioning the fixed-point cell. Above the silica-glass envelope, there were 12 Inconel

radiation shields (E) (4.8 cm o.d. by 0.08 cm thick) separated by 1 cm long silica-glass spacers (F), a matte-finished, silica-glass thermometer guide tube (D) and the matte-finished, silica-glass pumping

tube. The first radiation shield was 2 cm above the top of the silica-glass envelope. The space remaining above the top radiation shield was filled with Fiberfrax insulation (C). Radiation shields were used to create radiation traps that temper the thermometer and help provide adequate

immersion. The thermometer guide tube extended about 0.5 cm above the top of the protecting tube. The pumping tube extended about 2.5 cm above the Inconel protecting tube and then extended about

5 cm at a right angle for connection to a vacuum/gas-handling system. This fixed-point cell assembly was placed inside a sodium-filled heat-pipe furnace for evaluation of the aluminum. A description of the sodium-filled heat-pipe furnace may be found in Refs. [2] and [3].

2.3 Assembling of the Fixed-Point Cell Any handling procedure of high-purity material is apt to introduce contamination. The "shot" form

is convenient for handling and filling during freezing-point cell construction, while a solid cylinder sample may require cutting and cleaning. By using one-time use polyethylene gloves, every possible

effort was made to maintain the purity of the aluminum and other fixed-point cell components that come in contact with the aluminum. Also, a clean laboratory environment was maintained.

Prior to filling the graphite crucible with the aluminum metal "shot," the graphite crucible assembly

(crucible, cap and re-entrant well) was placed in a silica-glass furnace tube and "baked-out" at 975 °C

under vacuum for 4 hours. This "bake-out" of the graphite was a final purification to remove

hydrocarbons and other contaminants that might have been present from the fabrication process.

Additionally, the silica-glass tape used for cushioning the graphite crucible in the silica-glass envelope was inserted into the furnace tube and "baked-out" at 975 °C at a partial pressure (34 kPa) of oxygen

4
for 4 hours to remove any organic binders that may have been used during the manufacturing of the

tape. The vacuum system used during the fabrication of the aluminum freezing-point cells is described in Ref. [4].

This silica-glass furnace tube used in the assembly process is a 4.8 cm o.d. test tube with a silica-glass pumping tube placed near the top ofthe furnace tube to allow for evacuation and filling with purified argon during use. The open end ofthe furnace tube is sealed with either a solid silicon-rubber stopper

or a stopper with a vacuum seal that allows a silica-glass push rod to be used for pushing the graphite

well into place.

After the graphite assembly was heated and cooled to ambient under vacuum, the furnace tube was

purged with purified argon and the graphite assembly removed and placed inside a clean polyethylene

bag for storage. To remove any contaminants from the "bake-out," the silica-glass furnace tube was

(1) cleaned in hot, soapy water; (2) rinsed with copious amounts of water; (3) the inside soaked in 20% nitric acid-80% distilled water (volume) for 1 hour; (4) rinsed with copious amounts of distilled water and then (5) dried. After each use ("bake-out" or "fill"), the silica-glass furnace tube was

cleaned.

High-purity argon gas is necessary for the during both the construction and use of the aluminum

fixed-point cell. The gas purification system for argon described in Ref. [4] was designed to remove

any hydrocarbons, oxygen and water that would contaminate and react with the molten aluminum.

In order to introduce 356 g of aluminum into the crucible and insert the graphite re-entrant well, two

fillings were required. The aluminum "shot" was poured directly into the graphite crucible. Approximately, 290 g of aluminum "shot" could be poured into the graphite crucible for the first

"fill." The graphite crucible with the graphite cap and the first filling were placed into the cleaned

furnace tube and the furnace tube was placed in the furnace. The system was evacuated for 1 hour and then back-filled with purified argon to a pressure of 34 kPa. This process of pumping and

flushing the system was carried out three times with the system finally under vacuum. Upon the final evacuation, the furnace was turned on and the temperature was brought to 670 °C to melt the aluminum sample. After about 2 hours the sample was completely melted, and the furnace was

allowed to cool to ambient under vacuum. For the second "fill," purified argon was introduced, the graphite crucible was removed from the furnace tube, and the remaining metal was added (about 66 g) to metal in the graphite crucible from

the first "fill." The graphite re-entrant well was inserted in the graphite crucible as far as possible through the hole in the graphite cap and the assembly was placed in the newly cleaned furnace tube. A silica-glass push rod extended from the bottom of the graphite re-entrant well through a vacuum

seal at the top ofthe furnace tube and for a sufficient distance above the seal at the top of the furnace

tube to allow the graphite well to be pushed into place when the aluminum was molten. Finally, the

furnace tube was placed in the furnace and pumped and flushed with purified argon using the method

described above. When the sample was melted, the graphite re-entrant well was slowly inserted into the molten aluminum by pushing down the silica-glass push rod. When the graphite well was fully

5
inserted, the furnace was turned off and the system allowed to cool to ambient under a partial pressure (34 kPa) of purified argon. The introduction of argon during the cooling of the graphite crucible assembly was used to prevent breakage of the graphite crucible. For unknown reasons, cooling the completed graphite assembly under a vacuum may cause the graphite crucible to split. The graphite crucible assembly was ready to be placed into its silica-glass envelope and fixed-point

cell assembly that was described earlier.

3. Experimental Evaluation and Analysis of Results

3 .1 Measurement Procedure The first step in evaluating the fixed-point cells was to obtain three freezing and three melting curves

for each of the specimens in the cells, using a 25.5 Q standard platinum resistance thermometer

(SPRT). A commercially-available 30 Hz ac resistance ratio bridge with an ac/dc 100 Q reference

resistor was used to measure the SPRT. A description of the measurement system used in the PRT

Laboratory may be found in Ref [3].

In the realization of the freezing point, the recommended "induced inner freeze" method [5, 6] was

used. With the metal completely melted, the furnace was set at about 2 °C below the freezing-point temperature. After supercooling and recalescence were observed with a 25.5 Q SPRT in the fixed- point cell, the furnace temperature was set to about 1 °C below the freezing-point temperature for the duration of the freeze. Next, the thermometer was removed and a silica-glass rod was twice

successively inserted into and removed from the thermometer well at 1 minute intervals to induce

freezing of a mantle of metal around the graphite re-entrant well. Finally, the thermometer was then

reinserted into the cell and, after equilibrium was obtained, the recording of readings was started, using an excitation current of 1 mA.

3.2 Analysis of Freezing Curves

Figures 2 to 4 show the freezing curves for each of the three fixed-point cells (the region of

supercooling and recalescence is not shown, as the curves begin after the reinsertion of the thermometer). Using an excitation current of 1 mA, thermometer readings were recorded

continuously until the freezing was complete. The average length of a freeze for the three cells was

16 hours. The calculated temperature depression when 50% of the metal was frozen and the shapes from the freezing-curve plateaus were used to estimate and confirm the overall purity of the sample

in the cell.

In most cases, including the two known impurities in the present high-purity samples will cause a depression in the temperature of a freezing point. Using Raoult's Law of dilute solutions, calculations may be made to estimate the expected temperature depression when 50% of the metal has frozen

during a freezing curve realization [7]. Using the known type and amount of impurities given in the emission spectrograph^ assay, the molar fraction of impurities was calculated. The total impurity

6
level (2.98 X 1(T

7 mol) and Raoult's Law of dilute solutions give a calculated depression in temperature estimated to be 0.2 m°C. Another method of estimating the expected temperature

depression when 50% of the metal has frozen during a freezing curve realization, was made using

binary phase diagrams [8]. The binary phase diagrams for Al-Ca and Al-Si give a calculated depression in temperature estimated to be 0.3 m°C. As determined from the freezing curve plateaus

in figures 2 to 4, the average estimated temperature depression from the extrapolated 0% frozen

metal (time of recalescence) to the 50% frozen metal ofthe flat section of the plateaus were 0.6 m°C,

0.5 m°C and 0.6 m°C for Al 94-1, Al 94-2 and Al 94-3, respectively, which are several times larger than the calculated values. Differences between the calculated and the experimentally derived temperature depressions may indicate either an uncertainty in using Raoult's Law of dilute solutions,

in using binary phase diagrams, the sample contained additional impurities not accounted for in the emission spectrographic assay or perhaps additional impurities were added to the metal during the construction of the fixed-point cells. In estimating the expected temperature depression, Raoult's Law of dilute solutions and binary phase diagrams are intended to provide a guideline and should not be strictly applied. Additionally, there is an uncertainty in the extrapolation method chosen to experimentally derive the temperature depression.

Comparisons of the average temperature depressions when 50% of the metal of the three fixed-point

cells was frozen were made with Al 78-1. The average temperature depression from the extrapolated 0% frozen metal (time of recalescence) to the 50% frozen metal of the flat section of the plateaus was

0.7 m°C for Al 78-1. The smaller temperature depressions for the three cells relative to the

laboratory standard indicate that the SRM metal is of slightly higher purity than that of the reference

cell.

3.3 Analysis of Melting Curves

After the aluminum was slowly and completely frozen in the above manner, the furnace temperature was set at about 1 °C above the freezing-point temperature to slowly melt the metal over a time of

approximately 10 hours. Using an excitation current of 1 mA, thermometer readings were recorded

continuously until the melting was complete. Figures 5 to 7 show the melting curves for each of the

three fixed-point cells. From the graphs, the average temperature range of the melting curves were

1.4 m°C, 1.0 m°C and 1.2 m°C for Al 94-1, Al 94-2 and Al 94-3, respectively.

During the melting ofthe aluminum, two liquid-solid interfaces are formed. One liquid-solid interface

is next to the inner wall of the graphite crucible and the second liquid-solid interface is near the graphite re-entrant well. This second liquid-solid interface is formed where the lower-purity metal

solidified at the end ofthe previous slow freeze. The lower-purity metal has a slightly lower freezing and melting temperature causing this second liquid-solid interface to form during the melt. The height ofthe freezing range and the melting range are about the same which is expected if an inner melt is formed during the melt [9].

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The temperature range of a melting curve is not indicative of the purity of the metal. Using the melting curve to indicate purity is complicated by the fact that the shape and range of a melting curve

will depend upon the history ofthe previous freezing ofthe metal in the fixed-point cell [10]. A slow

freeze causes the impurities to be segregated which in turn causes a large melting range. A fast freeze causes a homogenous mixture of the impurities with metal sample which in turn causes a small melting range.

3 .4 Direct Comparison of Fixed-Point Cells The second part ofthe evaluation was a direct comparison of the fixed-point cells under test with the

laboratory standard fixed-point cell (Al 78-1) to determine their freezing-point temperatures relative to that of the reference cell. This was obtained by realizing simultaneous freezes for the two cells (Al 94-X and Al 78-1, where X was 1, 2 or 3) and making three sets of alternate measurements, at equal time intervals, on their freezing-curve plateaus, using an SPRT. Measurements of each cell were made using an SPRT with excitation currents of 1 mA and 1.414 mA to permit extrapolation to zero-power dissipation (0 mA). Corrections were made for any differences in pressure and

hydrostatic head effects in each cell. Each cell was measured using an SPRT three times during the

direct comparison and this procedure was repeated two times. Figure 8 shows the results of the

direct comparison of the three SRM cells with the laboratory standard. The set of matching symbols

(open and closed) are for the direct comparison measurements of Al 94-X (where X is 1, 2 or 3) compared with Al 78-1 . Each set of symbols (open or closed) connected by lines are for the direct comparison measurements made during the simultaneous freezes. The average temperature difference of the first readings of each direct comparison showed that Al 94-1, Al 94-2 and Al 94-3 were

0.1 9 m°C, 0.3 2 m°C and 0.

5 m°C hotter than the laboratory standard. An expanded uncertainty of

1.0 m°C (k=2) is assigned to the laboratory reference cell. The basis for that uncertainty is given in Refs. [11] and [12]. For the metal to be certified as a freezing-point standard, the fixed-point cells containing samples of

the metal must have a freezing-point temperature that is in agreement with the laboratory reference

cell containing high-purity metal to within the uncertainties of the measurements. If the purity of the metal in the new fixed-point cell is greater than that of the reference cell, then it may be of even

higher quality, as indicated by being "hotter" than the laboratory standard. A cell that is "hotter"

usually has fewer impurities, since impurities in these samples will usually decrease the freezing-point temperature.

3.5 Application

In assigning a temperature value to realizations of the aluminum freezing point, corrections must be

applied for the depth of immersion (C) of the thermometer sensing element below the surface of the metal (dt/dH = 1.6 x 10" 3 °C/m) [1], Also, if the pressure (p) over the cell during the measurements

is not controlled at 101 325 Pa (1 standard atmosphere), a correction (dt/dp = 7.0 x 10 8 °C/Pa) must

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be made for the difference in pressure [1]. The immersion depth for the three cells was 18 cm from

the sensor mid-point ofthe SPRT to the top of the liquid metal surface (distance from the bottom of

the graphite re-entrant well to the top ofthe liquid level is 20.5 cm). The pressure in the cells during use was 101 325 Pa ±27 Pa. For those constructing their own aluminum freezing-point cell containing SRM 1 744, it is necessary

to confirm that the purity of the metal was maintained during construction of that cell. This

confirmation is made by comparing the freezing and melting curves of the new cell with those shown

in figures 2-7. As a continuing check on the overall purity of the aluminum metal contained in the

fixed-point cell, melting and freezing curves should be obtained every 6 months and compared with

those obtained previously.

4. Conclusions The evaluation of SRM 1744 for use as an aluminum freezing-point standard has shown that the

material is of high-purity (>99.9999%) and is acceptable for use as a defining fixed point of the ITS-90 and has been so certified. Based on the results from the evaluation of the three fixed-point

cells containing SRM 1744, the average temperature range of melting of the bulk material is not expected to exceed 1.5 m°C. Plateau temperatures of the freezing curves for this material are expected to differ by not more than 0.5 m°C from each other. The results from the evaluation of the

three fixed-point cells using freezing and melting curves and direct comparisons of the freezing-point temperature with that of the laboratory standard (Al 78-1) indicates that the plateau temperature of

a freezing curve of the SRM 1744 aluminum is expected to differ by not more than 1 m°C from the ITS-90 assigned temperature. A copy of the certificate for SRM 1744 is given in the appendix.

Direct comparisons ofthe three fixed-point cells (Al 94-1, Al 94-2 and Al 94-3) with the laboratory

standard (Al 78-1) have shown that the aluminum of SRM 1744 is of slightly higher purity than any samples of aluminum previously obtained by us. Consequently, fixed-point cell Al 94-2 has been

selected to be the new laboratory standard at NIST for the ITS-90 realization of the freezing point of aluminum. Of the three cells, Al 94-2 has the smallest freezing-curve temperature depression,

smallest melting-curve temperature range and has the "hottest" freezing-point temperature relative to the old laboratory standard (Al 78-1). The other two fixed-point cells (Al 94-1 and Al 94-3), however, are also acceptable for use as laboratory standards and they will be kept in reserve for that purpose and for control checks of the laboratory standard Al 94-2.

16
5. References

[1] H. Preston-Thomas, "The International Temperature Scale of 1990 (ITS-90)," Metrologia,

Vol. 27, pp. 3-10, (1990); ibid. p. 107.

[2] J.P. Evans, "Evaluation of Some High-Temperature Platinum Resistance Thermometers,"

J. Res. Natl. Bur. Stands. (U.S.), 89, No. 5, pp. 349-373 (1984).

[3] G.F. Strouse, "NIST Implementation and Realization of the ITS-90 Over the Range 83 K

to 1235 K. Reproducibility, Stability, and Uncertainties," Temperature. Its Measurement and Control in Science and Industry, Edited by J.F. Schooley, Vol. 6, pp. 169-174, (American Institute of Physics, New York, 1992).

[4] G.T. Furukawa, J.L Riddle, W.R. Bigge and E.R. Pfeiffer, "Application of Some Metal SRM's as Thermometric Fixed Points," NBS SP260-77, 140 pp. (1982).

[5] E H. McLaren, "The Freezing Points of High Purity Metals as Precision Temperature

Standards," Canadian J. ofPhys., 35, pp. 1086-1106 (1957).

[6] B.W. Mangum and G.T. Furukawa, "Guidelines for Realizing the International Temperature Scale of 1990 (ITS-90)," NIST Technical Note 1265, 190 pp. (1990).

[7] S. Glasstone, Thermodynamicsfor Chemists, p. 322, (D. Van Nostrand Co., Inc., New

York, 1947).

[8] Binary Alloy Phase Diagrams, Editor-in-Chief T.B. Massalski, Vol. 1, (ASM International, 1990).

[9] Private communication with G.T. Furukawa.

[10] J.V. McAllan and M.M. Ammar, "Comparison of the Freezing Points of Aluminum and Antimony," Temperature. Its Measurement and Control in Science and Industry, Edited by H.H. Plumb, Vol. 4, pp. 275-285, (Instrument Society of America, Pittsburgh, 1972).

[11] B.N. Taylor and C.E. Kuyatt, "Guidelines for Evaluating and Expressing the Uncertainty ofNIST Measurement Results," NIST Technical Note 1297, 17 pp. (1993).

[12] G.F. Strouse and W.L. Tew, "Assessment of Uncertainties of Calibration of Resistance Thermometers at the National Institute of Standards and Technology," NISTIR 5319, 16 pp. (1994).

17
6. Appendix A

Certificate for SRM 1744: Aluminum Freezing-Point Standard.

18
Standard Reference Material 1744

Aluminum Freezing-Point Standard

660.323 °C

International Temperature Scale of 1990 (ITS-90)

The certified value of 660.323 °C is the temperature assigned to the freezing-point of pure al uminum as one of the defining fixed points of the International Temperature Scale of 1990 (ITS-90) [1]. The fixed point is realized as the plateau temperature (or liquidus point) of the freezing curve of slowly-frozen high-purity aluminum. The metal is in the form of millimeter-size "shot" and is packaged in 200 g units in plastic bottles in the presence of argon.

Certified Freezing-Point Temperature: 660.323 ± 0.001 °C

Based on samples tested, the temperature range of melting of the bulk material is not expected to exceed 0.0015 °C. Temperatures of freezing curve plateau (see Figure 2) for samples of this material are expected to differ by not more

than 0.0005 °C from each other and by not more than 0.001 °C from the assigned temperature. The expanded

uncertainty (k=2), gives a 95.45% level of confidence for the fixed point measurements and is described in detail in reference 2. The aluminum for this Standard Reference Material (SRM) is of high-purity, with the total of all elements that affect the freezing-point temperature being less than one mg/kg (part per million). Source of Material: The aluminum metal (Lot M2075) for this SRM was obtained from Johnson Matthey Co., Spokane, WA 99216.

Notice and Warnings to Users: Any handling procedure for high-purity material is apt to introduce contamination. The "shot" form of this SRM minimizes the need for handling during freezing-point cell construction. Nevertheless, every possible effort should be made to maintain the purity of this SRM through the use of polyethylene gloves during

handling. Also, a clean laboratory environment is essential.

In assigning a temperature value to realizations of the aluminum freezing-point for calibration purposes, corrections must be applied for the average depth of immersion (t) of the thermometer sensing element below the surface of the metal (dt/df = 1.6 x 10" 3 °C/m). Also, if the pressure (p) over the cell during the measurements is not controlled at

1 standard atmosphere (101325 Pa), a correction (dt/dp = 7.0 x 10" 8 °C/Pa) must be made for the difference in pressure.

Temperature studies on freezing-point cells prepared using metal from randomly-selected bottles were performed by G.F. Strouse of the NIST Process Measurements Division. The technical and support aspects involved in the preparation, certification, and issuance of this SRM were

coordinated through the Standard Reference Materials Program by J.C. Colbert.

Gaithersburg, MD 20899 Thomas E. Gills, Chief November 4, 1994 Standard Reference Materials Program

(over)
Certification Testing: The thermal tests for the certification of this SRM were performed on three freezing-point

cells prepared in a manner similar to that described in reference 3 listed below. Each cell contained approximately 356 g of aluminum obtained from randomly-selected bottles of the lot. The freezing-point temperature was determined using the recommended "induced inner freeze" method [4]. With the metal completely melted, the furnace temperature was set at about 2 °C below the freezing-point temperature. After supercooling and recalescence had been observed with a 25.5 Q standard platinum resistance thermometer (SPRT)

in the cell, the thermometer was removed and a silica-glass rod was twice' successively inserted into and removed from

the thermometer well at one min intervals to induce freezing of a mantle of metal around the well. The thermometer was then reinserted into the cell and, after equilibrium was obtained, recording of readings was begun. After equilibrium was established, the measured temperatures of the plateaux of the nine freezing curves of the three samples decreased by no more than 0.0007 °C during the first 50 percent of the freezing curves. A typical freezing curve obtained under such conditions is shown in Figure 1 (the region of supercooling and recalescence is not shown,

as the curve begins after the reinsertion of the thermometer); some of the same data are plotted at greater resolution

in Figure 2.

After the metal was slowly and completely frozen in the above manner, the furnace temperature was set at about 2 °C

above the freezing-point temperature to slowly melt the metal over a time of approximately 10 h. Thermometer

readings were recorded continuously until the melting was complete. A typical melting curve obtained under such

conditions is shown in Figure 3; some of the same data are plotted at greater resolution in Figure 4. During the freezing and melting curve measurements, an environment of argon gas at one standard atmosphere (101325 Pa) pressure was maintained in.the cells.

Following the freezing and melting curve measurements, the plateau temperature of a freezing curve of each test cell was compared direcdy with that of the standard al uminum freezing-point cell of the Platinum Resistance Thermometer

Calibration Laboratory, using a 25.5 O SPRT. REFERENCES

[1] Preston-Thomas, H., "The International Temperature Scale of 1990 (ITS-90)," Metrologia 27, 3-10

(1990), Metrologia 27, 107, 1990.

[2] Strouse, G.F. and Tew, W.L., "Assessment of Uncertainties of Calibration of Resistance Thermometers at the National Institute of Standards and Technology," NISTIR 5319, 16 pages, 1994.

[3] Fumkawa, G.T., Riddle, J.L., Bigge, W.R., and Pfeiffer, E.R., "Standard Reference Materials: Application of Some Metal SRM's as Thermometric Fixed Points," Natl. Bur. Stand. (U.S.), Spec.

Publ. 260-77, 140 pages, 1982,.

[4] Mangum, B.W. and Furukawa, G.T., "Guidelines for Realizing the International Temperature of 1990 (ITS-90)," Natl. Inst. Stand. Tech. Tech. Note 1265, 190 pages, 1990.

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I I I
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