NBS SPECIAL PUBLICATION 260-53

(1988 Edition)

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

Standard Reference Materials:

Standardization

of pH Measurements

Yung Chi Wu, William F. Koch, and Richard A. Durst
Tm he National Bureau of Standards* was established by an act of 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 to assure international competitiveness and leadership of U.S.

industry, science arid technology. NBS work involves development and transfer of measurements, standards and related science and technology, in support of continually improving U.S. productivity, product quality and reliability, innovation

and underlying science and engineering. The Bureau's technical work is performed by the National Measurement

Laboratory, the National Engineering Laboratory, the Institute for Computer Sciences and Technology, and the Institute for Materials Science and Engineering.

The National Measurement Laboratory

Provides the national system of physical and chemical 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; provides advisory and research services to other Government agencies; conducts physical and chemical research; develops,

produces, and distributes Standard Reference Materials; provides

calibration services; and manages the National Standard Reference Data

System. The Laboratory consists of the following centers:

• Basic Standards^

• Radiation Research

• Chemical Physics

• Analytical Chemistry

The National Engineering Laboratory

Provides technology and technical services 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. TTie Laboratory consists of the following centers:

AppUed Mathematics

Electronics and Electrical Engineering^

Manufacturing Engineering

Building Techiiology

Fire Research

Chemical Engineering'

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 Pubhc 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 divisions:

Information Systems Engineering

Systems and Software

Technology

Computer Security

Systems and Network

Architecture

Advanced Computer Systems

The Institute for Materials Science and Engineering

Conducts research and provides measurements, data, standards, reference

materials, quantitative understanding and other technical information

fundamental to the processing, structure, properties and performance of

materials; addresses the scientific basis for new advanced materials technologies; plans research around cross-cutting scientific themes such as nondestructive evaluation and phase diagram development; oversees Bureau-wide technical programs in nuclear reactor radiation research and

nondestructive evaluation; and broadly disseminates generic technical information resulting from its programs. The Institute consists of the following Divisions:

• Ceramics

• Fracture and Deformation'

• Polymers

• Metallurgy

• Reactor Radiation

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address

Gaithersburg, MD 20899. 'Some divisions within the center are located at Boulder, CO 80303.

'Located at Boulder, CO, with sonne elements at Gaithersburg, MD
Standard Reference Materials:

Standardization of pH IVIeasurements

Yung Chi Wu, William F. Koch, and Richard A. Durst

Center for Analytical Chemistry

National Measurement Laboratory

National Bureau of Standards

Galthersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued February 1988
Library of Congress Catalog Card Number: 75-61 9344

National Bureau of Standards Special Publication 260-53

Natl. Bur. Stand. (U.S.), Spec. Publ. 260-53, 56 pages (Feb. 1988)

CODEN: XNBSAV

Supersedes NBS Spec. Publ. 260-53 (Dec. 1978)

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1988

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

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

Bureau of Standards (NBS) 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. 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 is 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

The 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

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 author(s). 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

Gaithersburg, MD 20899

Stanley D. Rasberry, Chief

Office of Standard Reference Materials

iii
OTHER NBS PUBLICATIONS IN TfflS SERIES

Seward. R. W., ed., NBS Standard Reference Materials

Catalog 1988-89, NBS Spec. Publ. 260 (January 1988)

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

Materials: Preparation of White Cast Iron

Spectrochemical 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 CopperBase Spectrochemical Standards, NBS Misc. Publ. 260-2 (October 1964). COM74-1 1063**

Michaelis, R. E., Yakowitz, H., and Moore, G. A., Stan- dard Reference Materials: Metallographic Characterization of an NBS Spectrometric Low-Alloy Steel Standard, NBS Misc. Publ. 260-3 (October 1964).

COM74-11060**

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. 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

Spectrochermical 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., 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 Standars, NBS Misc. Publ. 260-9

(November 1965). COM74-11065**

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

Michaelis, R. E., Standard Reference Materials:

Homogeneity Characterization ofNBS Spectrometric

Standards 11: 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., Stan- dard Reference Materials: Homogeneity Characterization of NBS Spectrometric Standards III: White

Cast Iron and Stainless Steel Powder Compact, NBS

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

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).

Yakowitz, H., Michaelis, R. E., and Vieth, D. L., Stan- dard Reference Materials: Homogeneity Characterization of NBS Spectrometric Standards IV:

Preparation and Microprobe Characterization of W20% Mo Alloy Fabricated by Powder Metallurgical

Methods, NBS Spec. Publ. 260-16 (January 1969).

COM74-11062**

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

Materials: Analysis of Interlaboratory Measurements

on the Vapor Pressure of Gold (Certification of Stan- dard Reference Material 745). NBS Spec. Publ. 260-

19 (January 1970). PB190071**

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

Materials: Analysis of InterlaboratcMy 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., Stan- dard Reference Materials: Homogeneity Characterization 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). C0M71-

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., and

Michaelis, R. E., 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: National

Bureau of Standards-US Steel Corporation Joint Prognam for Determining Oxygen and Nitrogen in Steel, NBS Spec. Publ. 260-26 (February 1971). PB

81176620**

Gamer, E. L., Machlan, L. A., and Shields, W. R., Stan- dard Reference Materials: Uranium Isotopic Standard

Reference Materials, NBS Spec. Publ. 260-27 (April

1971). COM74-11358**

iv
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 Gold-Copper Alloys for

Microanalysis, NBS Spec. Publ. 260-28 (August

1971). COM71-50365**

Geller, S. B., Standard Relerence 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). C0M71-

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). COM74-50018**

Wagner, H. L., Standard Reference Materials: Comparison of Original and Supplemental SRM 705, Nar- row Molecnlar Weight Distribution Polystyrene,

NBS Spec. Publ. 260-33 (May 1972). COM72-

50526**

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

Materials: Thermal Conductivity of Austenitic Stainless Steel, SRM735 from 5 to 280 K, NBS Spec. Publ. 260-35 (April 1972.) 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. I., Bell, R. K., Rains, T. C, and Menis, O. , Stan- dard Reference Materials: Methods of Analysis of

NBS Clay Standards, NBS Spec. Publ. 260-37 (June

1972). COM72-50692**

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., Gam, P. D., and Menis, O., Standard

Reference Materials: Selection of Thermal Analysis

Termperature Standards Through a Cooperative

Study (SRM 758, 759, 760), NBS Spec. Publ. 260-40

(August 1972) COM72-50776**

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.. Stan- dard Reference Materials: Preparation and

Homogeneity Characterization of an Austenitic Iron- Chromium-Nickel Alloy. NBS Spec. Publ. 260-43

(November 1972). COM73-50760**

Schooley, J. P., 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^ (December 1972). COM73-

50037**

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

S. D., Standard Reference Materials: Powdered LeadBased Paint, SRM 1579, 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, SRM's 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 Thermometers for die Clinical Laboratory, SRM 933 and

SRM 934, NBS Spec. Publ. 260-48 (May 1974).

COM74-50533**

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

Materials: Calibrated Glass Standards for Fission

Track Use, NBS Spec. Publ. 260-49 (November

1974). COM74-5 1185**

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

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). COM75-10698**

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

Reference Materials: Glass Filters As a Standard

Reference !vlaterial for Spectrophotometry-Selec

tion. Preparation, Certification, 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 Standard Reference Materials 730 and

799, from 4 to 3000 K, NBS Spec. Publ. 260-52 (September 1975). COM75-1 1 193**

Durst, R. A., Standard Reference Materials: Standardization of pH Measurements, NBS Spec. Publ. 260-

53 (December 1975, Revised). PB248127**

V
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., and

E>ouglas, T. B., Standard Reference Materials: Enthalpy and Heat C2^acity; 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**

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**

Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel, J., and Moody, J. R., 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

Polyethylene (SRM 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., Mandel, J., Machlan, L. A., and Gramlich, J. W., 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 Standard

Reference Material Quinine Sulfate Dihydrate (SRM

936), NBS Spec. Publ. 260-64 (January 1980). PB80-

132046**

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

Standard Reference Materials: Micro-Homogeneity

Studies of NBS Standard Reference Materials, NBS

Research Materials, and Other Related Samples, NBS

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

Venable, W. H., Jr., and Eckerle, K. L., Standard

Reference Materials: Didymium Glass Filters for Calibrating the Wavelength Scale of

Spectrophotometers-SRM 2009, 2010, 2013, and

2014. NBS Spec. Publ. 260-66 (October 1979). PB80-

104961**

Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel, J., Murphy, T. J., and Gramlich, J. W., Standard

Reference Materials: A Reference Method for the Determination ofChloride in Serum, NBS Spec. Publ. 260-67 (November 1979). PB80-110117**

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

Reference Materials: Metal-On-Quartz Filters as a

Standard Reference Material for Spectrophotometry

SRM 2031, NBS Spec. Publ. 260-68 (April 1980).

PB80-197486**

Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel, J., Machlan, L. A., Gamer, E. L., and Rains, T. C, Stan- dard Reference Materials: A Reference Method for the Determination of Lithium in Serum, NBS Spec.

Publ. 260-69 (July 1980). PB80-209117**

Marinenko, R. B., Biancaniello, F., Boyer, P. A., Ruff, A. W., and DeRobertis, L., Standard Reference

Materials: Preparation and Chanicterization of an Iron-Chromium-Nickel Alloy for Microanalysis,

NBS Spec. Publ. 260-70 (May 1981). PB84-

165349**

Seward, R. W., and Mavrodineanu, R., Standard

Reference Materials: Summary of the Clinical

LaboratOTy Standards Issued by the National Bureau

of Standards, NBS Spec. Publ. 260-71 (November

1981).PB82-135161**

Reeder, D. J., Coxon, B., Enagonio, D., Christensen, R.

G., Schaffer, R., HoweU, B. F„ Paule, R. C, and Mandel, J., Standard Reference Materials: SRM 900, Antiepilepsy Drug Level Assay Standard, NBS Spec.

Publ. 260-72 (June 1981). PB81-220758

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). PB82-

215559**

Marinenko, R. B., Standard Reference Materials:

Preparation and Characterization ofK-4 1 1 and K-4 14 Mineral Glasses for Microanalysis: SRM 470, NBS

Spec. Publ. 260-74 (April 1982). PB82-221300**

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

Materials: Preparation and Calibration of First Sur- face Aluminum Mirror Specular Reflectance Stan- dards (SRM 2003a), NBS Spec. Publ. 260-75 (May

1982).PB82-221367**

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). PB83-1 15568**

Furukawa, G. T., Riddle, J. L., Bigge, W. G., and Pfieffer, E. R., Standard Reference Materials: Application

of Some Metal SRM's 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 (SRM487), NBS

Spec. Publ. 260-78 (September 1982). PB83-

115576**

vi
Richmond, J. C, Hsia, J. J., Weidner, V. R., and Wilmering, D. B., Standard Reference Materials: Second

Surface Mirror Standards of Specular Spectral

Reflectance (SRM's 2023, 2024, 2025), NBS Spec.

Publ. 260-79 (October 1982). PB84-203447**

Schaffer, R., Mandel, J., Sun, T., Cohen, A., and Hertz, H. S., Standard Reference Materials: Evaluation by

an ID/MS Method of the AACC Reference Method

for Serum Glucose, NBS Spec. Publ. 260-80 (October

1982). PB84-2 16894**

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

Reference Materials: Accuracy in Analytical

Spectrophotometry, NBS Spec. Publ. 260-81 (April

1983). PB83-214536**

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

Opal Glass Diffuse Spectral Reflectance Standards

for the Visible Spectrum (SRM's 2015 and 2016),

NBS Spec. Publ. 260-82 (April 1983). PB83-

220723**

Bowers, G. N., Jr., Alvarez, R., Cali, J. P., Eberhardt, K.

R., Reeder, D. J., Schaffer, R., and Uriano, G. A., Standard Reference Materials: The Measurement of

the Catalytic (Activity) Concentration of Seven En- zymes in NBS Human Serum SRM 909, NBS Spec.

Publ. 260-83 (June 1983). PB83-239509**

GiUs, T. E., Seward, R. W., CoUins, R. J., and Webster,

W. C, Standard Reference Materials: Sampling,

Materials Handling, Processing, and Packaging of

NBS Sulfur in Coal Standard Reference Materials

2682, 2683, 2684, and 2685, NBS Spec. Publ. 260-84

(August 1983). PB84-109552**

Swyt, D. A., Standard Reference Materials: A Look at Techniques for the Dimensional Calibration of Stan- dard Microscopic Particles, NBS Spec. Publ. 260-85

(September 1983). PB84- 112648**

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

Materials: A Standard Reference Material Containing Two and One-Half Percent Austenite, SRM 488,

NBS Spec. Publ. 260-86 (December 1983). PB84-

143296**

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). PB84-149996**

Gladney, E. S., Bums, C. E., Perrin, D. R., Roelandts, I., and GiUs, T. E., 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). PB84-218338**

Hust, J. G., Standard Reference Materials: A FineGrained, Isotropic Graphite for Use as NBS Thermophysical Property RM's 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 Conduciivity and

Electrical Resistivity of Electrolyiic Iron, Tungsten,

and Stainless Steel, NBS Spec. Publ. 260-90 (September 1984). PB85-1 15814**

Goodrich, L. F., Vecchia, D. F., Piuman, E. S., Ekin, J. W., and Clark, A. F., Standard Reference Materials:

Critical Current Measurements on an NbTi Superconducting Wire Standard Reference Material, 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). PB85-1 13025**

Ehrstein, J.R., Standard Reference Materials: I*reparation and Certification of Standard Reference

Materials for Calibration of Spreading Resistance

Probes, NBS Spec. Publ. 260-93 (January 1985).

PB85-177921**

GiUs, T. E., Koch, W. F., Stolz, J. W.. Kelly, W. R., Paulsen, P. J., Colbert, J. C, Kirklin, D. R., Pei, P.T.S., Weeks, S., Lindstrom, R. M., Fleming, R. F., Greenberg, R. R., and Paule, R. C, Standard Reference

Materials: Methods and Procedures Used at the National Bureau of Standards to Certify Sulfur in Coal

SRM's 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.,

Marx, E., and Lettieri, T. R., Standard Reference

Materials: Development of a 1 |im Diameter Particle Size Standard, SRM 1690, NBS Spec. Publ. 260-95

(May 1985). SN003-003-026654*

Carpenter, B. S., Gramlich, J. W., Greenberg, R. R., Machlan, L. A., DeBievre, P., Eschbach, H. L., Meyer, H., Van Audenhove, J., Connolly, V. E., Trahey, N. M., and Zook, A. C, Standard Reference

Materials: Unanium-235 Isotopic Abundance Stan- dard Reference Materials for Gamma Spectrometry

Measurements, NBS Spec. Publ. 260-96 (September

1986). PB87- 108544**

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). SN003-003-02688-3*

Hust, J. G., Standard Reference Materials: Glass Fiber- board 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., Stan- dard Reference Materials: Feasibility Study for the Development of Standards Using Differential Scanning Calorimetry, NBS Spec. Publ. 260-99 (August

1985). SN003-003-02675-1*

vii
Taylor, J. K., Standard Reference Materials: Handbook

for SRM Users, NBS Spec. Publ. 260-100 (September 1985). PB86- 110897**

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). SN003-003-

02722-7*

Weidner, V. R., Mavrodineanu, R., Mielenz, K. D., Velapoldi, R. A., Eckerle, K. L., and Adams, B., Stan- dard Reference Materials: Holmium Oxide Solution

Wavelength Standard from 240- 650 nm, SRM 2034,

NBS Spec. Publ. 260-102 (July 1986). PB86-

245727**

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

Fiberblanket SRM for Thermal Resistance, NBS

Spec. Publ. 260-103 (September 1985). SN003-003-

02687-5*

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).

SN003-003-02704-9*

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). SN003-003-02725-1*

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). PB86-

247483**

Hartman, A. W., Standard Reference Materials: SRM

1965, Microsphere Slide (10 (im Polystyrene

Spheres), NBS Spec. Publ. 260-107 (In Preparation).

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).

Candela, G. A., Chandler-Horowitz, D., Novotny, D. B., Marchiando, J. F., and Belzer, B. J., Standard

Reference Materials: Preparation and Certification of an Ellipsometrically Derived Thickness and Refractive Index Standard of a SiUcon Dioxide Film (SRM

2530), NBS Spec. Publ. 260-109 (In Preparation).

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

Materials: Portland Cement Chemical Composition

Standards (Blending, Packaging, and Testing), NBS

Spec. Publ. 260-110

Gladney, E. S., O'Malley, B. T., Roelandts, I., and Gills, T. E., 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).

*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.

viii
Abstract

One of the most widely perfommed analytical measurements In chemical

laboratories Is that of pH using the glass electrode. In order to ensure the

consistency of these measurements, the National Bureau of Standards has

recommended an operational scale of pH, maintained in terms of the pH(S) of a series of standard solutions. Certified samples of buffer materials, from which

the standard reference solutions of reproducible pH can be prepared, as well as solutions in the case of rainwater, are issued by the National Bureau of

Standards as Standard Reference Materials. This report is concerned primarily

with a discussion of the method used at NBS for the assignment of pH(S) values

to these standard solutions, a description of the NBS measurement facilities

for pH, and a summary of the characteristics of the buffer materials. A brief

discussion of the types of electrodes used and the calibration of pH

instrumentation is also presented.

Key words: activity; activity coefficients; buffers; buffer solutions;

certification; glass electrode; hydrogen electrode; hydrogen ion;

measurement; pH; Standard Reference Materials; standards;

thermodynamics

ix
Foreword to the First Edition

December 1973

The certification of samples of buffer materials which define points on

the National pH scale has evolved over a period of several decades primarily

under the direction of Roger G. Bates. While this process is still not

complete and the future will likely see pH standards developed for specialized

purposes, e.g., pH in sea water and biological fluids such as blood, we have

currently reached a point where the pjrimary standards available and the

certification techniques involved are relatively well established and few

changes are anticipated in terms of the basic technology and philosophy of the

NBS pH scale. Consequently, the time seemed opportune to prepare this addition

to the "260 Series" of NBS Special Publications which was established for the

purpose of reporting technical work done at NBS in the area of Standard

Reference Materials.

This report is concerned primarily with a description of the concept of

pH, its definition, and the method and facilities used at NBS for the assignment of pH values to the standard buffer solutions. A brief discussion is also

included of pH instrumentation and some of the potential pit- falls associated

with the measurement of pH and the use of these standards. One additional

point is worth noting. The pH certificates reproduced in the appendix of this

publication refer to specific stocks of the Standard Reference Materials and,

since batch-to-batch pH variability exists , the use of these values for other

sources of these buffer materials is not recommended. It is anticipated that

in the near future the International Union of Pure and Applied Chemistry will

recommend a revised pH scale defined by these standards but of sufficiently

reduced precision that analytically pure reagents may be used for routine pH

calibration.

Finally, certain commercial materials and equipment are identified in this

report in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National

Burea of Stnadards, nor does it imply that the material or equipment identified

is necessarily the best available for the purpose.

Richard A. Durst

Analytical Chemistry Division

X
Foreword to the Second Edition

Since the first edition of this document was published in December, 1975,

several changes have occurred that affect the applicability of the original

document, both technically and practically. For instance, we have added the

certifications of SEIM 2185, the pD standard of potassium hydrogen phthalate,

and of SRM 2694, the pH standards for acidic rainwater. We are no longer

recommending the use of SRM's 922 and 923, [ tris (hydroxymethyl) -aminomethane

and tris (hydroxymethyl) aminomethane hydrochloride, respectively] as a pH

standard, because of the large residual liquid junction potentials associated

with this buffer system when used in an operational cell, even though it is

possible to assign an exact pH value to this buffer system in a cell without

liquid junction. However, the Tris system is still an excellent buffer in the

physiological range; the difficulty is assigning an exact pH value in an operational cell. Moreover, most of the pH SRM's listed in the first edition

have been recertified. The new pH values for these new lots of materials

differ, albeit slightly, from those of the previous lots. In addition, the

apparatus and instrumentation have been modified to improve the accuracy of the

measurements. All these changes and improvements necessitate that the document

be revised in order to fulfill its intended purpose.

The purpose of this document is to provide the user with a description of

the concept of pH, its definition, and the methods and facilities used at NBS

for the assignment of pH values to the buffer materials and solutions issued by

NBS as Standard Reference Materials. A brief discussion of some of the

pitfalls associated with the measurement of pH and the use of these standards

is also included. It should be noted that the pH certificates reproduced in

the appendix of this publication refer to specific stocks of the Standard

Reference Materials and, since batch-to-batch variability exists, the use of

the certified values for other sources of materials is not recommended.

Finally, certain commercial materials and equipment may be identified in

this document to specify adequately 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 identifies is necessarily the best available for the purpose.

Yung Chi Wu, William F. Koch and Richard A Durst

Center for Analytical Chemistry

xi
Table of Contents

Page

Abstract ix

Foreword to the First Edition x

Foreword to the Second Edition xi

1. Introduction 1

2. Definition of pH 1

3. Thermodynamic Foundation of pH 3

4. NBS Standardization of pH Solutions 6

5. The Operational Determination of pH and the Problem of Liquid

Junction 7

6. NBS Facilities for pH Certification Measurements 9

7. Recommended Standards 11

8. pH Instrumentation and Electrodes 16

9. Conclusion 19

10. Acknowledgement 20

11. References 20

xii
1. Introduction

Measurement and control of acidity and alkalinity are frequently essential

in both industrial processes and research work. Commercial pH meters with

electrodes of the glass type, some of which it is claimed can measure pH values

with a reproducibility of 0.001 unit, are used in nearly every laboratory where

chemical analyses or control tests are performed. These instruments, in

effect, compare the pH values of the test or sample solutions with those of

standard solutions of known pH. These standard solutions are used to calibrate

the readings of the electrode measurement system. It is very important to have

universal agreement on a standard scale of pH, and to adopt standard reference

solutions of known pH to maintain and to describe this scale, to ensure measurement comparability and compatibility.

The National Bureau of Standards is actively engaged in research to

advance and maintain standards for pH measurements. It advocates the adoption

of an operational pH scale based on thermodynamic principles and rational

conventions, and described in terms of the pH(S) of a series of standard

solutions whose assigned pH values are related to the negative of the common logarithm of the hydrogen ion activity, -log ag , in the respective solutions.

Certified samples of buffer materials, from which standard reference solutions

of reproducible pH can be prepared, or in some cases, certified solutions of

known pH, are issued by the National Bureau of Standards as Standard Reference

Materials (SRM's).

This report is concerned primarily with a description of the method used

at the National Bureau of Standards to assign pH values to the standard

reference solutions. The characteristics of these solutions are summarized and

the theory, definitions, and conventions used to arrive at the assigned values

are discussed. A brief review of types of electrodes and methods of calibration of pH assemblies is given, but for more details on these subjects,

specific monographs (see references) and the technical literature of the

equipment manufacturer should be consulted. Finally, some of the limitations

and pitfalls of pH measurements and subsequent interpretation of data are discussed.

2. Definition of pH

The negative logarithm of the hydrogen ion activity has been given the

symbol pH [1]^. It should be noted here that the original definition was in

terms of hydrogen ion concentration [2]. The present definition of pH is

associated with the "effective" concentration of hydrogen ion. The importance

of this quantity lies in the fact that it is the measure of the chemical

reactivity of the acid and alkali in aqueous solutions. It is used extensively

in biomedical studies and in monitoring agricultural and industrial processes.

Functionally, pH lies at the base of many chemical synthetic processes and is

^Numbers in brackets indicate literature references at the end of this

paper

1
essential In many analytical measurements. In biological and biochemical

research, pH is important because it is fundamental to natural processes.

The determination of pH is based on physicochemical principles and can be

performed by various techniques such as colorimetry, conductivity, and potentiometry. Of these, the potentiometric or the electromotive force (emf) method

is the simplest, the most accurate, and hence the most widely used. This is

particularly true since the invention and commercialization of the hydrogen ion

sensitive glass electrode. The employment of the glass electrode for

determining pH generally requires the following arrangement:

Glass Electrode I Test Solution||Salt Bridge Solution | Reference Electrode (I)

where the single vertical bar is the electrode-solution interface and the

double vertical bar is a liquid junction denoting an interface between the test

solution and the salt bridge solution, the latter having an ion in common with

the reference electrode.

From the measured electromotive force of cell (I), the pH can be computed

via the following equation:

E - (Ep'e + Ej) E - Ep'^

pH ^ -log aH = RT In 10/F = k

where a^ = the hydrogen ion activity (the charge, +, is dropped for

convenience)

E = the observed emf of the cell

Epg = a constant, dependent on temperature and pressure and the types of

electrodes used

Ej = the liquid junction potential of a given cell

R = the gas constant, 8.314510 J mol'^ K'

T = the kelvin temperature, 273.15 + "C

F = the Faraday constant, 96485.309 C mol"^

k = (RT In 10)/F

= EpH + Ej

The last quantity, E'g , cannot be accurately evaluated. However, if E'g is

assumed to be constant for a given system [3], and if a solution of known pH

[pH(S)] is available, then the pH of an unknown solution [pH(X)] can be

determined by using cell (I) twice: first with solution S, then with solution X and calculating the pH(X) from the difference in the respectively

measured emf's as follows:

Ex ~ Eg

pH(X) = pH(S) + ^ (2)

If pH(X) differs significantly from pH(S) or if the emf response of the system

differs from the ideal Nernstian response of 1 pH unit per volt (RT In 10/F)

2
then two pH standards (SI and S2) should be used. The value of pH(X) can be

computed according to eq (3), assuming linearity between the standards: ^ ~ 1 pH(X) = pH(Sl) + [pH(S2) - pH(Sl)] (3)

2 " 1 The choice of the two buffers SI and S2 should be such that pH(X) falls between

the values pH(Sl) and pH(S2). Such is the practical determination of pH today.

Therefore, the availability of standard pH solutions is a necessary

condition for the application of eq (2) or eq (3). Moreover, according to the

application of Henderson's or Planck's equation for the approximation of the

liquid junction potential [4-6] , the standard pH solution should be made as similar to the test solution as possible with respect to pH and composition.

Under such conditions the assumption regarding the constancy of E°q is sound.

Since in practice the conventional pH scale spans 14 pH units, several pH

standards are required for establishing calibration points over the entire pH

range

In the United States, pH standards are developed and maintained by the

National Bureau of Standards (NBS) . Many foreign countries have also adopted

the NBS approach to pH standardization. The early research in pH standards

began at NBS in the late 1930s [7] , and is manifest today through the issuance

of Standard Reference Materials. This approach, as well as the materials and

values for the NBS pH buffer standards, have been accepted internationally.

The theory and practices which are at the base of the certification process

undergo periodic critical examination at NBS to remain current with new

developments and technology and to be responsive to changing and expanding

national needs. A detailed discussion of the theory, the process, and the

refinement of the evaluation and certification of pH standards at the NBS

follows

3. Thermodynamic Foundation of pH

The use of pH in the expression pH = -log a^ is purely a formalism because

ag , a single (hydrogen) ion activity, is indeterminate. The ultimate definition of pH should be made in terms of determinable quantities. One way of

determining the pH of a weak acid (HA) may be from the dissociation constant,

Kg , of the acid, assvuning is known or can be independently determined.

Activity coefficients still enter into the expression, but in this case, as ratios. Therefore, uncertainties involving exact knowledge of activity

coefficients tend to be minimized to insignificance.

K. = = (4)

%A ShaTha

3
and

pH = pK. + log — + log — , (5)

'HA

and with the aid of the Debye-Huckel equation [8,9],

log 7i = -AzfTl/d + Ba^yi) + bl (6)

where ni = molality of the constituents

7jL = activity coefficient of the ith species

I = hSm^zl . (This is the definition of ionic strength only if the

electrolyte is fully dissociated. Otherwise, the degree of

dissociation, a, should be included. Thus I = hSam^zl; e.g., I for 0.05 m potassium hydrogen phthalate at 25 °C is not 0.05 but

0.0533, due to the partial dissociation of the biphthalate [10].) = ionic charge of the ith ion

A and B = constants dependent on the temperature and dielectric constant of

the solvent

a^^ = the ionic size parameter

b = an adjustable parameter.

On the other hand, the pH of the same acid can be determined without the

knowledge of K^, , using a galvanic cell without liquid junction, sometimes

called the Harned cell:

Pt, H2 (1 atm)|HA(m), KCl(m' )| AgCl , Ag (II)

The emf of this cell at 101.325 kPa (one atmosphere) of hydrogen pressure is

given by:

E = EA%.AgCi - (RT/F)ln mHmci7H7ci (7)

where m's are molalities, 7's the activity coefficients, R,T,F have their usual

significance, and E is the measured emf. E^^ AgCi potential of the

silver/silver chloride electrode measured relative to the "standard hydrogen

electrode," [Pt, H2 (1 atm) | ag=l], for which the reaction is:

hR^ig) = H-^ + e- (8)

and

Eg = E| - (RT/F)ln (9)

'H2

where Pg2 is the pressure of the H2 gas. When pg2 = Ph2 = ^ atmosphere, and

ag = 1 , then. Eg = Eg . Since half-cell potentials cannot be measured independently, it has been established by convention that Eg = 0 at all temperatures.

All other half-cell potentials are computed in relation to

4
By collecting all the known quantities on the right hand side, eq (7) may

be rewritten as

-log a^yci = [(E - E°)/k] + log 13^1 (10)

Since, by definition, 7^^^ = 1 as 10^.^ 0, by plotting the right hand side of

eq (10) vs and extrapolating to 50^1^ =0, the limiting value of the acidity

function can be expressed as -log (ag7)j [10]. This value corresponds to the

acidity function at the particular ionic strength, I, of the weak acid HA and

pH = -log(aH7)' + log 7(.t !)• (11)

The last term of eq (11) may be evaluated from eq (6). Therefore, all the

quantities related to pH, either from the dissociation constant or from the emf

of galvanic cells without liquid junction, can be determined or calculated.

Hence, within this framework, the term pH is defined.

It should be noted that the application of eq (6) for the evaluation of

the activity coefficients requires assumptions outside the domain of thermodynamics. However, it has been experimentally proven that this equation is a suitable approximation for the activity coefficients of strong electrolytes up

to 1 molal concentration [11,12] and, therefore, the use of this equation is

justified for the determination of pH at ionic strength lower than 1 molal.

As the ionic strength decreases, the last term in eq (6) becomes

insignificant, and the influence of a also decreases. By selecting a value for

Bdj^ (for example 1.5, as recommended by the Bates -Guggenheim convention [13,14]

at the ionic strength of around 0.1 molal), the resultant uncertainty in pH is

less than 0.005 pH unit, even if Ba^ is varied by as much as 10%. The ionic

strength of the standard pH solutions certified by NBS is less than 0.1 molal.

Thus the Bates -Guggenheim convention,

log 7i = -AzfTl/d + l.syT) (12)

is justified within the present experimental uncertainty (see table 1), and is

applicable to the certification of the NBS primary buffer standards.

With the adoption of the Bates -Guggenheim convention, it has become

possible to assign pH(S) values to the primary standard solutions with an uncertainty of ±0.005 pH units. The assigned values of pH(S) for the standard

solutions at temperatures from 0 to 50 °C, and even to 95 °C for certain

buffers, are smoothed values calculated by the equation:

pH(S) = (A/T) + B + CT + DT2 (13)

where T is the temperature in kelvin. Inasmuch as each lot of material is

individually certified, the pH(S) values of different lots may differ slightly

in the third decimal place.

New demands for pH standards of ionic strength greater than 0.1 molal,

such as that for seawater, render the use of Bates -Guggenheim convention

inapplicable. In these cases, one is faced with two choices, both equally

5
difficult. The first is to evaluate the single ion activity coefficient; the

other is to minimize and fully characterize the liquid junction potential.

Both of these are classical problems; their solutions are long overdue. We are currently working on a conventional procedure and will test it experimentally.

At present, in solutions with ionic strengths between 0.1 and 1.0 molal, the

certainty of a pH standard is limited to ±0.02 pH unit.

4. NBS Standardization of pH Solutions

The standardization of pH solutions was initiated at NBS in the late

1930' s when Hamer [15] suggested the use of a galvanic cell without a liquid

junction, similar to cell II, for the purpose. Later Hamer and Acree [7]

proved experimentally the applicability of cell II for the determination of pH.

Early in the 1940' s, Bates et al. [16] published a list of provisional pH

values for standard buffers. In the following years the experimental setup and

the treatment of data were significantly refined as the state of the art of the

measurement advanced.

The principle of the determination of pH and the thermodynamics of the

establishment of a pH scale have been discussed in section 3. Because of the

complexity of ionic interactions in electrolyte solutions, the knowledge of

ionic activity coefficients has been advanced little in recent years. The only

improvements that have occurred in the determination of pH have been in the

methods of measurement. Among various methods for determining pH, Hamer et al.

[7,15] and later Bates [13,17,18] have ably demonstrated that a galvanic cell

without a liquid junction, such as cell II, is best suited for the purpose.

Since eq (7) is used for the determination of pH, it is necessary to know

^Ag Agci • value of E^g Agci determined using the following cell [11],

Pt.HgCl atm) |HCl(m) |AgCl,Ag (III)

and the equation,

^Ag.AgCi = ^cell '" (1^)

where Eg = 0, under the conditions described in the previous section. However,

the best values of E" reported for this cell disagree by 0.2 mV or more

[19,20]. This indicates that not all silver-silver chloride electrodes behave

identically. Therefore, for the most accurate pH work, the E° value of each

individual Ag,AgCl electrode must be determined in cell III and the stability

of each electrode firmly established before it may be used in cell II.

Using cell II, relying on thermodynamic principles, and adopting the

Bates -Guggenheim convention, the NBS has certified six primary and two secon- dary pH standards. They are listed in table 1. The uncertainty associated

with each of the primary standards is 0.005 pH and the uncertainty of the

secondary standards is 0.01 pH.

The widespread use of cell I for practical pH measurements necessitates

the use of pH standards that are traceable to the primary standards for

6
calibration purposes. The work at NBS satisfies this need by providing

definitively calibrated and certified pH standards. The advantages and shortcomings of cell I have been discussed in section 2. Nevertheless, it is of

interest to evaluate the data obtained with cell I by using a combination glass

electrode to test the internal consistency of the NBS pH standards. The

reference junction was a ceramic frit. Measurements of emf were made in five

of the NBS pH standard solutions under an argon atmosphere . The emf values

were plotted versus the certified pH values and fitted by a least squares

procedure. The results are shown in table 2. The maximum deviation among all

pH(S) listed in table 2 is 0.002 pH units. These results were corroborated

with another set of experiments using a glass electrode and a saturated calomel

reference electrode with a fine flowing liquid junction. In this set, the

electrode system was calibrated with one buffer solution, and then all others

measured, assuming Nernstian response. Thus, it is concluded that the NBS pH

scale is internally consistent to at least 0.003 pH unit, confirming the

usefulness of the approximations and conventions described earlier in this

report.

5. The Operational Determination of pH and the Problem of Liquid Junction

The operational definition of pH entails the measurement of pH with cell I and the comparison of the pH of the unknown solution with that of the standard

as shown in eq (2). Such comparison assumes the constancy of the liquid

junction potential, Ej , in eq (1). This assumption has only limited validity.

In some instances significant errors in the pH determination may be introduced

by this assumption.

The problem of liquid junction potential has been the subject of numerous studies [3,15,21,22] which indicate that the liquid junction potential is not a thermodynamic quantity, i.e., that the liquid junction potential cannot be

specifically defined. For example, Ej depends on the concentration of the salt

bridge and on the temperature, but no functional correlation has been found.

Ej also varies with specific electrolytes, such as strong and weak acids.

Furthermore, it changes with the geometrical structure and the manner in which

the junction of the salt bridge is formed [15,21]. Therefore, Ej cannot be

determined exactly and eq (2) cannot be considered a rigorous thermodynamic

definition of pH.

For the reasons outlined above, one cannot in general assume Ej to be

constant when determining pH in solutions of unknown nature. It has been

reported that the uncertainty in determining pH in acid rain [22,23] and in

biological fluids [24], using a combination glass electrode in cell I, is

significant, often as large as 0.5 pH.

7
Table 1. NBS pH Standards

Solution composition

(molality)

Pr imary S tandards :

potassium hydrogen tartrate

(KHC^H^Og, sat'd. at 25 °C)

(KHC^H^Og, 0.01 m)

potassium dihydrogen citrate

(KH2C6H5O7 , 0.05 m)

potassium hydrogen phthalate

(KHCgH^O^ , 0.05 m)

potassium dihydrogen phosphate/

disodium hydrogen phosphate

(KH2PO4, 0.025 m + NazHPO^ , 0.025 m)

(KH2PO4, 0.0087 m + Na2HP0^, 0.0304 m)

sodium tetraborate decahydrate, (borax)

(Na2B^07 • IOH2O, 0.01 m)

sodium bicarbonate/sodium carbonate

(NaHCOg , 0.025 m + Na2C03 , 0.025 m)

Secondary Standards:**

potassium tetroxalate

(KHC2O4 •H2C2 0« •2H2O, 0.05 m)

calcium hydroxide

(Ca(0H)2 , sat'd. at 25 °C)

pH(S) Temperature

at 25 "C range (°C) SRM

3,.557 25 to 95 188

3,.639 0 to 60 188

3,.776 0 to 50

4,.006 0 to 50 185f

6,,863 0 to 50 186Id/186IId

7 410 0 to 50 186Id/186IId

9,,180 0 to 50 187c

10,,010 0 to 50 191a/192a

1,,679 0 to 50 189

12,.454 0 to 60 in prep.

* Uncertainty: ±0.005 pH

** Uncertainty: ±0.01 pH

8
Table 2. Internal consistency of the NBS pH scale

Standard Solution SRM g 25 °C ApH

Tartrate (0.01 m) 188

pH(S) 3.639

pH(meas) 3.638 +0.001

Phthalate 185e

pH(S) 4.004

pH(meas) 4.006 -0.002

1:1 Phosphate 186Ic/186IIc

pH(S) 6.863

pH(meas) 6.861 +0.002

Borax 187b

pH(S) 9.183

pH(meas) 9.184 -0.001

Carbonate 191/192

pH(S) 10.014

pH(meas) 10.014 +0.000

6. NBS Facilities for pH Certification Measurements

The NBS emf measurement system has gone through several modifications

during the last three decades. The current setup is essentially the same as it

was in the 1930s, but the instrumentation has advanced to the state of the art,

which is more precise, more accurate and more convenient. The system is

described as follows:

A. Apparatus

1 . Constant Temperature Bath

A double-walled, thermally insulated with rock wool, stainless steel

rectangular enclosure, with an open top and 200 liter (50 gallon)

capacity, (operating volume of 150 liters of water) constitutes the

bath. A quartz thermometer (Hewlett-Packard Co., Palo Alto, CA) with a frequency meter (Data Precision, Inc, Danvers, MA) calibrated with the

NBS temperature standard to 0.001 "C was used for sensing the temperature in the bath. The bath temperature was controlled with a dial

setting temperature control unit (Yellow Springs Instrument Co., Yellow

Springs, OH), which was connected to a 1200 watt quartz heater (Corning

Co., Corning, NY). The unit consists of a thermistor and a Wheatstonetype bridge. A cooling system is required to offset the heating.

Coiled copper tubing connected to a chilled water (12 "C) source was satisfactory for bath temperatures above 15 °C. For bath temperatures

at or below 15 "C, a refrigeration unit was used in the bath. The high

cooling capacity of the refrigeration unit was balanced by three

auxiliary 1200 watt quartz heaters which were controlled by variable

transformers. For rapid heating or cooling, hot water or ice was used,

respectively, to bring the bath temperature close to the dial setting

9
at which the control unit operated effectively. Four submersion

circulating pumps with nozzles set at different directions were used

for uniform stirring. The bath temperature was controlled to within

±0.005 °C between 10 and 40 °C; at 0, 5, 45, and 50 "C the temperature

control was limited to ±0.01 "C.

Measuring Instruments

A digital barometer (Setra Systems, Acton, MA) calibrated against the

NBS standard barometer was used to record the atmospheric pressure. A

digital multimeter (Model 3500, Data Precision, Danvers , MA) with 10

Mohm input impedance, calibrated by the NBS Electricity Division, was used to determine emf. Temperature was measured as described above.

Electrodes

The platinized platinum electrodes were made with platinum foils (1/2 x

2 1/2 cm) attached to 22 gauge platinum wires which were sealed into

soft glass tubes which in turn were joined to standard- taper glass

joints. Each electrode was platinized at a current density of 100

mA/cm^ for three min, in a 2% chloroplatinic acid (H2PtClg) solution to

which a small amount of lead acetate (10 mg Pb(C2H3 02 )£ * 3H2O per liter) was added to improve the plating characteristics. The plating was uniformly black and firmly adhered to the electrode. The electrode

served as the hydrogen electrode and was stable and reproducible (±0.02

mV) .

The silver/silver chloride electrode was prepared on two different

supporting wires, viz, silver (99.999% pure) or platinum, by the

thermal electrolytic method (19). A thoroughly washed Ag20 paste was coated onto the wires. These were heated at 500 °C for 3 to 4 h to

convert the silver oxide to a spongy, porous silver metal and then were cooled under an inert, argon atmosphere. They were anodized in 1 molal

HCl solution which was purged with argon gas, at 10 mA for the time

required to convert 15% of the spongy silver to AgCl. The electrodes

were washed with distilled water and were stored in 0.05 m HCl solution

(also purged with argon) . The E** of each electrode was determined in

0.05 m HCl solution [7 = 0.8304 (10)]. The E** values of platinum wire

supported and silver wire supported electrodes were statistically

identical. The average E" for a set of electrodes produced at NBS was determined to be 0.22244 V with a standard deviation of 0.00003 V.

Another set of electrodes was provided by Radiometer A/S (Copenhagen,

Denmark). The E° value provided with the electrodes was 0.22240 V

(Std. dev. 0.00002 V) and was confirmed in our laboratory. This set of

electrodes appeared to be more stable than those produced at NBS after

two months of use and storage. Further details of the preparation of

the platinized (or pallidized) platinum and the silver/silver chloride

electrodes can be found in references [13], [19], and [25].

10
4, Galvanic Cell without Liquid Junction

The specially designed emf measurement cell shown in figure 1 consists

of a hydrogen gas electrode compartment, a silver/silver chloride

electrode compartment, and a series of gas -dispersion compartments for

humidifying the incoming hydrogen gas by passage through the buffer

solution. Six measurement cells, containing each of the three levels

of added chloride in duplicate, are placed in the controlled temperature water bath as shown in figure 2. Also shown in this figure are the coiled copper tubing hydrogen gas inlets.

B. Materials

The materials used in the production of Standard Reference Material pH

standards are of high quality as specified on the pH certificates (see

Appendix) . In performing a pH certification, the first requirement is

to prepare the solutions and measurement cells with utmost care. In

the case of the buffer salts and as specified in the pH certificates,

the carefully dried and weighed salts are dissolved in high-purity,

distilled/deionized water having a conductivity of less than 2 /xS/cm at

25 "C. For buffers in the neutral and basic regions, carbon dioxide

must also be removed from the water prior to dissolution of the buffer

salts. The hydrogen gas must be free of oxygen and may be purified of

oxygen by passage through a catalytic reduction tube or through a palladium purifier.

C. Calculation of pH(S)

The data reduction consists of a series of operations ( vide supra ) including correcting the experimental emf values to the standard

partial pressure of hydrogen, calculating the acidity functions,

extrapolating to 15^1^ = 0 to obtain the limiting acidity functions,

evaluating the chloride ion activity coefficient using the

Bates -Guggenheim convention, and finally calculating the pa^ values.

These exper-imental values are then smoothed with respect to temperature (eq 13) by the method of least squares to give the certified

pH(S).

7. Recommended Standards

The pH scale recommended by the National Bureau of Standards is maintained

in terms of the pH(S) assigned to the six primary and two secondary reference

solutions listed in table 1. The composition and properties of the standard

solutions are given in table 3. The phthalate, phosphate, borax, carbonate,

and tetroxalate buffer solutions are prepared by dissolving the indicated

weights of the pure reference materials in water and diluting to 1 dm^ (1

liter) . The directions for the preparation of the molal solutions are given on

a volume basis, i.e., grams per liter of solution, to simplify the preparation

of the buffer solutions. However, the weights of the salts indicated in table

3, and in the NBS certificates for these pH standards are corrected for

solution density and air buoyancy and thus correspond to the nominal

11
Table 3. Composition and Properties of Standard Solutions

Solution

Butter

Material

Mass (g) in

air/dm^

Dilution

Value^

Butter

Value^

Temp

Coeff .3

Tetroxalate, 0.05m KH3(C20,)2-2H20 12 .61 +0.186 0.070 +0.001

Tartrate, sat'd KHC^H^Og >7 +0 . 049 0.027 -0.0014

Citrate, 0.05m ^2 ^6 ^5 ^7 11 .41 +0.024 0.034 -0.0022

Phthalate, 0.05m KHCgH^O^ 10 . 12 +0.052 0.016 +0.0012

Phosphate , 1:1

0.025m

U . Uz jm

KH2 P04

XT UT>A Na2 nrU^

3

3

.387

.533

+0.080 0 . 029 -0 . 0028

Phosphate , 1:3.5 +0.07 0.016 -0.0028

0 . 008695m

0.03043m

KH2 P04

Na2HP0^

14.179

.303

Rnrpv 0 01

Na R 0 • 1 OH 0 3 .80 +0 . 01 0 . 020 -0 0082

Carbonate , 1:1

0.025m

0.025m

NaHCOa

Na2 CO3

22

.092

.640

+0.079 0.029 -0.0096

Calcium hydroxide,

sat'd

Ca(0H)2 >2 -0.28 0.09 -0.033

Dilution value is the change in pH observed when a buffer solution is diluted

with an equal volume of water [1]; ApH^ = pH (C/2) - pH(C)

^Buffer value, ^, is a differential ratio db/dpH where db is an increment of

strong base expressed in moles of hydroxide ion per liter [1].

•'Temperature coefficient is the change in pH with temperature, 6pH/5T, expressed

as pH/K.

concentration on the molal scale. The NBS certificates also give the weights

of the pure reference materials needed to make the indicated molal solutions by

dilution with 1.000 kilogram of pure water. The tartrate and calcium hydroxide

solutions are made by shaking an excess of the pure material with water in a glass -stoppered bottle. The excess, insoluble material should be removed by

filtration or decantation [13]. The calcium hydroxide standard solution is

best stored in a polyethylene bottle and made fresh daily from CaO. The

tartrate solution is about 0.034 mol/kg, and the calcium hydroxide solution

about 0.021 molAg12
Figure 1. Emf measurement cell used for the certi

fication of pH buffer solutions.

13
14
Prior to weighing, a few of the reference materials should be oven-dried

for two hours: potassium hydrogen phthalate at 110 "C, potassium dihydrogen

phosphate and disodium hydrogen phosphate at 110-130 "C, and the sodium car- bonate at 275 "C. The other materials do not require oven drying but should

not be exposed to excessively high humidity. Storing in a desiccator is recom- mended. When drying the salts, only a sufficient quantity for the preparation

of the buffer solution should be sampled and dried in a weighing bottle. It is

not recommended to dry the entire bottle of salt at once. Depending on user preference, the weighed samples of reference materials should be diluted either

to the appropriate weight or volume, as specified on the certificates, with

distilled or deionized water at 25 °C. The distilled water should have a conductivity no greater than 2 /iS/cm. In addition, the phosphate, borax and

carbonate buffers should be prepared using water purged with carbon dioxide

free nitrogen or argon, or using freshly boiled water of pH 6.7 to 7.3. Boiled

water should be protected from contamination by atmospheric carbon dioxide by

cooling and storing in a bottle fitted with a tube of carbon dioxide absorbent,

e.g., soda lime. Exposure of the prepared buffer solutions to the atmosphere

should also be minimized. The more acidic buffers as well as the saturated

calcium hydroxide solution may be prepared from air-equilibrated water, pH 5.6

to 6.0. It is recommended that the buffer solutions be replaced about once a month, and sooner if mold, particulate, or other evidence of contamination is

observed. The tartrate buffer is particularly subject to mold growth, accom- panied by an increase in pH. The addition of a small amount of thymol has been

found to be an effective preservative, although this treatment should be

avoided for measurements of the highest accuracy [13].

In addition to the primary and secondary buffer materials, a new class of

pH Standard Reference Materials is now available, intended to simulate the

matrices of select solutions. The purpose of these new standards is to improve

the accuracy and comparability of pH measurements in solutions in which the

residual liquid junction potential between the customary buffer standards and

the sample solution is suspected to be significant, variable, and/or unpredictable. The first set of such standards was developed for acid rain studies,

SRM 2694, Simulated Rainwater. A detailed description of the production,

analysis, and uses of this SRM is given in NBS Special Publication 260-106

[26]. As needs develop, additional matrix-specific pH SRM's will be issued.

In the research stage at present is the development of pH standards for

seawater and for biological fluid applications.

A unique class of matrix specific pH standards are those intended for use

in heavy water, D2O. These standards are called pD standards. At present,

there are three recommended pD standards issued as SRM's, including the new

standard in the acidic region, potassium hydrogen phthalate, SRM 2185 [27].

A complete list of the pH and pD Standard Reference Materials presently

available from the National Bureau of Standards is given in table 4, and their

certificates are given in the Appendix. Although considerable work has gone

into the determination of pH values for the citrate and calcium hydroxide

solutions, reference materials have not yet been certified.

15
Table 4. NBS pH and pD Standard Reference Materials*

Material SRM Designation

Potassium hydrogen phthalate 185f

Potass iiim dihydrogen phosphate 186-I-d

Disoditun hydrogen phosphate 186-II-d

Sodium tetraborate decahydrate 187c

Potassium hydrogen tartrate 188

Potassium tetroxalate 189a

Sodium bicarbonate 191a

Sodium carbonate 192a

Simulated Rainwater 2694

Potassium hydrogen phthalate (pD) 2185

Potassium dihydrogen phosphate (pD) 2186-1

Disodium hydrogen phosphate (pD) 2186-11

Sodium bicarbonate (pD) 2191a

Sodium carbonate (pD) 2192a

*Available from the Office of Standard Reference Materials, National Bureau of

Standards, Gaithersburg, Maryland 20899. Request NBS Special Publication 260

for price list and ordering information.

8 . pH Instrumentation and Electrodes

A comprehensive discussion of pH instrumentation and electrodes is beyond

the scope of this report, and for more details, the referenced monographs and

the equipment manufacturers' literature should be consulted. However, we present a brief discussion to give general information to the practitioner

about the type of instrumentation and electrodes available.

In recent years, many excellent pH meters capable of providing pH values

with a reproducibility of a few thousandths of a pH unit have become readily

available. However, while modern digital pH meters can be read to the nearest

0.001 pH unit, the fundamental meaning of these measured values is considerably less certain than the precision of the measurement. In making pH

16
measurements, it is important to remember that the pH assembly is designed to

indicate the difference between a standard buffer and a test solution, both of

which should be at the same temperature. To affirm the proper functioning of

the glass electrode and the accuracy of the instrument over the pH range of the

test solutions, the assembly should be calibrated with two standards which

bracket the pH of the samples. With this procedure, it is assiimed that the emf

of the cell varies linearly with the pH within the range of the standards but

that the pH electrode response slope is not necessarily exactly Nernstian,

i.e., (RT In 10)/F volts per pH unit. The pH of the test solution is given by

eq (3). In actual practice, with most modern direct-reading instruments, the

above calibration is made by adjusting the standardization and sensitivity

(slope) controls on the instrument. Errors caused by fluctuations in the

residual liquid- junction potential and temperature are minimized by

standardizing the assembly at a pH close to that of the test solution.

There are four principal types of pH indicator electrodes:

The hydrogen electrode is the ultimate standard for the determination of pH,

but special precautions must be taken to ensure obtaining the most reliable and

theoretically correct results. Its use, therefore, is limited, owing to

experimental difficulties. The other pH electrodes, in particular the glass

electrode , are commonly used for routine measurements

The glass electrode is the most widely used pH- indicator electrode. It is

applicable over a wide pH range and exhibits high stability, reliability and

reproducibility. It is the only one of the four types of pH electrodes that is

completely free of oxidation- reduction interferences. The construction and

response of this electrode has been discussed in detail elsewhere [13].

Suffice it to say that the pH-responsive surface consists of a thin membrane

formed from a special glass, which after suitable conditioning, develops a surface potential that is an accurate indicator of the hydrogen ion activity of

the test solution. The glass membrane is usually made as thin as is consistent

with adequate mechanical strength. Even so, its electrical resistance is very

high, usually in the range from tens to hundreds of megohms, which is the

reason that electronic amplifiers must be used to obtain adequate measurement

accuracy. The exact mechanism of operation of the glass electrode is still a matter of some conjecture. However, there seems to be no doubt that during the

conditioning process of the glass membrane, there is an exchange between alkali

metal ions, such as sodium or lithium, in the interstices of the glass matrix

with hydrogen ions (protons) from the solution. The exchange of labile protons

between the stable sites in the conditioned glass surface (gel layer) and the

solution phase appears to be the mechanism by which the surface phase -boundary

potential mirrors changes in hydrogen ion activity in the solution. The fact

that this process is one of proton transfer rather than electron transfer is

the reason for the freedom from redox perturbations.

12

3

4

the hydrogen (gas) electrode,

the glass electrode,

the metal-oxide electrodes, and

the quinhydrone electrode.

17
Although the glass electrode exhibits remarkable specificity for hydrogen

ion, it is by no means perfect. Departures from ideal behavior can be found in

alkaline solutions containing high concentrations of alkali metal ions and also

in solutions of strong acids. By modifying the composition of pH glasses,

electrodes have been developed which show a reduction in both the alkaline and

acid errors. This should be considered when selecting glass electrodes; that

is, the type of electrode chosen should be substantially error-free under the

conditions of measurement. The glass electrode has been fabricated into a wide

variety of designs for special applications. The most common type is the

immersion electrode, but capillary and microelectrodes are also used rather

widely, particularly in biological studies [28, 29].

The metal-oxide electrodes, e.g., antimony, bismuth, iridium, and

tungsten, develop a potential by a redox reaction involving the metal and a thin layer of oxide present on the surface of the metal. In practice, their pH

response is variable and non- linear. Careful calibration is critical. The

electrode response is sensitive to dissolved oxygen concentration, the composition of the buffer and test solutions, and to stirring rate. It is also

affected by oxidizing and reducing agents, certain complexing anions, and

traces of some cations. The main advantages of the metallic pH electrodes are rapid response and rugged construction. They can be used under extreme

pressure and/or temperature conditions. They are also relatively inexpensive,

and exhibit low impedance, thus requiring less sophisticated (and less

expensive) read-out devices.

The quinhydrone electrode is also a redox electrode that produces an emf

dependent upon the pH. It is subject to a variety of limitations which greatly

restricts its use. For example, it is subject to an alkaline error that limits

its application to solutions of pH less than 8. Of less importance is the salt

error resulting from unequal changes in the activity coefficients of hydroquinone and quinone. However, this error becomes important only at high ionic

strengths. Finally, this electrode is poisoned by proteins, and cannot be used

in the presence of strong oxidizing or reducing agents. The use of the quinhydrone electrode has all but been abandoned. Its inclusion in this report is

primarily for completeness and for historical reasons.

The electrical circuit of the pH cell is usually completed via a salt

bridge and reference electrode. The three main requirements for a satisfactory

reference electrode are reversibility, reproducibility and stability [19]. The

four principal reference electrodes are the mercury/mercury (I) chloride

(calomel), the silver/silver chloride, the mercury/mercury (I) sulfate, and the

thalliiim amalgam/thallium(I) chloride electrodes. The most widely used refer- ence electrode is probably the calomel electrode which is highly reproducible

but suffers from instability at elevated temperatures (>80 °C) caused by

disproportionation of the mercury (I) chloride. The silver/silver chloride and

thallium amalgam/thallium(I) chloride are less sensitive to temperature effects

and are recommended for measurements above 80 "C.

Probably the greatest source of difficulty in a pH cell is the liquid

junction which is formed at the area of contact between the salt bridge and the

test solution. The magnitude of the liquid- junction potential depends on the

design and characteristics of the liquid junction, e.g., the composition and

18
concentration of the salt-bridge solution, and the type of junction (capillary,

porous plug, cracked bead, sleeve, membrane, flowing). Unfortunately, measure- ments made with the best reference electrodes and the most reproducible liquid

junctions are still subject to a residual liquid- junction potential error. This error results from a difference in the liquid- junction potentials at the

boundaries between the salt bridge and the test and standard buffer solutions.

In general, the residual liquid- junction potential can be minimized by correct

design of the liquid junction, properly selected salt-bridge solution, ionic

strengths of the test and standard solutions approximately equal or less than

0.1, the pH between 3 and 11, the proper choice of standards, and the exclusion

of other phases such as colloids and suspensions. A more detailed discussion

of these and other sources of error in potentiometric measurements can be found

in reference 30.

The most commonly used salt-bridge solution is potassium chloride

saturated at room temperature. It has been demonstrated experimentally, and

can also be shown by calculations using the Henderson equation for

liquid- junction potentials [13], that the higher the concentration of KCl in

the salt-bridge solution, the more effective it is in reducing the

liquid- junction potential. The saturated KCl salt bridge is not without its

disadvantages, however. The most serious problem is the blockage of flow at

the liquid junction by crystals of KCl which form upon lowering of the temperature or, over longer periods, caused by evaporation of water from the

salt-bridge solution. The blockage raises the resistance of the cell and

impairs the reproducibility of the liquid- junction potential. For this reason,

a slightly unsaturated solution, but not less than 3.5 mol/L, is recommended.

9. Conclusion

The definition and measurement of pH, as we know it today, is the result

of eight decades of research and evolutionary development. This process is

continuing as the feasibility of providing matrix- specific pH standards for

selected applications is being tested. The first application of such standards

is embodied in SRM 2694, Simulated Rainwater. The effort at NBS will be

expanded to include matrices such as seawater, biological fluids, and

eventually non- aqueous media. This new generation of pH standards will be

consistent with the current NBS pH scale and will be as thermodynamically

meaningful as possible. This should avoid the confusion that would be caused

by several inconsistent pH scales. This work will necessarily involve a re- evaluation of the presently accepted definitions and conventions incorporated

in the pH values of the SRMs issued by NBS and of the recommended standards of

organizations such as the International Union of Pure and Applied Chemistry

[31].

The ultimate goal of the NBS research program on pH is to develop and

maintain a unified pH scale based on clearly stated thermodynamic criteria,

with a wide range of applicability to practical pH measurements. The present

projects form the foundations for this goal and will lead to intensive investigations into liquid junction potentials, activity coefficients, and the

concept of single ion activities.

19
1 0 . Acknow1 edgemen

The authors wish to acknowledge the contributions of Dr. Roger G, Bates

and Dr. Walter J. Hamer, whose efforts over many years at NBS resulted in the

basis for the national pH scale described in this report.

11. References

[1] S^rensen, S. P. L. and Linderstrom-Lang, K. , Compt. Rend. Trav. Lab.

Carlsberg 15, 40 (1924).

[2] S<^rensen, S. P. L. , Compt. Rend. Trav. Lab. Carlsberg 8, 1 (1909).

[3] Hitchcock, D. I., and A. C. Taylor, J. Am. Chem. Soc. 59, 1912 (1937).

[4] Guggenheim, E. A., J. Am. Chem. Soc. 52, 1315 (1930).

[5] Henderson, P., Z. Physik. Chem. 59, 118 (1907).

[6] Plank, M. , Wied. Ann. 39, 161 (1890); 40, 561 (1890).

[7] Hamer, W. J., and S. F. Acree, J. Res. Natl. Bur. Stand. 23, 647 (1939).

[8] Debye, P., and E. Huckel, Physik. Z. 24, 185 (1923).

[9] Huckel, E.. Physik. Z. 26, 93 (1925).

10] Hamer, W. J., and S. F. Acree, J. Res. Natl. Bur. Stand. 32, 215 (1944).

11] Harned, H. S., and B. B. Owen, The Physical Chemistry of Electrolytic

Solutions, 3rd ed. , Reinhold Book Corp., New York, (1958).

12] Robinson, R. A., and R. H. Stokes, Electrolyte Solutions, 2nd ed.

Butterworths Scientific Publ., London, (1959).

13] Bates, R. G. , Determination of pH: Theory and Practice, 2nd ed. , John

Wiley & Sons, New York, (1973).

14] Bates, R. G. , and E. A. Guggenheim, Pure & Appl. Chem. i, 163 (1960).

15] Hamer, W. J., Trans. Electrochem. Soc. 7, 45 (1937).

16] Bates, R. G. , W. J. Hamer, G. G. Manov, and S. F. Acree, J. Res. Natl.

Bur. Stand. 29, 183 (1942).

17] Bates, R. G. , CRC Crit. Rev. Anal. Chem. 247 (1981).

18] Bates, R. G., Pure &Appl. Chem. 50, 1701 (1978).

19] Ives, D. J. G. and G. Janz, eds. Reference Electrodes, Academic Press,

New York, (1961)

20
[20] Clark, W. M, , The Determination of Hydrogen Ions, 3rd ed. , Williams and

Wilkins Co., Baltimore, (1928).

[21] Maclnnes, D. A., The Principles of Electrochemistry, Reinhold Publ.

Corp. , New York (1939)

[22] Koch, W. F., and G. Marinenko, ASTM Special Technical Publication 823,

10-17 (1981).

[23] Illingworth, J. A., Biochem. J. 195, 259-262 (1981).

[24] Bates, R. G. ; E. A. Guggenheim, H. S. Harned, D. J. G. Ives, G. J. Janz,

C. B. Monk, R. A. Robinson, R. H. Stokes, and W. F. K. Wynne-Jones,

J. Chem. Phys. 25, 361 (1956).

[25] Wu, Y. C, Koch, W. F. and Marinenko, G. , J. Res. Natl. Bur. Stand. 89,

395-400 (1984).

[26] 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. Natl. Bur. of

Stand. (U.S.), Spec. Publ. 260-106, pp. 78 (July 1986).

[27] Wu, Y. C. and Koch, W. F. , J. Soln. Chem. 15, 481-493 (1986).

[28] Eisenman, G. , Ed., Glass Electrodes for Hydrogen and Other Cations,

Marcel Dekker, New York (1967).

[29] Lavallee, M. , Schanne, 0. F. and Hebert, N. C, Eds., Glass

Microelectrodes , John Wiley & Sons, New York (1969).

[30] Durst, R. A. , "Sources of Error in Ion-Selective Electrode

Potentiometry" , Ion-Selective Electrodes in Analytical Chemistry,

H. Preiser, ed. , Plenum Publ. Corp., NY, 311-338 (1978).

[31] Covington, A. K. , Bates, R. G. and Durst, R. A., Pure & Appl. Chem. 52,

531-542 (1985).

21
U. S. Departrtt^irU M ComnKrce

Malcolnti&aldrige

((Itvtriitnit

Standard Reference Material 185f

Potassium Hydrogen Phthalate

pH Standard

This Standard Reference Material (SRM) is intended for use in preparing solutions for calibrating electrodes for pH

measuring systems. SRM 185f, Potassium Hydrogen Phthalate (KHC8H4O4), was prepared to insure high purity and

uniformity.

The pH value of the standard pH(S), corresponds to log (l/an) where aH is a conventional activity of the hydrogen

(hydronium) ion referred to the standard state on the molai scale. The certified values listed below were derived from emf

measurements of cells without liquid junction by the method of calculation described in the Journal of Research of the National Bureau of Standards 66A, 179(1962). The uncertainty of the pH(S) of SRM 1 85f is estimated not to exceed ±0.005 unit for the temperature range 0 to 50 °C. The O.OS-molal solution is recommended for the standardization of pH equipment. The pH(S) of this solution as a

function of temperature is given below: These certified values apply only to SRM 185f.

°c pH(S) °C pH(S)

0.0 4.006 30.0 4.012

5.0 4.001 35.0 4.021

10.0 3.999 37.0 4.025

15.0 3.999 40.0 4.031 20.0 4.001 45.0 4.043

25.0 4.006 50.0 4.057

The values of pH(S) of SRM 185f were not measured above 50 °C. For pH(S) values for the temperature range 55-95 °C

for potassium hydrogen phthalate, refer to National Bureau of Standards Special Publication 260-53, "Standardization

of pH Measurements," December 1975, pp 25-26. The estimated uncertainty of these values as applied to SRM 185f is ±0.01 unit for the temperature range 55-95 °C. The potassium hydrogen phthalate was obtained from Mallinckrodt. inc. It meets the specifications of the American

Chemical Society for reagent-grade material, but may not be entirely free of impurities such as traces of occluded water,

free acid or alkali, chlorides, sulfur compounds, or heavy metals. The analytical measurements were performed by Yung-Chi Wu and William F. Koch of the Inorganic Analytical Research Division. The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of E.L. Garner, Chief, Inorganic Analytical Research 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 W.P. Reed.

Washington, DC 20234 Stanley D. Rasberry, Chief January 31, 1984 Office of Standard Reference Materials

(over)

22
Directions for Use

Preparation of the 0.05-molal solution: The salt should be dried for 2 hours at 110 "C before use. Add 10.21g of

SRM 185f to lOOO.Og of distilled water and mix thoroughly. The distilled water should have a conductivity not greater

than 2xlO~* siemens/cm. If volumetric apparatus is to be used, transfer I0.12gof SRM 185ftoa l-liter volumetric flask. Add distilled water to dissolve the salt, fill to the mark with distilled water at 25 °C, and mix thoroughly by shaking.

The water used in the preparation of this pH buffer solution need not be protected from atmospheric carbon dioxide, and

elaborate precautions for the exclusion of air from the solution are not necessary. The solution should, however, be

protected against evaporation and contamination by molds. This buffer solution should be replaced, at least every month or whenever mold is detected, if the highest accuracy is required.

23
U. S. OapartiilMl of Commerce

Blaurie* H. SUni

Natiaul B^t*ii ot SUndardi

L. M. BrmOMsarii. Dlraetsr Certificate

Standard Reference Materials

Potassium Dihydrogen Phosphate (186-I-c)

Disodium Hydrogen Phosphate (186-II-c)

These standard reference materials are intended for use in deHning the pH(S) scale. The lots of potassium dihydrogen phosphate (KH2PO4) and disodium hydrogen phosphate (NajHP04) were

prepared to ensure high purity and uniformity. They meet the specifications of the American

Chemical Society for reagent-grade materials, but should not be considered as entirely free from

impurities such as traces of water, free acid or alkali, carbon dioxide, chlorides, sulfur compounds

and heavy metals. The pH(S) values Hsted below correspond to log(l/aH ), where oh is a conventional activity of the hydrogen (hydronium) ion referred to the standard state on the scale of molality. The values were

derived from the cmf of cells without liquid junction by the method of calculation described in the Journal of Research of the National Bureau of Standards, 66A, 179 (1962). The uncertainty of the

assigned values of pH(S) is estimated not to exceed ± 0.005 unit from 0 to 50 "C. The values listed below apply only to the lots here certified. Minor variation of pH(S) (of the order of a few

thousandths of a unit) may be expected to occur between different lots. The solution 0.025 molal with respect to both KHjP04 and Na}HP04 is recommended for the

calibration of pH equipment. The pH(S) of this solution as a function of temperature is given

below:

°C pH(S) -C pH(S) °C pH(S)

0 6.982 20 6.878 37 6.839

5 6.949 25 6.863 40 6.836

10 6.921 30 6.851 45 6.832

15 6.898 35 6.842 50 6 831 For pH measurements in the physiologically important range pH 7 to 8 a solution 0.008695

molal in KH2PO4 and 0.03043 mulal in Na2HP04 is also useful. The pH(S) values for this solution

as a function of a temperature follow: [See Journal of Research of the National Bureau of

Standards, 65A, 267(1961).]

°C pH(S) °C pH(S) X pH(S)

0 7.534 20 7.430 37 7.392 5 7.501 25 7.415 40 7.388

10 7.472 30 7.403 45 7.385 15 7.449 35 7.394 50 7.384

The potaiisium dihydrogen phosphate was obtained from the Mallinkrodt Chemical Works of

St. Louis, Missouri; the disodium hydrogen phosphate from the J. T. Baker Chemical Co., of

Phillipsburg, New Jersey. The experimental work leading to the certification of this material was

carried out by M. Paabo and B. R. Staples.

The overall direction and coordination of technical measurements leading to the certification were performed under the chairmanship of R. A. Durst. 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 T. W. Mears.

Washin};t«>n, 1). C 202U J. Paul Cali, Acting Chief

Seplcinlier 1, 1070 Office of Standard Reference Materials

(over)

24
PREPARATION OF THE 0.025-MOLAL SOLUTION

Transfer 3.387 g of the potassium dihydrogen phosphate (186-I-c) and 3.533 g of the disodium

hydrogen phosphate (186-II-c) to a 1-Iiter volumetric flask. Dissolve and fill to the mark with

distilled water at 25 ''C. The distilled water should not contain dissolved carbon dioxide and should

have a conductivity no greater than 2 X 10'^ ohm-' cm-'. Carbon dioxide-free water can be

prepared by boiling a good grade of distilled water for 10 minutes and guarding it with a soda-lime

tubt' while cooling. The salts should be dried for 2 hours at 110 °C to 130 °C before use. Although

elaborate precautions to prevent contamination of the buffer solution with atmospheric carbon

dioxide are u:»ually unnecessary, the container should be kept tightly stoppered at all times when a sample is not actually being removed. (The figures given are weights in air.)

The solution should be replaced after a few weeks or sooner if molds or sediment appear, or if it has been exposed repeatedly to air containing carbon dioxide.

PREPARATION OF THE PHYSIOLOGICAL STANDARD SOLUTION

The physiological standard solution pH(S) = 7.415 at 25 "C, is prepared by transferring 1.179 g

of potassium dihydrogen phosphate ( 186-1 -c) and 4.303 g of disodium hydrogen phosphate

(186-II-c) to a 1-liter volumetric flask, dissolving, and filling to the mark with water of the quality

described above. (The figures given are weights in air.)

This buffer solution is more sensitive to contamination with carbon dioxide than is the

0.025-molal solution. If the solution is to maintain the assigned pH(S) tor a few weeks, exclusion of

carbon dioxide may be essential.

25
Rational ^ureau of ^tmidutdB

(K^rttftcat^

Standard Reference Material 187c

Sodium Tetraborate Decahydrate

(Borax)

This Standard Reference Material (SRM) is intended for use in preparing solutions for calibrating electrodes for pH

measuring systems. SRM 187c, Borax (Na2B407 - IOH2O) meets the specifications of the American Chemical Society

for reagent grade material. The water content of this salt, stored under ordinary conditions, is less than theoretical. This does not affect the use of this salt as a pH standard, but could lead to erroneous results if the partially desiccated salt is used as a boron or acidimetric standard. The pH(S) values listed below correspond to log ( I an), where an is a conventional activity of the hydrogen (hydronium)

ion referred to the standard state (p° = I atmosphere) on the scale of molality. The values were derived from emf

measurements of cells without liquid junction by the method of calculation described in the Journal of Research of the National Bureau of Standards, 66A, 179 (1962). The uncertainty of the pH(S) of Standard Reference Material 187c is estimated not to exceed ± 0.005 unit from 0 to 50 "C. The O.OI-molal solution is recommended for the calibration of pH equipment. The pH(S) of this solution as a function of temperature is as follows: These certified values apply only to SRM 187c.

"C pH(S) °C pH(S) °C pH(S)

0.0 9.463 20.0 9.226 40.0 9.070

5.0 9.395 25.0 9.180 45.0 9.042

10.0 9.333 30.0 9.139 50.0 9.018

15.0 9.277 35.0 9.102

The borax was obtained from Mallinckrodt, Inc. The experimental workleading to the certification ofthis material was

carried out by W.E. Koch, and Yung-Chi Wu of the Inorganic Analytical Research 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 W.P. Reed.

Washington, DC 20234 Stanley D. Rasberry, Chief March 22, 1984 Office of Standard Reference Materials

(Over)

26
Directions for Use

Preparation of the O.OI-molal solution: Crush gently any large lumps of salt. The salt must be dried in an oven before

use. Transfer 3.8 1 g of the borax to a flask and dissolve in 1 .000 kg of distilled carbon dioxide-free water. Alternatively, if volumetric apparatus is to be used, transfer 3.80g to a 1-liter volumetric flask, dissolve in distilled carbon dioxide-free

water, and fill to the mark with water at 25 **C. Water, sufficiently carbon dioxide-free, can be prepared by boiling for 10 min. and cooling in a vessel guarded by a soda-lime tube. The water should have a conductance less than

2x lO^ohm 'cm . To avoid contamination ofthe buffer solution with atmospheric carbon dioxide, keep the stopper in place except when removing a portion of the solution. If desired, the solution may be protected with a soda-lime tube. Prepare fresh solutions at least once a month. For the highest accuracy, prepare fresh solutions on a weekly basis.

27
Rational ^mtm of Standards

Olerttftcate

Standard Reference Material 188

Potassium Hydrogen Tartrate

(pH Standard)

This Standard Reference Material (SRM) is intended for use in preparing solutions for calibrating electrodes for pH

measuring systems. SRM 188, potassium hydrogen tartrate (KHC4H4O6), is a material of high purity and uniformity;

however, it should not be considered entirely free from impurities such as traces of free acid or alkali, occluded water,

chlorides, sulfur compounds, and heavy metals.

The certified values listed in the table below are the assigned pH(S) numbers for solutions of Standard Reference

Material 188 as functions of temperature. These values correspond to log (l/an), where an is the conventional activity of the hydrogen ion referred to the standard state on the scale of molality. They were derived from emf measure- ments of cells without liquid junction by the method of calculation described in the Journal of Research of the National Bureau of Standards 66A, 179(1962). The uncertainty in the pH(S) values listed is estimated not to exceed ±0.005 unit from 0 to 60 °C and ±0.01 unit from 70 to 95 "C. The certified values apply only to SRM 188. A solution saturated with potassium hydrogen tartrate at 25 "C ± 1 °C is recommended as a standard for the calibra- tion of pH equipment between 25 and 95 "C.

Table 1

•C pH(S) "C

25 3.557 45 30 3.552 50 35 3.549 55 38 3.548 60 40 3.547

pH(S) 'C pH(S)

3.547 70 3.580

3.549 80 3.609 3.554 90 3.650 3.560 95 3.674

The 0.01 molal solution is recommended as a standard for the range 0 to 60 °C.

Table 2

•C pH(S) "C

0 3.711 20

5 3.689 25

10 3.671 30 15 3.657 35

38

Gaithersburg, MD 20899 May 18, 1987

(Revision of certificate dated 1-10-64)

pH(S) •c pH(S)

3.647 40 3.632

3.639 45 3.635 3.635 50 3.639 3.632 55 3.644

3.631 60 3.651

Stanley D. Rasberry, Chief

Office of Standard Reference Materials

28
Directions for Use

Preparation of the saturated solution: Add an excess of undried SRM 188 to distilled water in a glass-stoppered

bottle or flask and shake vigorously. With 100 percent excess of the salt, a few minutes of shaking is sufficient for saturation. (Note: 100 mL of water at 25 °C will dissolve about 0.7 g of potassium hydrogen tartrate.) Allow the solid to settle and decant the clear solution, or fdter if necessary. Store the solution in a glass-stoppered borosilicate glass

bottle.

Preparation of the 0.01 -molal solution: Add 1.883 g (air weight) of undried SRM 188 to 1000.0 g of distilled water and mix thoroughly. If volumetric apparatus is to be used, transfer 1.878 g (air weight) of Standard Reference

Material 188 to a 1-liter volumetric flask. Fill to the mark with distilled water and shake until all of the salt is dis- solved and the solution thoroughly mixed.

Solutions of tartrate are very susceptible to mold growth, which is usually accompanied by an increase of a few

hundredths in pH. For accurate results, tartrate standards should be prepared fresh each day. The distilled water used in preparation of these solutions should have a conductivity not greater than 2 x 10"^ ohm'^ cm'^ at 25 °C, but dis- solved carbon dioxide need not be removed.

29
Rational '^mmn of Standards

(H^rtificate

Standard Reference Material 189a

Potassium Tetroxalate

pH Standard

This Standard Reference Material (SRM) is intended for use in calibrating pH measuring systems. SRM 189a, Potassium Tetroxalate Dihydrate (KHC204*2H2C204«2 H2O), is a highly pure and uniform material and its assay is close to 100 percent. The pH value of this SRM, pH(S), corresponds to log (1/aH) where aH is a conventional activity of the hydrogen ion referred

to the standard state on the molal scale. The certified values listed below were derived from emf measurements of cells without liquid junction by the method of calculation described in the Journal of Research of the National Bureau of Standards

89, 395 (1984). The uncertainty of the pH(S) of SRM 189a is estimated not to exceed ±0.005 unit for the temperature range 5 to 50 'C. The liquid-junction potential of the common pH cell displays a considerably greater variability in solutions of pH less than

2.5 than in solutions of pH between 2.5 and 1 1.5. For this reason, solutions of poussium tetroxalate are not recommended

as primary standards of pH. They are useful, however, as secondary standards and for confirmatory purposes, with the ex- pectation that the experimental pH may differ by 0.02 to 0.05 unit from the values of pH(S) given below. They are also recommended when standards of pH are needed and a liquid junction is not involved. The 0.05-mola] solution is recommended for the standardization of pH equipment. The pH(S) of this solution as a function of temperature is given below. These certified values apply only to SRM 189a.

"C pH "C pH

5 1.669 30 1.686

10 1.671 35 1.692

15 1.673 40 1.697 20 1.677 45 1.706

25 1.681 50 1.714 The values of pH(S) of SRM 189a were not measured above 50 °C for potassium tetroxalate, refer to the National Bureau

of Standards Special Publication 260-53, "Standardization of pH Measurements," December 1975, pp 25-26. (The esti- mated uncertainty of these values as applied to SRM 189a is ±0.01 for the temperature range 55-95 "C.) The potassium tetroxalate was obtained from Fluka Chemical Corp. It meets the specifications of the American Chemical

Society for reagent-grade material, but may not be entirely free of impurities such as traces of occluded water, free acid or

alkali, chlorides, sulfur compounds, or heavy metals. The analytical measurements were performed by Yung-Chi Wu and W.F. Koch of the Inorganic Analytical Research

Division. The overall direction and coordination of the technical measurements leading to certification were performed under the chair- manship of J.R. DeVoe, Chief, Inorganic Analytical Research 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.W. Seward.

St'Cfclary

Nalmniil Buranu 6rStiindard& F.rimrAmhlcr. Director

Gaithersburg. MD 20899

April 21, 1986

(over) 30

Stanley D. Rasberry, Chief

Office of Standard Reference Materials
Directions for Use

Preparation of the 0.05-molal solution: Add 12.70 g of SRM 189a to 1000.0 g of distilled water and mix thoroughly. The

distilled water should have a conductivity not greater than 2xlO~* siemens/cm. If volumetric apparatus is to be used,

transfer 12.61 of SRM 189a to a I-liter volumetric flask. Add distilled water to dissolve the salt, fill to the mark with distilled water at 25 °C, and mix thoroughly by shaking.

The water used in the preparation of this pH buffer solution need not be protected from atmospheric carbon dioxide, and ela- borate precautions for the exclusion of air from the solution are not necessary. The solution should, however, be protected

against evaporation and contamination by molds. This buffer solution should be replaced, at least every month or whenever

mold is detected, if the highest accuracy is required.

31
I). S. Departmerit of Commerce MaIcolmjBaldrige "Secmary

National lurtati oPSiandards

Ernest l^rnb^r, ^rector Rational ^umu of ^tanttards

Certificate

Standard Reference Materials

191a Sodium Bicarbonate

192a Sodium Carbonate

These Standard Reference Materials are intended for use in preparing buffer solutions to calibrate electrodes for pH

measuring systems. The lots of sodium bicarbonate (NaHCOs) and sodium carbonate (NaaCOa) were prepared to ensure

high purity and uniformity. They meet the specifications of the American Chemical Society for reagent-grade materials, but should not be considered entirely free from impurities such as traces of water, free alkali, silica, chlorides, sulfur compounds and heavy metals.

pH(S) Values

The pH(S) values listed below correspond to log ( 1 / an), where aH is a conventional activity of the hydrogen ion referred to the standard state on the scale of molality. The values were derived from the emf of cells without liquid junction by the method of calculation described in the Journal of Research of the National Bureau of Standards, 66A, 179(1962). The

uncertainty of the assigned values of pH(S) is estimated not to exceed ± O.OOS unit from 0 to SO "C. The certified values

listed below apply only to these lots. A solution 0.025 molal with respect to both NaHCOa and Na2C03 is recommended for the caUbration of the glass electrode and pH meter used for pH measurements. The pH(S) of this solution as a function of temperature is given below.

°C pH(S) °C pH(S) "C pH(S)

0.0 10.316 20.0 10.062 40.0 9.884

5.0 10.244 25.0 10.011 45.0 9.850

10.0 10.178 30.0 9.965 50.0 9.819

15.0 10.118 35.0 9.922

The sodium bicarbonate and sodium carbonate were obtained from the Mallickrodt, Inc., St. Louis, Mo. The experi- mental work leading to the certification of this material was carried out by Y.C. Wu, G. Marinenko and W.F. Koch, NBS

Inorganic Analytical Research Division. The overall direction and coordination of technical measurements leading to the certification were performed under the chairmanship of E.L. Garner, Chief, NBS Inorganic Analytical Research 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 W.P. Reed and L.J. Powell.

Gaithersburg, MD 20899 November 21, 1984

(Revision of Certificate dated 5-1 1-83)

Stanley D. Rasberry, Chief

Office of Standard Reference Materials

(over)

32
Directions for Use

Preparation of the 0.025-molal solution: Transfer 2.092 g of sodium bicarbonate (SRM 191a) and 2.640 g of Na2C03

(SRM 192a) (weights in air) to a 1-liter volumetric flask. Dissolve and fill to the mark with distilled water at 2S °C. The

distilled water should contain no dissolved carbon dioxide and should have a conductivity no greater than 2 x 10 siemens/cm. Carbon dioxide-free water can be prepared by boiling a good grade of distilled water for 10 min. and

guarding it with a soda lime tube while cooling. The sodium bicarbonate should not be dried by heating; the sodium

carbonate should be dried for 2 hr. at 275 **C before use. Although elaborate precautions to prevent contamination of the

buffer solution with atmospheric carbon dioxide are usually unnecessary, the container should be kept tightly stoppered at

all times when a sample is not actually being removed.

The solution should be replaced after a few weeks or sooner if sediment appears or if it has been exposed repeatedly to air containing carbon dioxide.

33
U. S. DepartnKiit of Commerce

MalcoliB-fialdrige Secretary ^ _ National Buroau 6r Standards

Ernest tAmbler. Director Rational ^nrtm of ^tandarrfg

Certificate of ^nalgsis

Standard Reference Material 2694

Simulated Rainwater

This Standard Reference Material (SRM) has been developed to aid in the analysis of acidic rainwater by providing

homogeneous materials as control standards at each of two levels of acidity. SRM 2694 consists of four 30 mL solutions, two at each level, in polyethylene bottles. It was prepared by the dissolution of high-purity salts and acids in high-purity

distilled /deionized water. Certified values and method(s) used for certification are given in Table 1. Values in parentheses are not certified, but are included for information only. Transition and heavy metals are not included in SRM 2694 as they are not stable in solutions at this pH level. Analysts are referred to SRM 1643b "Trace Elements in Water^ for applications involving trace metal determinations.

Table 1

Simulated Rainwater

Constituent Elements/ Methods 2694-1 2694-11 Parameters pH @ 25 °C c 4.27 ±0.03 3.59 ±0.02

Specific Conductance b 26 ±2 130 ±2

(MS/cm (5 25.0 "C)

Acidity, meq/L d 0.050 ±0.002 0.284 ±0.005

Fluoride, mg/L a,c 0.054 ±0.002 0.098 ±0.007

Chloride, mg/L a,c (0.24) (1.0) Nitrate, mg/L a,f 7.06 ±0.15

Sulfate, mg/L a.e 175 ±0.05 10.9 ± .2 Sodium, mg/L a,g.h 0.205 ±0.009 0.419 ±0.015

Potassium, mg/L a,g.h 0.052 ±0.007 0.106 ±0.008 Ammonium, mg/L a,c (10)

Calcium, mg/L g,i.lc 0.014 ±0.003 0.049 ±0.011 Magnesium, mg/L gJ 0.024 ±0.002 0.051 ±0.003 The certified values are based on proven reliable methods of analysis. The estimated uncertainties are 2 standard

deviations of the certified values, except for uncertainties associated with SO4, acidity, pH, and specific conductance which are based on scientific judgment and are roughly equivalent to 2 standard deviations of the certified value.

Note: The nitrate value for 2694-1 is not certified because of instability, contributes to that instability.

It is believed that bacterial or fungal activity

Techniques:

a. Ion Chromatography

b. ConduCtimetry

c. Potentiometry/Ion Selective Electrodes

d. Coulometry

e. Isotope Dilution Thermal Ionization Mass

Spectrometry k.

Spectrophotometry

Laser Enhanced Ionization Spectrometry

Flame Emission Spectrometry

Inductively Coupled Plasma Spectrometry

Flame Atomic Absorption Spectrometry

Direct Current Plasma Spectrometry

The overall direction and coordination of the technical measurements leading to this certificate were performed under

the direction of J.R. DeVoe, Chief, NBS Inorganic Analytical Research Division. The technical and support aspects involved in the preparation, analysis, and issuance of this Standard Reference

Material were coordinated through the Office of Standard Reference Materials by T.E. Gills and L.J. Powell.

Gaithersburg, MD 20899

April 23, 1986

(Revision of Certificate dated 9-19-85)

Stanley D. Rasberry, Chief

Office of Standard Reference Materials

(Over)

34
The preparation and analyses of these solutions were performed by W.F. Koch.G. Marincnko, T.C. Rains, R.W. Burke,

M. Knoerdel, W.R. Kelly, G.C. Turk, Mo De-Ming, M.S. Epstein, T.A. Rush, T.A. Butler, K. Han and M.V. Smith in the NBS Inorganic Analytical Research Division.

The statistical analysis of the certification data was performed by R.C. Paule of the National Measurement Laboratory.

Notice and Warnings to Users:

Expiration of Certification: The data included in this Certificate of Analysis are valid for 6 months from the shipping

date.

Precautions: The solutions of SRM 2694 are very dilute, unbuffered, and as such are very susceptible to contamination.

Therefore, the solutions should be used immediately upon opening. No assurances can be made as to the composition or

stability of the solutions after being opened and recapped. SRM 2694 should be stored in an area free from acid and / or ammonia fumes or vapors. Refrigeration of the solutions is not necessary; however, they should not be exposed to extreme heat.

Additional Information: If conductance and pH are to be measured on the same sample, then conductance must be

measured first to prevent leakage from the pH reference electrode filling solution from affecting the conduaance. The

measurement of pH should be performed according to the guidelines set forth in the attachment to this document to minimize the bias caused by residual liquid junction potentials. Acidimetric titrations should be performed on samples

that have been purged of dissolved carbon dioxide to prevent drifting endpoints and high results. Care must be exercised

in the determination of chloride and fluoride by ion chromatography to avoid possible errors associated with the

negative water-dip in the baseline. Ammonium ion is not certified in these solutions because of the instability of ammonium ion, particularly in 2694-1. The concentration of ammonium ion in 2694-1 was originally 0.1 itg/L, but is gradually decreasing. Thus no value is given for ammonium in this solution. The concentration of ammonium in 2694-11 appears to be more stable than in 2694-1. Thus a value of 1.0 mg/L is given for information only. However the user should be aware of the possible unreliability of this value. The cause ofthe instability is not known at this time. It does not appear to affect significantly any

other component in the solutions. The temperature coefficient for the specific conductance of both solutions in SRM 2694 has been determined experimentally to be 1.5 percent per degree Celsius at 25 *C. The density of the solutions is 0.997 g/mL at 23 "C.

SRM 2694

Page 2

35
GUIDELINES FOR THE MEASUREMENT OF pH IN ACIDIC RAINWATER

This report presents a recommended procedure for the measurement of pH in acidic rainwater. The intent of this guideline is to improve the accuracy and precision of the pH measurement with special emphasis on reducing the effect of the

residual liquid junction potential. It consists of three major parts: Calibration Sequence, Control Sequence, and Rainwater Measurement Sequence. The purposes of the Calibration Sequence are to accurately calibrate the pH measure- ment system with robust buffer solutions, to accurately set the slope, and to verify that the measurement system is functioning properly. The purpose of the Control Sequence is to quantitatively determine the magnitude of the residual

liquid junction potential bias for a particular set of electrodes which must be applied in the rainwater measurement

sequence to obtain more reliable and intercomparable results. Each sequence should be executed in stepwise order with

strict adherence to detail.

Note: This guideline is applicable only to the measurement of pH in acidic rainwater and acidic low ionic strength aqueous solutions. It should not be used for other applications as inaccuracies may ensue.

General Directions Record the solution temperature to within 1 "C. Record all pH values to at least 0.01 pH unit. Make all measurements

in a quiescent solution. Fully document all calibration and control standards.

Calibration Sequence

1) Standardize the pH electrodes and meter using SRM 18SF, Potassium Hydrogen Phthalate [pH(S) 4.006 at 2S.0°C,

0.05 molal], or equivalent. Refer to ASTM D 1293, "Standard Test Methods for pH of Water" for guidance. Record

the value. Rinse the electrodes with distilled water (ASTM Type II or better).

2) With the slope adjustment of the meter set at 100 percent, and the temperature adjustment set at the temperature of the buffer solution, check the Nernstian response of the pH measurement system with a second buffer, SRM 186Ic^ 18611c, Potassium Dihydrogen Phosphate /Disodium Hydrogen Phosphate [pH(S) 6.863 at 25.0°C], or

equivalent. Refer to ASTM D1293 for guidance. If the reading for the second buffer is not within 0.03 pH units of

the prescribed value, recheck the calibration of the system.^ DO NOT CONTINUE until the conditions for calibra- tion and Nernstian response have been satisfied. If the reading for the second buffer is within 0.03 pH units of the

prescribed value, record the value and continue.

Control Sequence

3) Rinse the electrodes thoroughly with distilled water (ASTM Type II or better). Remove drops of water on the

electrode by blotting gently (Do Not Rub!) with a clean lab tissue.

4) Insert the electrodes into a clean beaker (10-20 mL capacity) containing a portion (10-20 mL) ofthe rainwater control standard (e.g., SRM 2694-1^). Be certain that the reference junction and glass bulb are completely immersed. Do

not insert the electrodes directly into the polyethylene bottles.

5) Stir or swirl the solution to ensure homogeneity and contact with the electrodes.

6) Allow the solution to settle to a quiescent state (approx. 30seconds). Record the pH after the reading has stabilized.

7) Discard this portion of the control standard. Do not use for subsequent control checks or for other analytical determinations such as specific conductance, anions, cations, and acidity. c

8) Repeat steps 3 through 7 with a second rainwater control standard (e.g., SRM 2694-11).

9) Calculate the differences between the true pH values of the rainwater control standard and the values as determined by the pH measurement system.^ Average the differences and apply this bias correction to subsequent rainwater measurements. (For example, if the pH measurement system displays the pH of the control 0. 13 pH units lower than

the true value, add 0.13 pH units to the subsequent pH measurements of rainwater.)

SRM 2694 Page 3

36
Rainwater Measurement Sequence

10) Rinse the electrodes thoroughly with distilled water (ASTM Type II or better). Remove drops of water on the

electrode by blotting gently (Do Not Rub!) with a clean lab tissue.

1 1) Insert the electrodes into a clean beaker containing a portion ( 1 0-20 mL) of the rainwater sample. Be certain that the reference junction and glass bulb are completely immersed.

12) Stir or swirl the solution to ensure homogeneity and contact with the electrodes.

13) Allow the solution to settle to a quiescent state (approx. 30 seconds). Record the pH after the reading has

stabilized.

14) Apply the bias correction as determined in step 9 and report this corrected value as the pH of the rainwater sample.

15) Discard this portion of the rainwater sample. Do no use it in other analytical tests.

16) Repeat steps 10 through IS for subsequent rainwater samples.

17) Repeat the Control Sequence at regular intervals, based upon quality control guidelines, performance history of

the measurement system, frequency of measurements, and required accuracy.

Storage of Elertrodes

1 8) When not in use, soak the electrodes in a solution which is 0. 1 mol / L potassium chloride and 1 x 1 0~^ mol / L hydrochloric acid. Do not store the electrodes in buffers, concentrated acids, concentrated potassium chloride, basic

solutions, or distilled water. Do not allow the electrodes to dry out.

19) Use these electrodes exclusively for rainwater measurements.

Notes A) For the highest accuracy and the most direct traceability to the National Bureau of Standards, it is recommended

that freshly prepared solutions of Standard Reference Materials (SRM*s) be used.

B) Possible causes for deviation from Nernstian response include:

i) improper calibration,

ii) old or contaminated buffer solutions,

iii) insufficient rinsing of the electrodes between solutions,

iv) plugged reference junction,

v) defeaive electrodes, and

vi) defective meter.

It may be necessary to replace the electrodes with a new pair if the non-Nemstian behavior -persists even with

accurately and freshly prepared buffer standards. Q Standard Reference Material 2694, Simulated Rainwater, is issued by the National Bureau of Standards, Office of Standard Reference Materials. D) Some systems may require five minutes or more to stabilize. If drifting persists, record the reading after 10 minutes and annotate the data accordingly. With such severe drift, it would be advisable to acquire a different type of pH

electrodes which do not exhibit this adverse characteristic.

E) The difference should agree to within 0.0S pH units. If not, repeat the Control Sequence.

SRM 2694 Page 4

37
A NOTE TO THE USER OF THIS SRM

Please take a moment to fill out and return this registration sheet. Frequently, we have difficulty

contacting the actual users of SRMs, because the addresses to which we ship are often those of procurement agents. The information on this registration sheet will enable us to inform you, the

user, directly of any changes in the Certificate, additions to the catalog, or other matters which

may be of interest to you. Return of this sheet will also entitle you to a copy of a NBS Special Publication, describing the production, methods of

analysis, and recommended uses of this SRM.

PLEASE TAKE A MOMENT TO REGISTER YOUR NEW

SRM 2694, SIMULATED RAINWATER

Date received: Invoice No.: _ Name:

Title: Address:

Phone:

Comments:

Thank you.

OFFICE OF STANDARD REFERENCE MATERIALS

ROOM B 316, CHEMISTRY BUILDING

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG, MD 20899

ATTN: T. E. GILLS

SRM 2694

Page 5

38
Rational ^ur^au of ^tatutarrfs

(Sertiftcat^

Standard Reference Material 2185

Potassium Hydrogen Phthalate

pD Standard

This Standard Reference Material (SRM) is intended for use in preparing buffer solutions to calibrate electrodes

for pD measuring systems. SRM 2185, Potassium Hydrogen Phthalate (KHC8H4O4), was prepared to ensure highpurity and uniformity. It meets the specifications of the American Chemical Society for reagent-grade material; however, it should not be considered to be entirely free of impurities such as traces of occluded water, free acid or

alkali, chlorides, sulfur compounds, or heavy metals.

pD(S) Values

The certified pD(S) values listed below correspond to log ( 1 / ao), where ao is a conventional activity of the deuterium ion

referred to the standard state on the molal scale. The certified values were derived from emf measurements of cells without liquid junction by the method of calculation similar to that described in the Journal of Research of the National Bureau of Standards, 66A. 1 79 ( 1 962). The uncertainty of the certified values of pD(S) is estimated not to exceed ± 0.005

unit from 5 to 50 °C . This uncertainty includes allowances for random error and all known sources of possible

systematic error. A 0.05-molal solution is recommended for the calibration of the glass electrode and pH meter used for pD measurement. The pD(S) of this solution as a function of temperature is given below:

Depart

Malc< ilalcolmBaldrige ^cretaryt'

National puraau ofSlandards

Erncil l(^mbrr. pircclor

pD(S) "C pD(S)

5.0 4.542 30.0 4.518

10.0 4.532 35.0 4.521

15.0 4.524 40.0 4.527

20.0 4.520 45.0 4.534

25.0 4.518 50.0 4.543 The potassium hydrogen phthalate was obtained from Mallinckrodt, Inc., St. Louis, Mo.

The analytical measurements leading to the certification of the material were performed by Y.C. Wu and W.F. Koch,

NBS Inorganic Analytical Research Division. The overall direction and coordination of technical measurements leading to certification were performed under the chairmanship of J.R. DeVoe, Chief, NBS Inorganic Analytical Research 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 W.P. Reed and L.J. PowelL

Gaithersburg, MD 20899 Stanley D. Rasberry, Chief November 28, 1984 Office of Standard Reference Materials

(over)

39
Directions for Use

Preparation of the 0.05-molal solution: SRM 2185 should be dried for 2 hours at 1 10*'C before use. Add 10.218 gofthe

dried SRM to 1000.0 g of deuterium oxide (weights in air) and mix thoroughly. If volumetric apparatus is to be used,

transfer 1 1.237 g of the dried SRM (weight in air) to a 1-liter volumetric flask. Dissolve and fill to the mark with

deuterium oxide at 25 **C. Mix thoroughly by shaking. The deuterium oxide should have an isotopic composition of at

least 99 mole percent D2O and should have a conductivity no greater than 2 x 10 Siemens/ cm. The buffer solution should be protected from air during storage and all transfers of the D2O solution should be done in an

inert atmosphere to avoid rapid isotope exchange. If the calibration process is completed within one hour, it is not

necessary to exclude air from the working solution. The buffer solution should be discarded after a few weeks, or sooner,

if mold is detected or if it has been exposed repeatedly to air.

40
U. S. Depart^^^oi Commerce

"jse^^rfir

Certificate of Snalps(i£(

Standard Reference Materials

Potassium Dihydrogen Phosphate (2186--I)

Disodium Hydrogen Phosphate (2186—11)

Purity

These lots of potassium dihydrogen phosphate (KH2PO4) and disodium hydrogen phosphate (Na2HP04) were

prepared to ensure high purity and uniformity. They meet the specifications of the American Chemical Society for reagent-grade materials but should not be considered as entirely free from impurities such as traces of water, free acid or alkali, carbon dioxide, chlorides, sulfur compounds and heavy metals.

pD(S) Values

The pD(S) values Hsted below correspond to log (l/ao),where ac is the conventional activity of the deuterium ion refened to the standard state on the scale of molality. The values were derived from the emf of cells without liquid junction by a method of calculation analogous to that described for the assignment of pH(S) values [Journal of Research of the National Bureau of Standards, 66A, 179 (1962)] . The uncertainty of the assigned values forpD(S) is estimated not to exceed 0.01 unit. The values listed below apply only to the lots here certified. Minor variations of pD(S) (of the order of a few thousandths of a unit) may be expected to occur between different lots. The solution 0.025 molal with respect to both KD2PO4 and Na2DP04 is recommended for the calibration of pH

meters to be used for the measurement of pD in deuterium oxide. These compounds are prepared in situ by

hydrogen-deuterium exchange between the protium salts, KH2PO4 and Na2HP04 and the deuterium oxide solvent. The pD(S) of this solution as a function of temperature is given below: t,°C pD(S) t,°C pD(S) t,°C pD(S)

5 7.539 25 7.428 40 7.387

10 7.504 30 7.411 45 7.381

15 7.475 35 7.397 50 7.377

20 7.449 DIRECTIONS FOR USE

The preparation of the 0.025 molal solution should be carried out by the addition of weighed quantities of the salts to weighed quantities of deuterium oxide in the following proportions (weights in vacuo): 0.003402g KH2PO4 and 0.003549g Na2HP04 per g of deuterium oxide. The deuterium oxide should have an isotopic composition of at least 99.5 mole percent D2O. It should not contain dissolved carbon dioxide or other gases and should have a conductivity no greater than 2 x IQ-^ fi-l cm-1. The salts should be dried for 2 hr at 100 to 130 oC before use. Although

elaborate precautions to prevent contamination of the buffer solution with atmospheric carbon dioxide are usually unnecessary, the container should be kept tightly stoppered at all times when a sample is not actually being removed. The development of the pD scale and the experimental work leading to the certification of these materials were performed by Maya Paabo and Roger G. Bates.

The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of Roger G. Bates.

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 T. W. Mears.

Washington, D. C. May 28, 1968 41

W. Wayne Meinke, Chief

Office of Standard Reference Materials
U. S. Department of Commerce Ma Icoltttr^Baldr ifie "Secfelaryf'

National puraBuorstandards

Ernest (Amblpr, Dfirector Rational ^umu of ^tanrfartts

Certificate

Standard Reference Materials

2191a Sodium Bicarbonate

2192a Sodium Carbonate

These Standard Reference Materials (SRM's) are intended for use in preparing buffer solutions to calibrate electrodes

for pD measuring systems. The lots of sodium bicarbonate (NaHCOs) and sodium carbonate (Na2C03) were prepared

to ensure high-purity and uniformity. They meet the specifications of the American Chemical Society for reagent-grade

materials; however, they should not be considered to be entirely free from impurities such as traces of water, free alkali,

silica, chlorides, sulfur compounds, and heavy metals.

pD(S) Values

The pD(S) values listed below correspond to log (1 /an), where ao is a conventional activity of the deuterium ion referred to the standard state on the scale of molality. The values were derived from the emf of cells without liquid junction by the method of calculation similar to that described in the Journal of Research of the National Bureau of Standards. 66A. 179

(1962). The uncertainty of the assigned values of pD(S) is estimated not to exceed ± 0.005 unit from 5 to 50 °C*. The

certified values listed below apply only to these lots. A buffer solution which is 0.025 molal with respect to both NaHCOs andNajCOsis recommended for the calibration of

the glass electrode and pH meter used for pD measurements. The pD(S) of this solution as a function of temperature is given below:

°C pD(S) . °C pD(S) °C pD(S)

5.0 10.993 25.0 10.732 40.0 10.60'

10.0 10.917 30.0 10.684 45.0 10.57*

15.0 10.849 35.0 10.641 50.0 10.54*

20.0 10.787

'Because of some uncertainty involved at high temperatures, the last three values are certifled to only two decimal

places. The estimated uncertainty is within ±0.01 unit for these temperatures.

The sodium bicarbonate and sodium carbonate were obtained from Mallinckrodt, Inc., St. Louis, Mo.

The analytical measurements leading to the certification of these materials were performed by Y.C. Wu and W.F. Koch,

NBS Inorganic Analytical Research Division. The overall direction and coordination of technical measurements leading to the certification were performed under the chairmanship of J.R. DeVoe, Chief, NBS Inorganic Analytical Research 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 W.P. Reed and L.J. Powell.

Gaithersburg, MD 20899 Stanley D. Rasberry, Chief November 15, 1984 Office of Standard Reference Materials

(over) 42
Directions for Use

Preparation of the 0.025-niolal solution: Add 2.101 g of sodium bicarbonate (SRM 2191a) and 2.651 g of sodium

carbonate (SRM 2192a) to 1000.0 g of deuterium oxide (weights in air) and mix thoroughly. If volumetric apparatus is to be used, transfer 2.321 g of sodium bicarbonate and 2.928 g of sodium carbonate (weights in air) to a 1-liter volumetric

flask. Dissolve and fill to the mark with deuterium oxide at 25 °C. Mix thoroughly by shaking. The deuterium oxide

should have an isotopic composition of at least 99 mole percent D2O. It should not contain dissolved carbon dioxide or other gases, and should have a conductivity no greater than 2 x 10* Siemens /cm. Carbon dioxide-free deuterium oxide

may be obtained by boiling while passing dry nitrogen or argon gas through the solution. The sodium bicarbonate

should not be dried by heating; the sodium carbonate should be dried for 2 hours at 275 °C before use. The buffer solution should be protected from air during storage and all transfers of the D2O solution should be done in an inert atmosphere to avoid rapid isotope exchange. If the calibration process is completed within one hour, it is not necessary

to exclude air from the working solution. The buffer solution should be discarded after a few weeks, or sooner, if sediment appears or if it has been exposed

repeatedly to air.

43
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No, 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO. SHEET (See instructions) NBS/SP-260/53 February 1988

4. TITLE AND SUBTITLE

Standard Reference Materials:

Standardization of pH Measurements (1988 Edition)

5. AUTHOR(S)

Yung Chi Wu, William F. Koch, and Richard A. Durst

6. PERFORMING ORGANIZATION (If joint or other than NBS. see /n struct/on s) NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9, SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State. ZIP)

Same as No. 6

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number 75-619344

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual sunnmary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

One of the most widely performed analytical measurements in chemical

laboratories is that of pH using the glass electrode. In order to ensure the

consistency of these measurements, the National Bureau of Standards has

recommended an operational scale of pH, maintained in terms of the pH(S) of a series of standard solutions. Certified samples of buffer materials, from which

the standard reference solutions of reproducible pH can be prepared, as well as solutions in the case of rainwater, are issued by the National Bureau of

Standards as Standard Reference Materials. This report is concerned primarily

with a discussion of the method used at NBS for the assignment of pH(S) values

to these standard solutions, a description of the NBS measurement facilities

for pH, and a summary of the characteristics of the buffer materials. A brief

discussion of the types of electrodes used and the calibration of pH

instrumentation is also presented.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

activity; activity coefficients; buffers; buffer solutions; certification;

glass electrode; hydrogen electrode; hydrogen ion; measurement; pH; Standard

Reference Materials; standards; thermodynamics

13. AVAILABILITY

Unl imited

I I For Official Distribution. Do Not Release to NTIS

RT^ 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

56

15. Price

*U.S. Government Printing Office : 1988 -201-597/82532 USCOMM-DC 6043-P80
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