The pH metrology program supports traceability for pH measurements. This traceability rests on primary pH measurements in an ideal electrochemical cell (Harned cell) and on an internationally agreed-upon convention for single-ion activity coefficient. NIST disseminates this traceability to customers through six pH Standard Reference Materials (SRMs), certified using this primary measurement. NIST pH SRMs serve as benchmarks for pH measurements in all fields of industry, government, and academia. International Key Comparisons organized by the Consultative Committee on Amount of Substance (CCQM) provide a framework for the equivalence of NIST primary pH measurements and NIST pH SRMs with corresponding measurements and reference materials of other national metrology institutes (NMIs).
The suite of NIST pH SRMs forms the basis for pH measurements in the United States, and pH is the most-often-measured chemical quantity. Hence this NIST work directly or indirectly impacts a large number of users in industry, government, and academia. One of the most critical set of users is in clinical chemistry: blood pH falls within a narrow range, and low uncertainties and traceability can be crucial factors in critical patient diagnoses.
The main objectives of the pH metrology program are to provide the fundamental framework for pH measurements in the United States, to disseminate traceability via this framework to users of practical pH measurements, and to ensure the international compatibility of primary pH metrology in the United States. The first two objectives are met through the issuance of a suite of six primary pH SRMs. The pH SRMs provide the links for practical pH measurements to the primary measurement of pH. Each of these SRMs is certified using the internationally-recognized primary measurement for pH, based on thermodynamics and an accepted convention. To assure that the realization of the primary measurement is compatible with international practice, NIST participates in a set of Key Comparisons of the CCQM. These comparisons assess the capability of NMIs worldwide to perform the primary measurement at the highest level, thereby defining the state of the art and the ultimate traceability for pH measurements to the Système International d'Unités (International System of Units, SI).
Each of the pH SRMs is directly certified using the primary pH measurement, which consists of the measurement of the potential of the SRM buffer in an ideal electrochemical cell (cell without transference). The pH value is obtained from the measured cell potential using thermodynamic theory and the application of an internationally-accepted convention. Separate measurements of the homogeneity with respect to pH are also performed, to ensure that the large batches of candidate SRM material are suited for issuance as the SRM proper. Traceability for the SRM is assured by evaluating the uncertainty of each applicable source in conformance with the Guide to the Uncertainty of Measurements (GUM).
CCQM Key Comparisons are performed using the same primary measurement, but in this case a homogeneous solution is distributed worldwide and is measured by each participant within a brief, agreed time period. Results for all participants, including agreement on the overall uncertainty of the primary measurement itself, are evaluated at meetings of the CCQM Electrochemical Analysis Working Group. In Key Comparisons coordinated by NIST, great pains are taken to validate the integrity of the shipped solution, to assure that the results of the Key Comparison are not compromised.
The last two SRM pairs in the list below are biological buffers certified by a variation of the primary pH measurement method.