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).