Traceable measurements of pH

The primary method for pH is based on the measurement of the potential diff erence of an electrochemical cell containing a platinum hydrogen electrode and a silver/silver chloride reference electrode, often called a Harned cel l. Assumptions must be made to relate the operation of this cell to the the rmodynamic definition of pH. National metrology institutes use the primary method to assign pH values to a limited number of primary standards (PS). T he required comparability of pH can be ensured only if the buffers used for the calibration of pH meter-electrode assemblies are traceable to there pr imary pH standards. To assess the degree of equivalence, comparisons of pri mary measurement procedures for pH were organized in co-operation with EURO MET. Typical results will be presented. In 1998, the Consultative Committee for Amount of Substance (CCQM) decided to include the field of pH in its w orking programme. The first key comparison for this quantity was recently c arried out on two phosphate buffer solutions.