Traceable measurements of pH

Presented at the International Conference on Metrology - Trends and Applications in Calibration and Testing Laboratories, 16-18 May 2000, Jerusalem, 

Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany 

Abstract The primary method for pH is based on the measurement of the potential difference of an electrochemical cell containing a platinum hydrogen electrode and a silver/silver chloride reference electrode, often called a Harned cell. Assumptions must be made to relate the operation of this cell to the thermodynamic definition of pH. National metrology institutes use the primary method to assign pH values to a limited number of primary standards (PS). The required comparability of pH can be ensured only if the buffers used for the calibration of pH meter-electrode assemblies are traceable to 

The users of pH meters thus need calibration solutions of long-time stability which are traceable to primary pH standards pH(PS) related as closely as possible to the definition of pH. Although pH measurements are carried out on a large scale, the problems posed by the traceability of pH have not yet been adequately solved. 

A hundred years ago, in 1909, Soerensen of the Carlsberg Laboratory in Copenhagen [1] defined pH in terms of the concentration with a scale of 0-14 (at 

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