On the validity of acidity function extrapolation to zero added ionic strength and its impact on the uncertainty of primary pH measurements using the Harned cell

The determination of pH is the most commonly made analytical measurement throughout the world and the vast majority of these measurements are made to fulfil quality assurance and control requirements where traceability is required. Therefore, primary measurements of pH are essential to establish traceability to the agreed definition of this important quantity and disseminate this traceability to end users. The extrapolation of the acidity function to zero added ionic strength is an essential step in the Harned cell methodology for primary pH measurement. This approach relies on a number of solution chemistry approximations. Potential limitations of the current extrapolation method including possible non-linearity, and the validity of some approximations derived from solution chemistry, are reviewed and explored. Implications for the uncertainty of pH measurements using the Harned cell are discussed.

Fundamental solution chemistry approximations associated with primary pH measurements are discussed.Figure optionsDownload as PowerPoint slideHighlights
► Solution chemistry approximations associated with primary pH measurements are discussed.
► The theoretical validity of the procedure used to extrapolate to zero chloride molality is evaluated.
► Possible reasons for the non-linearity of the extrapolation process are also highlighted.
► The discussion is illustrated with examples from recent international comparison exercises.
► Consideration is given to increase the uncertainty of primary pH measurements as a result.