Measurement of pH. Definition, standards, and procedures (IUPAC Recommendations 2002)

TLDR

The definition of primary and secondary pH measurement methods, including the Harned cell, and their incorporation into the SI system allows uncertainties to be linked through an unbroken chain of comparisons. This document explains IUPAC recommended definitions, procedures, and terminology for measuring pH in dilute aqueous solutions between 5–50 °C. It details primary and secondary methods for measuring pH and the rationale for assigning pH values with appropriate uncertainties to selected substances. The approach resolves the 1985 IUPAC compromise and enables analysts to choose procedures that achieve target sample‑pH uncertainties. Citation: Pure Appl.

Abstract

Abstract The definition of a “primary method of measurement ” has permitted a full consideration of the definition of primary standards for pH, determined by a primary method (cell without transference, Harned cell), of the definition of secondary standards by secondary methods, and of the question whether pH, as a conventional quantity, can be incorporated within the internationally accepted system of measurement, the International System of Units (SI, Système International d’ Unités). This approach has enabled resolution of the previous compromise IUPAC 1985 Recommendations [ Pure Appl. Chem. 57 , 531 (1985)]. Furthermore, incorporation of the uncertainties for the primary method, and for all subsequent measurements, permits the uncertainties for all procedures to be linked to the primary standards by an unbroken chain of comparisons. Thus, a rational choice can be made by the analyst of the appropriate procedure to achieve the target uncertainty of sample pH. Accordingly, this document explains IUPAC recommended definitions, procedures, and terminology relating to pH measurements in dilute aqueous solutions in the temperature range 5-50 °C. Details are given of the primary and secondary methods for measuring pH and the rationale for the assignment of pH values with appropriate uncertainties to selected primary and secondary substances.