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

Abstract : The definition of a “primary method of measurement” [1] has permitteda full consideration of the definition of primary standards for pH, determined by aprimary method (cell without transference, Harned cell), of the definition of sec-ondary standards by secondary methods, and of the question whether pH, as a con-ventional quantity, can be incorporated within the internationally accepted systemof measurement, the International System of Units (SI, Systeme Internationald’Unites). This approach has enabled resolution of the previous compromiseIUPAC 1985 Recommendations [2].Furthermore, incorporation of the uncertain-ties for the primary method, and for all subsequent measurements, permits theuncertainties for all procedures to be linked to the primary standards by an unbro-ken chain of comparisons. Thus, a rational choice can be made by the analyst ofthe appropriate procedure to achieve the target uncertainty of sample pH.Accordingly, this document explains IUPAC recommended definitions, proce-dures, and terminology relating to pH measurements in dilute aqueous solutions inthe temperature range 5–50 °C.Details are given of the primary and secondarymethods for measuring pH and the rationale for the assignment of pH values withappropriate uncertainties to selected primary and secondary substances.