Measurement of pH. Definition, standards, and procedures (IUPAC
R. P. Buck, S.
Rondinini,* A. K. Covington, F. G. K. Baucke, C. M. A. Brett, M.
F. Camoes, M. J. T. Milton, T. Mussini, R. Naumann, K. W. Pratt, P.
Spitzer, and G. S. Wilson
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.
* Corresponding author.
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