Republication or reproduction of this report or its storage and/or dissemination by electronic means is permitted without the Measurement of pH. Definition, standards, and procedures (IUPAC Recommendations 2002)Abstract: The definition of a "primary method of measurement" [1] 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 [2]. 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.

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upon Tyne) said: I suggest that the third explanation (c) of line broadening in trifluoroacetic acid discussed briefly by Kreevoy and Mead 1 at the end of their paper is the correct one. In anhydrous trifluoroacetic acid there is a band 2 at 1455 cm-1 which has been assigned3 to a C-0 deformation mode and may in part be due to dimers. In the deuterated acid it appears 23 3 at 1426 cm-1. Fig. 1 summarizes the line broadening (half-width) observed for the three acids.194 Some new measurements by Mr. Freeman, Mr. Lilley and myself on trifluoroacetic acid are included for comparison. The close agreement between these and Kreevoy and Mead's is striking. The two sets of measurements were made at very different slit widths but the half-width broadening (obtained by subtracting the extrapolated half-width at zero ion concentration) is independent of the slit width. However, from fig. 1, although the line broadening