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Physical and Biophysical Chemistry Division
Analytical Chemistry Division
Working Party on the Definition of pH Scales

 

Measurement of pH.
Definition, standards, and procedures

pH, defined by pH = - lg a_H, is a single ion quantity; hence, it is immeasurable unless evaluated by an associated convention. Nevertheless, pH can be incorporated within the SI system of measurements, because primary and secondary standards can be defined and determined by the primary method or by secondary methods. 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 adopt to achieve the target uncertainty of sample pH.

Accordingly, this new document gives the IUPAC recommended definitions, procedures, and terminology relating to pH measurements in dilute aqueous solutions (0.1 mol kg^-1) in the temperature range 0-50 ^oC.

• The primary method is the cell without transference, called the Harned Cell.
• The procedure for assigning pH(PS) values to primary buffer solutions is based on the Bates-Guggenheim convention.
• The seven primary buffer solutions, to which pH(PS) values can be assigned by the primary method, fulfill the required condition of "highest metrological" quality, and are as follows:
  1. potassium hydrogen tartrate (saturated at 25 ^oC), 0.0341 mol kg^-1
  2. potassium dihydrogen citrate, 0.05 mol kg^-1
  3. potassium hydrogen phthalate, 0.05 mol kg^-1
  4. disodium hydrogen orthophosphate, 0.025 mol kg^-1 + potassium dihydrogen orthophosphate, 0.025 mol kg^-1
  5. disodium hydrogen orthophosphate, 0.03043 mol kg^-1 + potassium dihydrogen orthophosphate, 0.00869 mol kg^-1
  6. disodium tetraborate, 0.01 mol kg^-1
  7. sodium hydrogen carbonate, 0.025 mol kg^-1 + sodium carbonate, 0.025 mol kg^-1.
     
     
• As there can be significant variations in the purity of samples of a buffer substance of the same nominal chemical composition, the primary buffer material used must be certified with values that have been measured with the Harned cell.
• The certified pH(PS) values have target expanded uncertainties U = 0.004, with a repeatability of 0.0015, reproducibility of 0.003, and variations between batches of 0.003.
• To these uncertainties, a further contribution of 0.01, the assigned uncertainty of the Bates-Guggenheim convention, must be added in order to make a pH value traceable to the internationally-accepted SI system of measurement.
• The pH(SS) values of secondary standard buffer solutions can be assigned by the following procedures:

(i) Those substances that do not fulfill all the criteria for primary standards but to which pH values can be assigned using the primary method are considered to be secondary standards (target uncertainty U = 0.01).
(ii) Secondary standards are also derived by secondary methods, which allow the assignment of pH(SS) values by comparison with primary standards, either of the same nominal composition or of different composition (target uncertainties range from 0.004 to 0.015).
(iii) When secondary standards are not compatible with the use of the platinum hydrogen electrode, the glass electrode can be used in secondary methods (target uncertainty U = 0.02).

• Secondary methods use cells with transference which involve a liquid junction potential contribution to the measured potential difference; hence, the measurement equations contain terms that, although small, are not quantifiable so that these methods are secondary, not primary, methods.
• The target uncertainties for measuring the pH of unknown solutions depend on the uncertainties of the standard buffer solutions and on the chosen calibration procedure: multipoint (5-point), bracketing (2-point), and 1-point + assumed practical slope calibration.

Details are given of the primary and secondary methods for measuring pH and of the rationale for the assignment of pH values with appropriate uncertainties to selected primary and secondary methods and to unknowns.

The present approach supersedes the previous compromise IUPAC Recommendations [A.K. Covington, R.G. Bates, and R.A Durst, Pure Appl. Chem. 57, 531 (1985)], in that the focus is shifted from standard solutions to standard methods and their associated uncertainties. The unifying principle follows the simple path:

method + quality of materials standard with known uncertainty
standard + calibration procedure measurement of unknown pH with stated uncertainty

This approach requires the evaluation of the uncertainty associated with each step in the process, starting from the assignment of pH(PS) to a primary standard solution down to pH measurements of unknowns, and to tracing the resulting combined uncertainty back to the primary standards. Examples of uncertainty budget calculations, given in the Annex, allow identification of the principal experimental parameters affecting the accuracy of the selected procedure (to the target uncertainty level), thus directing attention to the major error sources, and enabling the minor ones to be disregarded.

> Download full text of the Provisional Recommendations (pdf file - 180 KB)

Comments by 31 January 2002
Prof. Sandra Rondinini
Dipartimento di Chimica Fisica ed Elettrochimica
Universit� di Milano
Via Golgi 19
I-20133 Milano, Italy

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