Appendix V L. Determination of pH Values

The pH is a number which represents conventionally the hydrogen ion concentration of an aqueous solution. For practical purposes, its definition is an experimental one. The pH of a solution to be examined is related to that of a reference solution (pH_s) by the following equation:

in which E is the potential, expressed in volts, of the cell containing the solution to be examined and E_s is the potential, expressed in volts, of the cell containing the solution of known pH (pH_s), k is the change in potential per unit change in pH expressed in volts, and calculated from the Nernst equation.

The potentiometric determination of pH is made by measuring the potential difference between 2 appropriate electrodes immersed in the solution to be examined: 1 of these electrodes is sensitive to hydrogen ions (usually a glass electrode) and the other is the reference electrode (for example, a saturated calomel electrode).

Apparatus The measuring apparatus is a voltmeter with an input resistance at least 100 times that of the electrodes used. It is normally graduated in pH units and has a sensitivity such that discrimination of at least 0.05 pH unit or at least 0.003 V may be achieved.

Method Unless otherwise prescribed in the monograph, all measurements are made at the same temperature (20-25 °C). Table 2.2.3.-2 shows the variation of pH with respect to temperature of a number of reference buffer solutions used for calibration. For the temperature correction, when necessary, follow the manufacturer's instructions. The apparatus is calibrated with the buffer solution of potassium hydrogen phthalate (primary standard) and 1 other buffer solution of different pH (preferably one shown in Table 2.2.3.-2). The pH of a third buffer solution of intermediate pH read off on the scale must not differ by more than 0.05 pH unit from the value corresponding to this solution. Immerse the electrodes in the solution to be examined and take the reading in the same conditions as for the buffer solutions.

When the apparatus is in frequent use, checks must be carried out regularly. If not, such checks should be carried out before each measurement.

All solutions to be examined and the reference buffer solutions must be prepared using carbon dioxide-free water R.

Potassium Tetraoxalate 0.05 M Dissolve 12.61 g of C₄H₃KO₈,2H₂O in carbon dioxide-free water R and dilute to 1000.0 mL with the same solvent.

Potassium Hydrogen Tartrate saturated at 25 °C Shake an excess of C₄H₅KO₆ vigorously with carbon dioxide-free water R at 25 °C. Filter or decant. Prepare immediately before use.

Potassium Dhydrogen Citrate 0.05 M Dissolve 11.41 g of C₆H₇KO₇ in carbon dioxide-free water R and dilute to 1000.0 mL with the same solvent. Prepare immediately before use.

Potassium Hydrogen Phthalate 0.05 M Dissolve 10.13 g of C₈H₅KO₄, previously dried for 1 h at 110 ± 2 °C, in carbon dioxide-free water R and dilute to 1000.0 mL with the same solvent.

Potassium Dihydrogen Phosphate 0.025 M + Disodium Hydrogen Phosphate 0.025 M Dissolve 3.39 g of KH₂PO₄ and 3.53 g of Na₂HPO₄, both previously dried for 2 h at 120 ± 2 °C, in carbon dioxide-free water R and dilute to 1000.0 mL with the same solvent.

Potassium Dihydrogen Phosphate 0.0087 M + Disodium Hydrogen Phosphate 0.0303 M Dissolve 1.18 g of KH₂PO₄ and 4.30 g of Na₂HPO₄, both previously dried for 2 h at 120 ± 2 °C, in carbon dioxide-free water R and dilute to 1000.0 mL with the same solvent.

Disodium Tetraborate 0.01 M Dissolve 3.80 g of Na₂B₄O₇,10H₂O in carbon dioxide-free water R and dilute to 1000.0 mL with the same solvent. Store protected from atmospheric carbon dioxide.

Sodium Carbonate 0.025 M + Sodium Hydrogen Carbonate 0.025 M Dissolve 2.64 g of Na₂CO₃ and 2.09 g of NaHCO₃ in carbon dioxide-free water R and dilute to 1000.0 mL with the same solvent. Store protected from atmospheric carbon dioxide.

Calcium Hydroxide, Saturated at 25 °C Shake an excess of calcium hydroxide R with carbon dioxide-free water R and decant at 25 °C. Store protected from atmospheric carbon dioxide.

Store buffer solutions in suitable chemically resistant, tight containers, such as type I glass bottles or plastic containers suitable for aqueous solutions.

[Suitable glass electrodes and pH meters of both the analogue and digital type are described in British Standards 2586:1979 and 3145:1978.]