The pH value conventionally represents the acidity or alkalinity of an aqueous solution. In the Pharmacopoeia, standards and limits of pH have been provided for those pharmacopoeial substances in which pH as a measure of the hydrogen-ion activity is important from the standpoint of stability or physiological suitability. The determination is carried out at a temperature of 25°C ± 2°C, unless otherwise specified in the individual monograph.
The pH value of a solution is determined potentiometrically by means of a glass electrode, a reference electrode and a pH meter either of the digital or analogue type.
Operate the pH meter according to the manufacturer's instructions. Calibrate the apparatus using buffer solution D as the primary standard, adjusting the meter to read the appropriate pH value given in the table, corresponding to the temperature of the solution. To set the scale, use a second reference buffer solution, either buffer solution A, buffer solution E or buffer solution G and carry out a check with a third buffer solution of intermediate pH. The pH reading of the intermediate solution must not differ by more than 0.05 from the corresponding value indicated in the table.
Reference Buffer Solutions
NOTE - Prepare the following buffer solutions using carbon dioxide-free water. Buffer solutions should be stored in bottles made of alkali-free glass, and must not be used later than 3 months after preparation.
A. A 1.271 per cent w/v solution of potassium tetraoxalate.
B. A freshly prepared solution, saturated at 25°C, of potassium dihydrogen tartrate.
C. A freshly prepared 1.151 per cent w/v solution of potassium dihydrogen citrate.
D. A 1.021 per cent w/v solution of potassium hydrogen phthalate, previously dried at 110°C to 135°C for 2 hours.
E A mixture containing 0.348 per cent w/v of potassium dihydrogen phosphate and 0.355 per cent w/v of anhydrous disodium hydrogen phosphate, both previously dried at 110°C to 130°C for 2 hours.
F A mixture containing 0.1184 per cent w/v of potassium dihydrogen phosphate and 0.4303 per cent w/v of anhydrous disodium hydrogen phosphate, both previously dried at 110°C to 130°C for 2 hours.
G A 0.3814 per cent w/v solution of sodium tetraborate stored protected from carbon dioxide.
H. A mixture containing 0.2649 per cent w/v of sodium carbonate and 0.210 percent w/v of sodium bicarbonate.
I. Shake on excess of calcium hydroxide with carbon dioxide- free water and decant at 25°C.
Immerse the electrodes in the solution under examination and measure the pH at the same temperature as for the standard solutions. At the end of a set of measurements, record the pH of the solution used to standardize the meter and the electrodes. If the difference between this reading and the original value is greater than 0.05, the set of measurements must be repeated.
When measuring pH values above 10.0 ensure that the glass electrode is suitable for use under alkaline conditions and apply any correction that is necessary. All solutions and suspensions of substances under examination must be prepared using carbon dioxide-free water.
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