Calibrate the pH meter (4) at a temperature of 20 (± 1)°C, using the buffer solutions (3.1), (3.2) or (3.3). Pass a slow stream of nitrogen onto the surface of the solution and maintain this throughout the test.

Pour 100.0 ml of water onto 10 (± 0.01) grams of the sample in a 250 ml beaker. Remove the insolubles by filtering, decanting or centrifuging the liquid.

• Measure the pH value of the clear solution at a temperature of 20 (± 1)°C, according to the same procedure as for the calibration of the meter.