Determination of the pH value – Electrometric method

The method can be used for determining the pH value of a sample of ground water. This is the procedure for determining the pH value by the electrometric method, which gives a direct reading of the pH value of a soil suspension in water.

Required equipment

•        A balance readable to 0.01 g.

•        A pestle and mortar

•        Non-corrodible tray

•        A 2 mm test sieve with receiver

•        A pH meter fitted with a glass electrode and a calomel reference electrode covering the range pH 3.0 to pH 10.0 readable to 0.05 pH units.

•        3 nos. 100 ml glass or plastic beakers with cover glasses and stirring rods

•        2 nos. 500 ml volumetric flasks

•        A wash bottle containing distilled water

Reagents

Buffer solution pH 4.00. Dissolve 5.11 g of potassium hydrogen phthalate in distilled water and dilute to 500 ml, using a volumetric flask.

Buffer solution pH 9.20. Dissolve 9.54 g of sodium tetraborate (borax) in distilled water and dilute to 500 ml, using a volumetric flask.

Potassium chloride. Prepare a saturated solution and use to maintain the calomel electrode of the pH meter in accordance with the manufacturer’s instructions.

Sample preparation

1. Allow the initial soil sample to air-dry or sun-dry by spreading out on a tray.
2. Sieve the sample on a 2 mm test sieve and crush retained particles other than stones to pass the sieve.
3. Reject the stones, ensuring that no finer material adheres to them, e.g. by brushing. Throughout these and subsequent operations, ensure that there is no loss of fines.
4. Reduce the material passing the 2 mm test sieve by successive riffling through a 15 mm divider to produce a representative test sample of 30 g to 35 g.

Test Procedure

1. Weigh out 30 ± 0.1 g from the sample and place in a 100 ml beaker.
2. Add 75 ml of distilled water to the beaker, stir the suspension for a few minutes, cover with a cover glass and allow to stand for at least 8 h.
3. Stir the suspension again immediately before testing.
4. Calibrate the pH meter by using standard buffer solutions, following the procedure recommended by the manufacturer.
5. Wash the electrode with distilled water and immerse in the soil suspension. Take two or more readings of the pH of the suspension with brief stirring between each reading. These readings shall agree to within 0.05 pH units before being accepted.
6. Remove the electrodes from the suspension and wash them with distilled water. Re-check the calibration of the pH meter against one of the standard buffer solutions.
7. If the instrument is out of adjustment by more than 0.05 pH units, set it to the correct adjustment and repeat Steps 5 and 6 until consistent readings are obtained.