General:
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 is described here. This method can also be used for determining the pH value of a sample of ground water.
NOTE: Good practice in chemical testing requires that duplicate specimens should be tested. In each of the test methods the measurement of only one value of the overall result is described. It is recognized that it is necessary in many practical applications to make a number of tests in order to obtain a mean value and an indication of the reliability of the results.
Apparatus
1. Balance, readable to 0.001 g.
2. Pestle and mortar, or a suitable mechanical crusher.
3. Test sieve, of 2 mm aperture size, with receiver.
4. Non-corrodible tray.
5. pH meter, fitted with a glass electrode and a calomel reference electrode (which may be incorporated in one probe) covering the range pH 3.0 to pH 10.0. The scale shall be readable and accurate to 0.05 pH units.
6. Three 100 mL glass or plastics beakers with cover glasses and stirring rods
7. Two 500 mL volumetric flasks.
8. Wash bottle, preferably made of plastics, containing distilled water.
9. Buffer solution, pH 4.0. Dissolve 5.106 g of potassium hydrogen phthalate in distilled water and dilute to 500 mL with distilled water. Alternatively, a proprietary buffer solution of pH 4.0 may be used.
10. Buffer solution, pH 9.2. Dissolve 9.54 g of sodium tetraborate (borax) in distilled water and dilute to 500 mL. Alternatively, a proprietary buffer solution of pH 9.2 may be used.
11. Potassium chloride. Saturated solution (for maintenance of the calomel electrode).
Preparation of Test Specimen
Step 1: Obtain an initial sample and allow the sample to air-dry by spreading out on a tray exposed to air at room temperature.
Step 2: Sieve the sample on a 2 mm test sieve (if appropriate, guarded by a sieve of larger aperture) and crush retained particles other than stones to pass through the 2 mm test sieve.
Step 3: Reject the stones, ensuring that no fine material adheres to them, e.g. by brushing. Record the mass m2 (in g) of the sample passing the 2 mm test sieve to the nearest 0.1 %. Throughout these and subsequent operations ensure that there is no loss of fines.
Step 4: Divide the material passing the 2 mm test sieve by successive riffling through the 15 mm divider to produce a representative test sample of 30 g to 35 g.
Electrometric Method of pH Determination
Step 1: From the sample obtained as described in step 4 above, weigh out 30 ± 0.1 g of soil and place in a 100 mL beaker.
Step 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.
NOTE: The pH value of a soil suspension varies with the ratio of soil to water, an increase in dilution bringing the pH closer to 7.
Step 3: Stir the suspension again immediately before testing.
Step 4: Calibrate the pH meter by using the standard buffer solutions, following the procedure recommended by the manufacturer.
Step 5: Wash the electrode with distilled water and immerse in the soil suspension. Take two or three readings of the pH of the suspension with brief stirrings between each reading. These readings shall agree to within 0.05 pH units before being accepted.
NOTE: The pH readings of the soil suspension should reach a constant value in about 1 min. No readings should be taken until the pH meter has reached equilibrium.
Step 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.
Step 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.
Step 8: When not in use, leave the electrode standing in a beaker of distilled water.
Test Report
The test report shall state that the test was carried out in accordance with section 9.5 of BS 1377 3:1990 and shall contain the following information:
a) the method of test used;
b) the pH value of the soil suspension to the nearest 0.1 pH unit;
c) other necessary information
