Test Method for pH Determination in Soil
1) Principle:
pH of soil is determined by electrometric method which gives a direct reading of the pH value of soil suspension in water.
2) Reagents and Apparatus:
I. Balance
II. Pestle and mortar
III. Test sieve
IV. Non-corrodible tray
V. pH meter
VI. Buffer solutions
VII. 100 mL beakers
VIII. Two 500 mL volumetric flasks
IX. Wash bottle
X. Measuring cylinder
3) Procedure:
3.1) Preparation of test specimen
Obtain an initial sample. Allow the sample to air-dry by spreading out on a tray exposed to air at room temperature. 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. 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. 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.
3.2) Electrometric method of pH determination
Weigh 30+0.1 g of soil sample in a 100 mL beaker. Add 75 mL of distilled water, stir the suspension for few minutes, cover with cover glass and stand for at least 8 hours. Stir the suspension again immediately before testing. Calibrate the pH meter by using the standard buffer solutions, following the procedure recommended by the manufacturer. 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. 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. If the instrument is out of adjustment by more than 0.05 pH units, set it to the correct adjustment and repeat steps until consistent readings are obtained.
4) Reference:
BS 1377 Part 3:1990 Amd 9028 :1996