The Fertilisers (Sampling and Analysis) Regulations (Northern Ireland) 1991

D. Determination of the pH Value

1.    SCOPE AND FIELD OF APPLICATION

This method defines the procedure for measuring the pH value of a solution of a straight ammonium nitrate fertiliser containing more than 28% nitrogen by weight.

2.    PRINCIPLE

Measurement of the pH of an ammonium nitrate solution by means of a pH meter.

3.    REAGENTS

Distilled or demineralised water, free from carbon dioxide.

Buffer solution. pH 6.88 at 20°C

3.1  Dissolve 3.40 ± 0.01 grams of potassium dihydrogen orthophosphate (KH₂ PO₄) in approximately 400 ml of water. Then dissolve 3.55 ± 0.01 gram of disodium hydrogen orthophosphate (Na₂HPO₄) in approximately 400 ml of water. Transfer the two solutions without loss into a 1,000 ml standard flask, make up to the mark and mix. Keep this solution in an airtight vessel.

Buffer solution pH 4.00 at 20°C

3.2  Dissolve 10.21 ± 0.01 grams of potassium hydrogen phthalate (KHC₈O₄H₄) in water, transfer without loss into a 1,000 ml standard flask, make up to the mark and mix.

Keep this solution in an airtight vessel.

3.3  Commercially available pH standard solutions may be used.

4.    APPARATUS

pH meter, equipped with glass and calomel electrodes or equivalent, sensitivity 0.05 pH unit.

5.    PROCEDURE

Calibration of the pH meter

5.1  Calibrate the pH meter (4) at a temperature of 20 (± l)°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.

Determination

5.2  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.

6.    EXPRESSION OF RESULTS

Express the result in pH units, to the nearest 0.1 unit, and state the temperature used.