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.
Measurement of the pH of an ammonium nitrate solution by means of a pH meter.
Distilled or demineralised water, free from carbon dioxide.
3.1 Dissolve 3.40+0.01 grams of potassium dihydrogen orthophosphate (KH2PO4) 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 litre graduated flask, make up to the mark and mix. Keep the solution in an airtight vessel.
3.2 Dissolve 10.21+0.01 grams of potassium hydrogen phthalate (KHC8O4H4) in water, transfer without loss into a 1 litre 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.
pH meter, equipped with glass and calomel electrodes or equivalent, sensitivity of 0.05 pH unit.
Calibration of the pH meter
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.
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.
Express the result in pH units, to the nearest 0.1 unit and state the temperature used.