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

Total soluble and insoluble nitrogen

In the absence of nitrates

7.1.1.1 Digestion

7.1.1.1 Weigh to the nearest 0.001 g, a quantity of the prepared sample containing 100 mg of nitrogen at the most. Place it in the flask of the distillation apparatus (5.1). Add 10 to 15 g of potassium sulphate (4.1), the prescribed quantity of catalyst (4.27), and a few anti-bump granules (4.28). Then add 50 ml of dilute sulphuric acid (4.7), and mix thoroughly. First heat gently mixing from time to time, until foaming ceases. Then heat so that the liquid boils steadily and keep it boiling for one hour after the solution has become clear, preventing any organic matter from sticking to the sides of the flask. Allow to cool. Carefully add about 350 ml of water, with mixing. Ensure that the dissolution is as complete as possible. Allow to cool and connect the flask to the distillation apparatus (5.1).

7.1.1.2 Distillation of ammonia

7.1.1.2 Transfer with a pipette, into the receiver of the apparatus, 50 ml standard 0.2 N sulphuric acid (4.8). Add the indicator (4.29.1 or 4.29.2). Ensure that the tip of the condenser is at least 1 cm below the level of the solution.

Taking the necessary precautions to avoid any loss of ammonia, carefully add to the distillation flask enough of the concentrated sodium hydroxide solution (4.9) to make the liquid strongly alkaline (120 ml is generally sufficient: check by adding a few drops of phenolphthalein. At the end of the distillation the solution in the flask must still be clearly alkaline). Adjust the heating of the flask so as to distil 150 ml in half an hour. Test with indicator paper (4.30) that the distillation has been completed. If it has not, distil a further 50 ml and repeat the test until the supplementary distillate reacts neutrally to the indicator paper (4.30). Then lower the receiver, distil a few ml more and rinse the tip of the condenser. Titrate the excess acid with a standard solution of potasssium or sodium hydroxide 0.2 N (4.10) to the end point of the indicator.

7.1.1.3 Blank test

7.1.1.3 Make a blank test under the same conditions (omitting only the sample) and use this value in the calculation of the final result.

7.1.1.4 Expression of the result

7.1.1.4

where:

• a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out by placing in the receiver of the apparatus (5.1), 50.0 ml of standard solution of sulphuric acid (0.2 N) (4.8).

• A = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis.

• M = weight of the sample in grams.

where:

• a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out by placing in the receiver of the apparatus (5.1), 50.0 ml of standard solution of sulphuric acid (0.2 N) (4.8).

• A = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis.

• M = weight of the sample in grams.

In the presence of nitrate

7.1.2.1 Test sample

7.1.2.1 Weigh to the nearest 0.001 g, a quantity of the sample containing not more than 40 mg of nitric nitrogen.

7.1.2.2 Reduction of the nitrate

7.1.2.2 Mix the sample in a small mortar with 50 ml of water. Transfer with the minimum amount of distilled water into a 500 ml Kjeldahl flask. Add 5 g of reduced iron (4.2) and 50 ml of stannous chloride solution (4.11). Shake and leave it to stand for half an hour. During this time shake again after 10 and 20 minutes.

7.1.2.3 Kjeldahl digestion

7.1.2.3 Add 30 ml of sulphuric acid (4.12), 5 g of potassium sulphate (4.1), the prescribed quantity of catalyst (4.27) and some anti-bump granules (4.28). Heat gently with the flask slightly tilted. Increase the heat slowly and shake the solution frequently to keep the mixture suspended; the liquid darkens and then clears with the formation of a yellow-green anhydrous iron sulphate suspension. Continue heating for one hour after obtaining a clear solution. maintaining it at simmering point. Leave to cool. Cautiously take up the contents of the flask in a little water and add little by little 100 ml of water. Mix and transfer the contents of the flask into a 500 ml graduated flask. Rinse the flask several times with distilled water. Make up the volume with water and mix. Filter through a dry filter into a dry receiver.

7.1.2.4 Distillation of ammonia

7.1.2.4 Transfer by pipette, into the flask of the distillation apparatus (5.1), an aliquot part containing 100 mg of nitrogen at the most. Dilute to about 350 ml with distilled water, add a few anti-bump granules (4.28), connect the flask to the distillation apparatus and continue the estimation as described in paragraph 7.1.1.2.

7.1.2.5 Blank test

7.1.2.5 See 7.1.1.3.

7.1.2.6 Expression of the result

7.1.2.6

where:

• a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out by placing in the receiver of the apparatus (5.1), 50.0 ml of standard solution of sulphuric acid (0.2 N) (4.8).

• A = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis.

• M = weight of the sample, expressed in grams, present in the aliquot part taken for analysis.

where:

• a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out by placing in the receiver of the apparatus (5.1), 50.0 ml of standard solution of sulphuric acid (0.2 N) (4.8).

• A = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis.

• M = weight of the sample, expressed in grams, present in the aliquot part taken for analysis.