The Fertilisers (Sampling and Analysis) Regulations 1991

3b. DETERMINATION OF NITRIC AND AMMONIACAL NITROGEN—ARND METHOD

SCOPE

1.  This method is for the determination of nitric and ammoniacal nitrogen with reduction according to Arnd (modified for each of the variants (a), (b) and (c)).

FIELD OF APPLICATION

2.  See method 3a.

PRINCIPLE

3.  Reduction of nitrates and nitrites to ammonia in a neutral aqueous solution by means of metallic alloy composed of 60% Cu and 40% Mg (Arnd’s alloy) in the presence of magnesium chloride.

Distillation of the ammonia and determination of the yield in a known volume of standard sulphuric acid solution. Titration of the excess acid by means of a standard solution of sodium or potassium hydroxide.

REAGENTS

4.—4.1 Hydrochloric acid solution, 50% (V/V): dilute an appropriate volume of hydrochloric acid (d = 1.18 g/ml) with an equal volume of water.

4.2 Sulphuric acid, 0.1 N solution. } for variant (a) (page 15)

4.3 Sodium or potassium hydroxide, 0.1 N solution, carbonate free. } for variant (a) (page 15)

4.4 Sulphuric acid, 0.2 N solution. } for variant (b) page (16) (see Note on page 15)

4.5 Sodium or potassium hydroxide, 0.2 N solution, carbonate free. } for variant (b) page (16) (see Note on page 15)

4.6 Sulphuric acid, 0.5 N solution. } for variant (c) (page 16) (see Note on page 15)

4.7 Sodium or potassium hydroxide, 0.5 N solution, carbonate free. } for variant (c) (page 16) (see Note on page 15)

4.8 Sodium hydroxide solution, approximately 2 N.

4.9 Arnd’s alloy—powdered so as to pass through a sieve with apertures less than 1.00 mm square.

20% Magnersium chloride solution:

4.10 dissolve 200 g magnesium chloride (MgCL².6H²0) in approximately 600-700 ml water in a litre flat-bottomed flask. To prevent frothing, add 15 g magnesium sulphate (MgSO⁴.7H²O). After dissolution add 2 g magnesium oxide and a few anti-bump granules of pumice stone and concentrate the suspension to 200 ml by boiling, thus expelling any trace of ammonia from the reagents. Cool, make up the volume to 1 litre and filter.

Indicator solutions

4.11.1 Mixed indicator:

4.11.1 Solution A: dissolve 1 g methyl red in 37 ml 0.1 N sodium hydroxide solution and make up to 1 litre with water.

• Solution B: dissolve 1 g methylene blue in water and make up to 1 litre. Mix 1 volume of A with 2 volumes of B.

• This indicator is violet in acid solution, grey in neutral solution and green in alkaline solution.

• Use 0.5 ml (10 drops).

4.11.2 Methyl red indicator solution:

4.11.2 dissolve 0.1 g methyl red in 50 ml 95% ethanol, make up to 100 ml with water and filter if necessary. This indicator may be used (4 to 5 drops) instead of the preceding one.

4.11.3 Congo red indicator solution:

4.11.3 dissolve 3 g Congo red in 1 litre warm water and filter if necessary after cooling. This indicator may be used, instead of the two described above, in the neutralisation of acid extracts before distillation, using 0.5 ml per 100 ml of liquid to be neutralised.

4.12 Anti-bump granules of pumice stone wahed in hydrochloric acid and ignited.

4.13 Sodium nitrate.

APPARATUS

5.  See method 2.

PREPARATION OF SAMPLE

6.  See method 1.

PROCEDURE

Preparation of the solution for analysis

7.—7.1 See method 2.

Determination

7.2 According to the chosen variant, place in the receiving flask a measured quantity of standard sulphuric acid as indicated in the Table of Method 2. Add the appropriate quantity of chosen indicator solution (4.11.1 or 4.11.2) and if necessary water to give a volume of at least 50 ml. The end of the extension tube of the condenser must be below the surface of the solution.

• Using a pipette, take, according to the Table, an aliquot part of the clear solution and place in the distillation flask.

• Add sufficient water to obtain a total volume of about 350 ml (see Note), 10 g Arnd’s alloy (4.8), 50 ml magnesium chloride solution (4.10) and a few fragments of pumice stone (4.12). Rapidly connect the flask to the distillation apparatus. Heat gently for about 30 minutes. Then increase the heating to distil the ammonia. Continue the ditillation for about an hour.

• After this time, the residue in the flask ought to have a syrupy consistency. When the distillation has finished, titrate the quantity of excess acid in the receiving flask according to the procedure in Method 2.

  • Note: When the sample solution is acid (addition of 20 ml hydrochloric acid (4.1) to dissolve the sample) the aliquot part taken for analysis is neutralised in the following way:

  • to the distillation flask containing the aliquot part add about 250 ml water, the necessary quantity of one of the indicators (4.11.1, 4.11.2, 4.11.3) and shake carefully.

  • Neutralise with 2 N sodium hydroxide solution (4.8) and acidify again with a drop of hydrochloric acid (4.1). The proceed as indicated in 7.2.

Blank test

7.3 Carry out a blank test under the same conditions (omitting only the sample) and allow for this in the calculation of the final result.

Control test

7.4 Before analysis, check that apparatus is working properly and that the correct technique is applied using a freshly prepared solution of sodium nitrate (4.13) containing 0.050 g to 0.150 g nitrogen depending on the variant chosen.

EXPRESSION OF THE RESULTS

8.  Express the results of analysis as a percentage of nitric nitrogen or combined ammoniacal and nitric nitrogen contained in the fertiliser as received for analysis.