Method 61a Determination of total nitrogen in presence of nitrate and organic N with final determination by distillation


• Salicylic acid, 2-HOC6H4COOH

• Selenium powder

• Sulphuric acid, approximately 98% m/m H2SO4


• Micro-Kjeldahl digestion unit

• Kjeldahl digestion flasks, 50 ml

• Distillation unit - the Markham semi-micro distillation unit is suitable, or a proprietary automatic unit.

Procedure. Take a representative of the bulk fertilizer sample and reduce in size by cone and quartering, or use a sample divider. Then grind and sieve to 0.2 mm (No. 70 or 70 mesh), mix thoroughly and immediately transfer to an airtight container. Weigh 0.250 g (in duplicate) into a 50-ml micro-Kjeldahl flask, add 25 ml sulphuric acid (approximately 98% m/m H2SO4), 0.5 g salicylic acid (free of lumps), swirl until dissolved and allow the amber coloured solution to stand for 30 min. Add 0.5 g zinc dust, swirl to mix, and allow to stand until evolution of gas subsides. Add 0.1 g selenium powder and 2 or 3 granules of carborundum (pumice-stone is not suitable because it floats in sulphuric acid) to avoid bumping occurring on boiling. Boil gently on a micro-

Kjeldahl heating rack until clear or a pale straw colour, then for a further 30 min. Allow to cool, and then add slowly with stirring to about 100 ml water in a 250-ml beaker. Dissolve the needle-like crystals remaining in the flask and rinse well with water, add the washings to the beaker and allow to cool. Transfer to a 250-ml volumetric flask with beaker washings, and make up to the mark with water and mix. Carry out a blank digestion omitting the sample (if the reduction step is omitted because of absence of nitrate, the sample should be replaced with 0.25 g sucrose to ensure partial reduction of any nitrate in the reagents). The total N, now present in the sample solution as ammonium sulphate, can be determined on a 10-ml aliquot by the distillation procedure given in Method 5.6b., Determination of organic plus ammonium-N by digestion and distillation.

Calculation. Subtract the blank titre from the sample titre, multiply the difference by 7 and divide by the sample weight in grams. This gives the g kg-1 of nitrogen in the fertilizer sample. Check duplicate values are sufficiently concordant, and take the mean value. If not, repeat the distillations; if still not concordant, repeat the digestion, ensuring that the original sample has been adequately ground and mixed before weighing.

Example, ammonium nitrate - sample weight, 0.25 g (which contains 43.8 mg NO3-N); sample titre, 12.69 ml; blank titre, 0.20 ml of 0.01 M H2SO4. The calculation as given above is: (12.70 - 0.20) X 7/0.25 = 12.50 X 28 = 350.00 g kg-1 N in ammonium nitrate. This may be checked as follows: molar mass of NH4NO3 = 80.04 g; relative formula mass of 2 X N is 28.02 g. Thus the total weight of nitrogen in 1 kg ammonium nitrate is: 103 X (28.02/80.04) = 350.07 g kg-1.

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