• Salicylic acid, 2-HOC6H4COOH
• Selenium powder
• Sulphuric acid, approximately 98% m/m H2SO4
• Micro-Kjeldahl digestion unit
• Kjeldahl digestion flasks, 50 ml, scratched with a graduation line at 25 ml
• Autoanalysis equipment
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 0.0500 g (in duplicate) into a 50-ml micro-Kjeldahl flask, add 25 ml sulphuric acid (approximately 98% m/m H2SO4), 0.1 g salicylic acid, swirl until dissolved and allow the amber coloured solution to stand for 30 min. Add 0.1 g zinc dust, swirl to mix, and allow to stand until evolution of gas subsides. Add 0.02 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 (approximately 3-4 h), then for a further 30 min. Allow to cool, adjust to the 25 ml scratch line with sulphuric acid, then add slowly with stirring to about 15 ml water in a 100-ml beaker, and allow to cool. Rinse the flask into the beaker with a few millilitres of water, then transfer to a 50-ml volumetric flask with washings and make up to the mark with water and mix. Carry out a blank digestion omitting the sample. The total N, now present in the sample solution as ammonium sulphate, can be determined by autoanalysis; see Chapter 7 for details of standards, reagents and method.
Calculation. The concentration in mg N l-1 read from the standard curve, after subtracting the blank, is equal to the g kg-1 of nitrogen in the sample. Take the mean of duplicates; if they differ by more than the acceptable experimental error, repeat the autoanalysis, and, if still unacceptable, repeat the digestion procedure, ensuring adequate grinding and mixing to obtain a homogeneous sample powder, which is important with such a small sample weight.
Example: ammonium nitrate - sample weight, 0.05 g. The blank and sample peaks are compared with the standard curve in the normal way. The blank peak value (15 mg N l-1) is subtracted from the sample peak value (365 mg N l-1) to give 350 mg N l-1 in the 50 ml of diluted sample digest solution. The weight of N in 0.05 g sample is therefore:
(350/1000) x 50/1000 g, thus the weight of N per kg = (350/1000) x (50/1000) x (1000/0.05) = 350 g N kg-1.
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