Method 56ai Reduction of nitrate before digestion and colorimetric autoanalysis

Reactions involved

1. Sulphuric acid converts nitrate to nitric acid

2. Salicylic acid converts nitric acid to nitrosalicylic acid

3. Zinc dust converts nitrosalicylic acid to aminosalicylic acid

4. Hot sulphuric acid converts aminosalicylic acid to ammonium sulphate, which is the same form to which protein nitrogen is converted.

Reagents.

• Salicylic acid, 2-HOC6H4COOH.

• Sulphuric acid-selenium reagent, approx 98% m/m H2SO4 - Safety note wear PPE because this is highly corrosive. Add 4 g selenium powder to 400 ml sulphuric acid (approx. 98% m/m H2SO4), and heat until just fuming. Stir occasionally with a glass rod until all the selenium has dissolved to give a dark green solution. Carefully remove from the hotplate and allow to cool. Make up to approximately 1 l with sulphuric acid.

Procedure. Weigh 0.5 g air-dry soil (0.1000 g for herbage samples) into the digestion tube. Add 5 ml sulphuric acid containing 0.4% w/w selenium catalyst, followed by 0.10 g salicylic acid and mix by gentle agitation of the tube. After allowing to stand for 30 min, add approximately 0.1 g zinc dust. Mix by gentle agitation, and allow sufficient time for the hydrogen gas to evolve. This is best carried out in a fume cupboard, as the fumes are unpleasant. Follow by heating at 100°C until frothing ceases, and then heat at 310°C for 4.25 h. Analyse according to the procedure for herbage nitrogen in Method 7.7a.

Notes:

1. After diluting the Kjeldahl digest to give a 50% aqueous solution, one observes a residue of silica and a fine white mineral deposit at the bottom of the digestion tube. There may also be some cloudiness in the solution, which should be left to settle for 48-72 hours. Using a disposable pasteur pipette, transfer sufficient of the upper clear solution to almost fill the auto-analyser sample cup.

2. During analysis, a flocculent light brown precipitate may be produced, which can build up in the flowcell, causing noisy peaks and drifting. A dialyser module could prevent this situation; otherwise the flowcell could be 'scrubbed' by forcing an air-bubble through between samples.

3. The Kjeldahl digestion can convert up to 20% of any nitrate-N to ammo-nium-N. This would usually only increase the ammonium-N by an insignificant amount and may therefore be ignored.

4. The volume of residue in the graduated digestion tube has been measured for a number of soils. It appears bulky, but after filtration, washing with 90% ethanol, drying and transferring to a 5-ml measuring cylinder, and pipetting in 2 ml ethanol, the volume was found to vary between 0.20 and 0.30 ml for all types of temperate and organic soils. The residue means the soil organic-N is contained in 9.7-9.8 ml rather than 10 ml, and therefore the readings are reduced by dividing the mg l-1 by 510-515 (or multiplying by 0.00196-0.00194) where the units are % N. We have adopted the average value of 513 or 0.00195 respectively. Where the units are required as mg N g-1 or g N kg-1, these factors become 51.3 and 0.0195 respectively.

Calculation. For 0.5 g air-dry soil sample, digested and made up to 10 ml, the concentration in that solution is read from the standard graph as y mg

N l-1. The concentration of organic-N in the air-dry soil is expressed by:

Express results for oven-dry soil as in Method 5.2, Calculation (2).

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