Estimation of Nitrogen: Micro-Kjeldahl Method | Soil Analysis

In this article we will discuss about the estimation of nitrogen by Micro-Kjeldahl method for analysing soil.

Next to C, H and O, N₂ is the element found to be present in living beings. It occurs in amino acids, purine and pyrimidine bases, vitamins, amino sugars, etc. However, the major nitro­gen source is protein and N₂ constitutes al­most 16% of its total make up.

Principle:

During digestion with H₂SO₄, the nitrogen present in proteins and other organic material is converted to ammonium sulphate. On dis­tillation, ammonium sulphate liberates ammo­nia which is collected in boric acid solution and neutralised with NaOH and titrated against 0.1 (N) HCl.

Requirements:

1. Kjeldahl apparatus – Digestion rack and noitalics flasks.

2. Distillation apparatus – glass distillation assembly.

3. Treated soil solution/plant material.

4. Digestion mixture: 10 g Potassium sul­phate + 1.0 g copper sulphate. Grind to fine powder and mix. 

5. Cone. H₂SO₄

6. Boric acid solution – 4% in warm dis­tilled water.

7. 10 (N) NaOH.

8. 0.1 (N) standard HCl.

9. Methyl red (alcoholic).

Procedure:

1. Add 3.0 g digestion mixture (K₂SO4 + CuSO₄-5H₂O) to 5 ml of treated soil so­lution/plant material into a Kjeldahl flask and keep on flame until it is clear (green­ish-blue) and then continue heating for five more minutes.

2. Cool, dilute to 15 ml with 10(N) NaOH to under neutralise.

3. Chill the flasks in ice water, wipe the outside and attach it to a distillation ap­paratus.

4. Fix a 100 ml conical flask with 10 ml bo­ric acid and 2 drops of methyl red at the receiving end.

5. Add 20-30 ml of distilled water to the boric acid solution in the conical flask so as to make the receiving end of the fun­nel dip into boric acid solution.

6. Add 5-8 ml 10 (N) NaOH from the side tube into the Kjeldahl flask to over neu­tralize the solution.

7. Heat the flask for distillation till 20 ml of solution remains.

8. Draw the receiver conical flask down and stop heating. When distillation stops wash the end of the receiving funnel.

9. Titrate the contents of the conical flask with 0.1 (N) HCl and note the volume of HCl used.

10. Run a blank without soil, reduce this value from titration figure.

Calculation:

The result multiplied by 6.25 will give the value of crude protein.