Kjeldahl procedure involves digestion and distillation. When soil is heated with sulfuric acid and catalyst mixture, organic N is digested to Ammonium-N. Then the concentration of ammonia is measured by volumetric titration in the presence of an indicator after steam distillation under the alkaline condition.

Apparatus and reagents

·         Kjeldhal distillation assembly

·         Titration Apparatus

·         Distilled water

·         Beaker

·         Conical flask

·         Burette and pipette

·         Graduated cylinders

·         Catalyst mixture

·         Sulphuric acid (concentrated)

·         Sodium hydroxide solution (40%)

·         Zink granules.

·         Boric acid cum indicator solution- 

·         Hydrochloric acid (0.1N)

Procedure

·         Weigh 10 gm of air-dry soil/sediment in a 300 ml round bottom flask

·         Add 20 gm of catalyst mixture and 35 ml of sulphuric acid.

·         Heat the contents from bottom of the flask for about 2 hours.

·         Cool the contents (digest) and add about 100 ml of distilled water,

·         Wait for about 5 minutes, and deliver the supernatant into 1liter distillation flask A of Kjeldhal distillation assembly.

·         Wash the residue with a little of distilled water several times and transfer the supernatant each time to the same distillation flask.

·         Add 100 ml of sodium hydroxide solution and a few granules of zinc.

·         Take 25 ml of boric acid cum indicator solution in a 500 ml Erlenmeyer flask B and place it below distillation assembly so that the lower open ends of the condenser are dipped in solution.

·         Heat the distillation flask on a hot plate and collect about 150 ml of distillate in flask B.

·         Remove the flask with distillate and titrate the distillate (which has turned blue due to dissolution of ammonia) against 0.1N hydrochloric acid. Turning of blue colour to light brown-pink indicates the end point.

·         Run distilled water blank in the same manner.  

·         Calculation

                % N=[(a-b)/s]*M*1.4*mcf

Where:

      a: ml of HCl required for titration sample

      b: ml of HCl required for titration blank

      s: air-dry sample weight in gram

     M: Morality of HCl

     1.4: 14x10^-3x100% (14 atomic weight of Nitrogen)

     mcf: moisture correction factor 

References

1. George E., Rolf S., John R.; Methods of Soil, plant and water analysis;3^rd ed; p65

2.Sadhan C. Anand Dev G.; Handbook of Water, air and soil analysis; international E publication;2014

3.Haluschak P. Laboratory Methods of Soil Analysis Canada-Manitoba Soil Survey, April 2006