Kjeldahl's Determination of Nitrogen

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Introduction

Nitrogen content of a sample may be required for effluent treatment purposes, to determine the protein content of food, or to find the ammonium content of a fertilizer, The determination of nitrogen content of a sample using the kjeldahl procedure involves the destruction of the sample matrix and the conversion of nitrogenous matter to ammonium salts. This digestion is carried out with concentrated sulphuric acid at temperatures above its boiling point. (CARE! Sulphuric acid, particular when hot, is very corrosive and must be handled with care)

The ammonium salt is then converted to ammonia by reaction with sodium hydroxide, the ammonia is steam-distilled off and trapped in a boric acid solution, and its value expressed in the desired form,including NH₃-N, NO₃-N, crude protein etc, using the appropriate calculation.

Procedure

(a) Sample digestion

1. Place into the kjeldahl tubes provided, a weighed quantity of sample. The mass to be used will depend on the nitrogen content of the sample. Each sample will be analyzed in triplicate

2. Into each tube, place one catalyst tablet (note type and composition), followed by the required volume of sulphuric acid. Rack and leave in fume hood to predigest overnight

3. After predigestion, place the tubes into the heating block of the kjeldahl digester apparatus, put the vapor trap into place, and turn on the water vacuum pump to remove acid fumes from the sample tubes.

4. Switch on the heater set at 400°C, and allow the samples to digest completely, such that all the sample is dissolved and the extract is clear blue/green in color. Note the time taken for this process.

Automated Ammonia determination

1. At the end of the digestion period,lift the tubes clear off the heating block and allow to cool to near ambient temperature

Place one tube at a time into the sample port of the steam distillation unit and ensure that it is firmly held in place.

2. Follow the instructions provided for the operation of the distillation and titration unit,and note all the operating parameters.

3. Start the distillation and titration process and record the nitrogen value obtained.

4. Ensure that sample blanks, certified reference material, and samples are analzsed. Correct all sample values, using the blank value.

5. Calculate the mean % recovery ± s.d. of nitrogen from Certified Reference Material.

6. Express the nitrogen content of the sample as weight % Crude Protein.

What sources of error exist in this method of nitrogen analysis?
Suggest means of minimizing such errors.



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