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c30f Analytical Chemistry Labscript

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Determination of total nitrogen content of samples
with the Kjeldahl analyzer

1. 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 excess sodium hydroxide, the ammonia is steam-distilled off and trapped in a boric acid solution. The ammonia is determined by titration with a standardized acid solution, and its value expressed in the desired form, including NH3-N, NO3-N, crude protein etc, using the appropriate calculations.

2. Experimental procedure:

(A) Sample digestion

(a) 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 (see Demonstrator). Each sample will be analysed in triplicate.

(b) Into each tube, place one catalyst tablet (note type and composition), followed by the required volume of sulphuric acid, dispensed directly from the acid container (No pouring of acid allowed). Place in the sample rack for digestion.

(c) Place the tubes in the rack on the heating block of the Kjeldahl digestion apparatus, put the vapor trap into place, and turn on the water vacuum pump to remove acid fumes from the sample tubes.

(d) Switch on the heater set at 400C and allow the samples to digest completely, such that all the sample is dissolved and the extract is clear blue in color, without any dark particles remaining. Note the time taken for the digestion.

(e) Raise the tubes clear of the heating block and allow to cool, with the water vacuum still operating.

(B) Automated ammonia determination:

(a) To test the efficiency of the steam distillation process, weigh the recommended mass of ammonium salt (see Demonstrator) into a clean, dry digestion tube.

(b) Follow the instructions provided for the setup of the distillation and titration unit, and note the operating parameters and procedures used.

(c) Set up for standardization and enter the mass of ammonium salt used and its % N content.

(d) Start the distillation and titration process and record the % N recovery value obtained. If a value between 95-105% is obtained, the distillation efficiency may be considered satisfactory and samples may then be analyzed.

However, if the % recovery is outside of this range, repeat the recovery procedure with a fresh ammonium salt sample, until the recovery is satisfactory.

(e) Switch to sample analysis mode and measure the N content of each sample extract. Tabulate the titre value and %N value of each sample.

(f) Analyze the sample blank and correct the sample results using this value.

(g) Express the nitrogen content of the sample as weight % Crude Protein, using the appropriate conversion factor (quote value and source of your factor)

3. Exercise:

(A) List 3 possible sources of error in this experiment.
(B) Suggest how would you minimize such errors?

Anal-Chem Resources
Chem. Dept. UWI. St. Augustine Campus