The Kjeldahl method developed in 1883 by Johan Kjeldahl is a method for the quantitative determination of nitrogen.
The method consists of heating a substance with sulfuric acid, which decomposes the organic substance by oxidation to liberate the reduced nitrogen as ammonium sulfate. In this step potassium sulfate is added to increase the boiling point of the medium (from 337°F to 373°F / 169°C to 189°C). Chemical decomposition of the sample is complete when the initially very dark-colored medium has become clear and colorless.
The solution is then distilled with a small quantity of sodium hydroxide, which converts the ammonium salt to ammonia. The amount of ammonia present, and thus the amount of nitrogen present in the sample, is determined by back titration. The end of the condenser is dipped into a solution of boric acid. The ammonia reacts with the acid and the remainder of the acid is then titrated with a sodium carbonate solution by way of a methyl orange pH indicator.
- Degradation: Sample + H2SO4 → (NH4)2SO4(aq) + CO2(g) + SO2(g) + H2O(g)
- Liberation of ammonia: (NH4)2SO4(aq) + 2NaOH → Na2SO4(aq) + 2H2O(l) + 2NH3(g)
- Capture of ammonia: B(OH)3 + H2O + NH3 → NH4+ + B(OH)4–
- Back-titration: B(OH)3 + H2O + Na2CO3 → NaHCO3(aq) + NaB(OH)4(aq) + CO2(g) + H2O
In practice, this analysis is largely automated; specific catalysts (mercury oxide or copper sulfate) accelerate the decomposition.
The Kjeldahl method is the method of choice for estimating the protein content in foods and is the standard method by which all other methods are judged. It does however measure non protein nitrogen as well as nitrogen derived from proteins. Correction factors are required for different proteins to account for different amino acid sequences.
Example conversion factors known as N factors for foods range from 6.38 for dairy to 6.25 for meat, eggs and maize. 5.83 for most grains, 5.70 for wheat flour and 5.46 for peanuts.
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