Kjeldahl method

The Kjeldahl method in analytical chemistry is a method for the quantitative determination of nitrogen in chemical substances developed by Johan Kjeldahl.

Method
The method consists of heating a substance with sulfuric acid which decomposes the organic nitrogen present to ammonium sulfate. In this step potassium sulfate is added in order to increase the boiling point of the medium (from 337 to 373 °C). Chemical decomposition of the sample is complete when the medium has become clear and colorless (initially very dark).

The solution is then distilled with sodium hydroxide (added in small quantities) which converts the ammonium salt to ammonia. The amount of ammonia present (hence the amount of nitrogen present in the sample) is determined by back titration. The end of the condenser is dipped into a solution of hydrochloric acid or sulfuric acid of precisely known concentration. The ammonia reacts with the acid and the remainder of the acid is then titrated with a sodium carbonate solution with a methyl orange pH indicator.


 * Protein + H2SO4 → CO2 + (NH4)2SO4 + SO2


 * (NH4)2SO4 + 2NaOH → Na2SO4 + NH4OH


 * 2NH4OH + H2SO4 → (NH4)2SO4 + 2H2O

Nowadays, the Kjeldahl method is largely automated and makes use of specific catalysts (mercury oxide or copper sulfate) to speed up the decomposition.

Applications
The Kjeldahl method's universality, precision and reproducibility have made it the internationally-recognized method for estimating the protein content in foods and it is the standard method against which all other methods are judged. It does not, however, give a measure of true protein content, as it measures non-protein nitrogen in addition to the nitrogen in proteins. Also, different correction factors are needed for different proteins to account for different amino acid sequences. Additional disadvantages, such as the need to use concentrated sulfuric acid at high temperature and the relatively long testing time (an hour or more), compare unfavorably with the Dumas method for measuring crude protein content.