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The Kjeldahl method in analytical chemistry is a method for the quantitative determination of nitrogen in chemical substances developed by Johan Kjeldahl.
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 boric acid. 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.
2008-07-11 01:18:25 補充：