Kjeldahl method
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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 [1].
The method as described in Julius Cohen's Practical Organic Chemistry of 1910 consists of heating a substance with sulfuric acid which oxidizes nitrogen 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.
Nowadays, the Kjeldahl method is largely automated and makes use of specific catalysts (mercury oxide or copper sulfate) to speed up the decomposition.
[edit] External links
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[edit] References
- ^ Julius B. Cohen Practical Organic Chemistry 1910 Link to online text
