Determination of nitrogen content according to USP general chapter <461>

Kjeldahl method is used to determine the nitrogen content in organic and inorganic samples. Kjeldahl consists of three steps: digestion, distillation, and titration. During the catalytic digestion step, organic nitrogen1 is converted into ammonium. Sodium hydroxide is added just before the distillation step for converting ammonium into ammonia. Through steam distillation the latter is transferred into the receiver vessel containing an absorbing agent (e.g., boric acid). Finally, the separated ammonia is titrated against sulfuric acid.
Protein content in samples can also be determined from the nitrogen content obtained by Kjeldahl setup. USP<461> describes the titration method to determine nitrogen content in organic products using Kjeldahl nitrogen setup. This Application Note illustrates nitrogen determination in heparin sodium.

 

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1
Kjeldahl is not working for all nitrogen-containing compounds; nitro and azo groups and nitrogen in rings are not included.