Water for Kjeldahl Analysis
Application Overview
The Kjeldahl method is an analytical method for the quantitative determination of nitrogen in chemical substances. It was originally developed by Johan Kjeldahl in 1883 to measure the amount of protein in the grain used to produce beer. It is a standard method for estimating the protein content in foods, feed, beverages, and many other samples. One drawback of this method is that it does not give a measure of true protein content, as it measures non-protein nitrogen in addition to the nitrogen in proteins. The Kjeldahl method is also used to measure nitrogen in organic and inorganic samples in the chemical industry, the pharmaceutical industry and in environmental samples.
The method comprises three main steps:
- Digestion
The sample is heated with concentrated sulfuric acid, which decomposes the organic substances by oxidation to liberate the reduced nitrogen as ammonium sulfate. Potassium sulfate is then added in order to increase the boiling point of the medium.
- Distillation
The solution is then distilled with sodium hydroxide, which converts the ammonium salt to ammonia.
- Titration
The amount of ammonia present (hence the amount of nitrogen present in the sample) is usually determined by titration. A known amount of acid solution is added to the receiving flask. The excess acid is back-titrated using a base. Methyl orange is used as a pH indicator. Boric acid may be used for the titration.
More Information
- Science @ a distance: http://www.brooklyn.cuny.edu/bc/ahp/SDKC/Chem/SD_KjeldahlMethod.html
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