Kjeldahl method

Method

The method consists of heating a substance with 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 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 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.^[2]

See also

• Total Kjeldahl Nitrogen

References

1. ^ Julius B. Cohen Practical Organic Chemistry 1910 Link to online text
2. ^ Dr. D. Julian McClements. Analysis of Proteins. University of Massachusetts. Retrieved on 2007-04-27.