Kjeldahl method

Kjeldahl method

The Kjeldahl method in analytical chemistry is a method for the quantitative determination of nitrogen in chemical substances developed by Johan Kjeldahl during the 1800s. [Julius B. Cohen "Practical Organic Chemistry" 1910 [http://www.archive.org/details/PracticalOrganicChemistry Link to online text] ]

Method

The method consists of heating a substance with sulfuric acid, which decomposes the organic substance by oxidation to liberate the reduced nitrogen as 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.

: Degradation: Protein + H2SO4 → (NH4)2SO4(aq) + CO2(g) + SO2(g) + H2O(g)

: Liberation of ammonia: (NH4)2SO4(aq) + 2NaOH → Na2SO4(aq) + 2H2O(l) + 2NH3(g)

: Capture of ammonia: B(OH)3 + H2O + NH3 → NH4+ + B(OH)4–

: Back-titration: B(OH)3 + H2O + Na2CO3 → NaHCO3(aq) + NaB(OH)4(aq) + CO2(g) + H2O

Nowadays, the Kjeldahl method is largely automated and makes use of specific catalysts (mercury oxide or 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. This can be mirrored from 2007's pet food incident when melamine, a nitrogen-rich chemical, was added to raw material to fake high protein contents. 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. cite web
url=http://www-unix.oit.umass.edu/~mcclemen/581Proteins.html
author=Dr. D. Julian McClements
title=Analysis of Proteins
publisher=University of Massachusetts
accessdate=2007-04-27]

ee also

*Total Kjeldahl Nitrogen

References

External links

* [http://www.buchi.com/kjeldahl.74.0.html Kjeldahl equipment supplier info I]
* [http://www.cgerhardt.com/applications/blocke.htm Kjeldahl equipment supplier info II]
* [http://www.scpscience.com/products/Digestion/digiprep_ht.asp Kjeldahl equipment supplier info III]
* [http://www.foss.dk/Solutions/ProductsDirect/KjeltecSystems.aspx Kjeldahl equipment supplier info IV]
* [http://www.velp.it/en/prodotti/familyDesc.asp?id_linea=7&id=74 Kjeldahl equipment supplier info V]


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