kjeldahl method

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Kjeldahl method 1

Kjeldahl method

The Kjeldahl method or Kjeldahl digestion (Danish pronunciation: [khldl]) in analytical chemistry is a method for

the quantitative determination of nitrogen in chemical substances developed by Johan Kjeldahl in 1883.[1]

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 to increase the boiling

point of the medium (from 337F to 373F / 169C to 189C). Chemical decomposition of the sample is complete

when the initially very dark-colored medium has become clear and colorless.

The solution is then distilled with a small quantity of sodium hydroxide, which converts the ammonium salt to

ammonia. The amount of ammonia present, and thus 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 by way of a methyl orange pH indicator.

Degradation: Sample + H2SO

4 (NH4)2SO4(aq) + CO2(g) + SO2(g) + H2O(g)

Liberation of ammonia: (NH4)2SO

4(aq) + 2NaOH Na

2SO

4(aq) + 2H

2O(l) + 2NH

3(g)

Capture of ammonia: B(OH)3

+ H2O + NH

3 NH

4

++ B(OH)

4

Back-titration: B(OH)3

+ H2O + Na

2CO

3 NaHCO

3(aq) + NaB(OH)

4(aq) + CO

2(g) + H

2O

In practice, this analysis is largely automated; specific catalysts (mercury oxide or copper sulfate) accelerate 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 nonprotein nitrogen in addition to the

nitrogen in proteins. This is evidenced by the 2007 pet food incident and the 2008 Chinese milk powder scandal,

when melamine, a nitrogen-rich chemical, was added to raw materials 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.[2]
http://en.wikipedia.org/w/index.php?title=Dumas_methodhttp://en.wikipedia.org/w/index.php?title=Melaminehttp://en.wikipedia.org/w/index.php?title=2008_Chinese_milk_scandalhttp://en.wikipedia.org/w/index.php?title=2007_pet_food_recallshttp://en.wikipedia.org/w/index.php?title=File:Kjeldahl%27s_distillation.svghttp://en.wikipedia.org/w/index.php?title=File:Kjeldahl_digestion.svghttp://en.wikipedia.org/w/index.php?title=Copper_sulfatehttp://en.wikipedia.org/w/index.php?title=Mercury_oxidehttp://en.wikipedia.org/w/index.php?title=Catalysthttp://en.wikipedia.org/w/index.php?title=PH_indicatorhttp://en.wikipedia.org/w/index.php?title=Methyl_orangehttp://en.wikipedia.org/w/index.php?title=Sodium_carbonatehttp://en.wikipedia.org/w/index.php?title=Boric_acidhttp://en.wikipedia.org/w/index.php?title=Liebig_condenserhttp://en.wikipedia.org/w/index.php?title=Back_titrationhttp://en.wikipedia.org/w/index.php?title=Ammoniahttp://en.wikipedia.org/w/index.php?title=Chemical_decompositionhttp://en.wikipedia.org/w/index.php?title=Boiling_pointhttp://en.wikipedia.org/w/index.php?title=Boiling_pointhttp://en.wikipedia.org/w/index.php?title=Potassium_sulfatehttp://en.wikipedia.org/w/index.php?title=Ammonium_sulfatehttp://en.wikipedia.org/w/index.php?title=Sulfuric_acidhttp://en.wikipedia.org/w/index.php?title=Johan_Kjeldahlhttp://en.wikipedia.org/w/index.php?title=Chemical_substancehttp://en.wikipedia.org/w/index.php?title=Nitrogenhttp://en.wikipedia.org/w/index.php?title=Analytical_chemistryhttp://en.wikipedia.org/w/index.php?title=Help:IPA_for_Danish


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Kjeldahl method 2

Total Kjeldahl nitrogen

Total Kjeldahl nitrogen or TKN is the sum of organic nitrogen, ammonia (NH3), and ammonium (NH

4

+) in the

chemical analysis of soil, water, or wastewater (e.g. sewage treatment plant effluent). To calculate Total Nitrogen

(TN), the concentrations of nitrate-N and nitrite-N are determined and added to TKN.

Today, TKN is a required parameter for regulatory reporting at many treatment plants, and as a means of monitoring

plant operations.

Conversion factors

TKN is often used as a surrogate for protein in food samples. The conversion from TKN to protein depends on the

type of protein present in the sample and what fraction of the protein is composed of nitrogenous amino acids, like

arginine and histidine. However, the range of conversion factors is relatively narrow. Example conversion factors,

known as N factors, for foods range from 6.38 for dairy and 6.25 for meat, eggs, maize (corn) and sorghum to 5.83

for most grains; 5.70 for wheat flour, and 5.46 for peanuts.[3]

References[1] Julius B. CohenPractical Organic Chemistry 1910 Link to online text (http://www.archive. org/details/PracticalOrganicChemistry)

[2] Dr. D. Julian McClements. "Analysis of Proteins" (http://www-unix.oit.umass.edu/~mcclemen/581Proteins.html). University of

Massachusetts Amherst. . Retrieved 2007-04-27.

[3] http:/ /www.fao. org/docrep/006/y5022e/y5022e03. htm

Wastewater Engineering: Treatment and Reuse, Metcalf & Eddy
http://www.fao.org/docrep/006/y5022e/y5022e03.htmhttp://en.wikipedia.org/w/index.php?title=University_of_Massachusetts_Amhersthttp://en.wikipedia.org/w/index.php?title=University_of_Massachusetts_Amhersthttp://www-unix.oit.umass.edu/~mcclemen/581Proteins.htmlhttp://www.archive.org/details/PracticalOrganicChemistryhttp://en.wikipedia.org/w/index.php?title=Histidinehttp://en.wikipedia.org/w/index.php?title=Argininehttp://en.wikipedia.org/w/index.php?title=Amino_acidshttp://en.wikipedia.org/w/index.php?title=Proteinhttp://en.wikipedia.org/w/index.php?title=Ammoniumhttp://en.wikipedia.org/w/index.php?title=Ammoniahttp://en.wikipedia.org/w/index.php?title=Nitrogen


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Article Sources and Contributors 3

Article Sources and ContributorsKjeldahl method Source: http://en.wikipedia.org/w/index.php?oldid=530163858 Contributors: AbbyKelleyite, Adam Zivner, Angrense, Beckejac, Can't sleep, clown will eat me, Cyt, Eeverion,

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