Kjeldahl Method

Kjeldahl method
The Kjeldahl method or Kjeldahl digestion (Danish 2 Applications

pronunciation: [kld l]) in analytical chemistry is a
method for the quantitative determination of organic
nitrogen in chemical substances like ammonia. This The Kjeldahl methods universality, precision and re-
method was developed by Johan Kjeldahl in 1883.[1][2] producibility 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 is also used to assay soils, waste waters,
1 Method fertilizers and other materials. It does not, however, give
a measure of true protein content, as it measures non-
The method consists of heating a substance with sulphuric protein nitrogen in addition to the nitrogen in proteins.
acid, which decomposes the organic substance by oxida- This is evidenced by the 2007 pet food incident and the
tion to liberate the reduced nitrogen as ammonium sul- 2008 Chinese milk powder scandal, when melamine, a
phate. In this step potassium sulphate is added to increase nitrogen-rich chemical, was added to raw materials to
the boiling point of the medium (from 337 C to 373 C) . fake high protein contents. Also, dierent correction fac-
Chemical decomposition of the sample is complete when tors are needed for dierent proteins to account for dif-
the initially very dark-coloured medium has become clear ferent amino acid sequences. Additional disadvantages,
and colourless. The solution is then distilled with a small such as the need to use concentrated sulfuric acid at high
quantity of sodium hydroxide, which converts the ammo- temperature and the relatively long testing time (an hour
nium salt to ammonia. The amount of ammonia present, or more), compare unfavorably with the Dumas method
and thus the amount of nitrogen present in the sample, is for measuring crude protein content.[4]
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 or-
ange pH indicator. 2.1 Total Kjeldahl nitrogen
Degradation: Sample + H2 SO4 Total Kjeldahl nitrogen or TKN is the sum of nitrogen in
(NH4 )2 SO4 (aq) + CO2 (g) + SO2 (g) + bound in organic substances, nitrogen in ammonia (NH3 -
H2 O(g) N) and in ammonium (NH4 + -N) in the chemical analysis
of soil, water, or waste water (e.g. sewage treatment plant
Liberation of ammonia: (NH4 )2 SO4 (aq) + euent).
2NaOH Na2 SO4 (aq) + 2H2 O(l) + 2NH3 (g) Today, TKN is a required parameter for regulatory re-
porting at many treatment plants, and as a means of mon-
Capture of ammonia: B(OH)3 + H2 O + NH3 itoring plant operations.
NH4 + + B(OH)4
Back-titration: B(OH)3 + H2 O + Na2 CO3

NaHCO3 (aq) + NaB(OH)4 (aq) + CO2 (g) +
H2 O 2.2 Conversion factors
In practice, this analysis is largely automated; specic TKN is often used as a surrogate for protein in food sam-
catalysts accelerate the decomposition. Originally, the ples. The conversion from TKN to protein depends on the
catalyst of choice was mercuric oxide. However, while type of protein present in the sample and what fraction of
it was very eective, health concerns resulted in it being the protein is composed of nitrogenous amino acids, like
replaced by cupric sulfate. Cupric sulfate was not as e- arginine and histidine. However, the range of conversion
cient as mercuric oxide, and yielded lower protein results. factors is relatively narrow. Example conversion factors,
It was soon supplemented with titanium dioxide, which known as N factors, for foods range from 6.38 for dairy
is currently the approved catalyst in all of the methods of and 6.25 for meat, eggs, maize (corn) and sorghum to
analysis for protein in the Ocial Methods and Recom- 5.83 for most grains; 5.95 for rice, 5.70 for wheat our,
mended Practices of AOAC International.[3] and 5.46 for peanuts.[5]
1
2 5 EXTERNAL LINKS
2.3 Sensitivity Wastewater Engineering: Treatment and Reuse,

Metcalf & Eddy
The Kjeldahl method is poorly sensitive in the origi-
nal version. Other detection methods have been used
to quantify NH4 + after mineralisation and distillation, 5 External links
achieving improved sensitivity: in-line generator of hy-
dride coupled to a plasma atomic emission spectrome-
Solutions for automation of the Kjeldahl method
ter (ICP-AES-HG, 1025 mg/L),[6] potentiometric titra-
tion (>0.1 mg of nitrogen), zone capillary electrophoresis Solutions for automation of the Kjeldahl method
(1.5 g/ml of nitrogen),[7] and ion chromatography (0.5
g/ml).[8]
2.4 Limitations
Kjeldahl method is not applicable to compounds con-
taining nitrogen in nitro and azo groups and nitrogen
present in rings (e.g. [[pyridine],quinoline,isoquinoline])
as nitrogen of these compounds does not convert to
ammonium sulfate under the conditions of this method.
3 See also
Dumas method, another nitrogen analysis method
Devardas alloy, a powerful reducing agent for ni-
trate analysis
Bicinchoninic acid assay, a colorimetric assay for
protein-nitrogen
Combustion analysis another CHN analysis method
4 References
[1] Kjeldahl, J. (1883) Neue Methode zur Bestimmung
des Stickstos in organischen Krpern (New method
for the determination of nitrogen in organic substances),
Zeitschrift fr analytische Chemie, 22 (1) : 366-383.
[2] Julius B. Cohen Practical Organic Chemistry 1910 Link to

online text
[3] AOAC International
[4] Dr. D. Julian McClements. Analysis of Proteins.

University of Massachusetts Amherst. Retrieved 2007-
04-27.
[5] http://www.fao.org/docrep/006/y5022e/y5022e03.htm
[6] A.M.Y. Jaber, N.A. Mehanna, S.M. Sultan. Determina-

tion of ammonium and organic bound nitrogen by induc-
tively coupled plasma emission spectroscopy. Talanta, 78
(4-5) 1298-1302, 2009.
[7] http://blog.pharmaphysic.fr/ecz-dosage-azote-kjeldahl/
#more-592
[8] http://blog.pharmaphysic.fr/
eviter-distillation-methode-kjeldahl/#more-83
3
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Kjeldahl method

The Kjeldahl method or Kjeldahl digestion (Danish 2 Applications

Back-titration: B(OH)3 + H2 O + Na2 CO3

2.3 Sensitivity Wastewater Engineering: Treatment and Reuse,

[2] Julius B. Cohen Practical Organic Chemistry 1910 Link to

[3] AOAC International

[4] Dr. D. Julian McClements. Analysis of Proteins.

[6] A.M.Y. Jaber, N.A. Mehanna, S.M. Sultan. Determina-

6 Text and image sources, contributors, and licenses

6.3 Content license

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