Kjeldahl methodFrom Wikipedia, the free encyclopediaTheKjeldahl methodorKjeldahl digestion(Danish pronunciation:[kldl]) inanalytical chemistryis a method for the quantitative determination ofnitrogeninchemical substancesdeveloped byJohan Kjeldahlin 1883.[1][2]Contents[hide] 1Method 2Applications 2.1Total Kjeldahl nitrogen 2.2Conversion factors 2.3Sensitivity 2.4Limitations 3See also 4References 5External linksMethod[edit]The method consists of heating a substance withsulfuric acid, which decomposes the organic substance by oxidation to liberate the reduced nitrogen asammonium sulfate. In this steppotassium sulfateis added to increase theboiling pointof the medium (from 337C to 373C) .Chemical decompositionof 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 toammonia. The amount of ammonia present, and thus the amount of nitrogen present in the sample, is determined byback titration. The end of thecondenseris dipped into a solution ofboric acid. The ammonia reacts with the acid and the remainder of the acid is then titrated with asodium carbonatesolution by way of amethyl orangepH indicator.Degradation: Sample + 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)4Back-titration: B(OH)3+ H2O + Na2CO3 NaHCO3(aq) + NaB(OH)4(aq) + CO2(g) + H2OIn practice, this analysis is largely automated; specificcatalystsaccelerate the decomposition. Originally, the catalyst of choice was mercuric oxide. However, while it was very effective, health concerns resulted in it being replaced by cupric sulfate. Cupric sulfate was not as efficient as mercuric oxide, and yielded lower protein results. It was soon supplemented with titanium dioxide, which is currently the approved catalyst in all of the methods of analysis for protein in the Official Methods and Recommended Practices of AOAC International.[3]

Applications[edit]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 is also used to assay soils, waste waters, fertilizers and other materials. 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 the2007 pet food incidentand the2008 Chinese milk powder scandal, whenmelamine, 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 theDumas methodfor measuring crude protein content.[4]Total Kjeldahl nitrogen[edit]Total Kjeldahl nitrogen or TKN is the sum of organicnitrogen,ammonia(NH3), andammonium(NH4+) in the chemical analysis of soil, water, or wastewater (e.g. sewage treatment plant effluent).Today, TKN is a required parameter for regulatory reporting at many treatment plants, and as a means of monitoring plant operations.Conversion factors[edit]TKN is often used as a surrogate forproteinin 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 nitrogenousamino acids, likearginineandhistidine. 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.[5]Sensitivity[edit]The Kjeldahl method is poorly sensitive in the original version. Other detection methods have been used to quantify NH4+after mineralisation and distillation, achieving improved sensitivity: in-line generator of hydride coupled to aplasma atomic emission spectrometer(ICP-AES-HG, 1025mg/L),[6]potentiometric titration (>0.1mg of nitrogen), zone capillary electrophoresis (1.5g/ml of nitrogen),[7]andion chromatography(0.5g/ml).[8]Limitations[edit]Kjeldahl method is not applicable to compounds containing nitrogen innitroandazo groupsandnitrogenpresent in rings (e.g.pyridine) as nitrogen of these compounds does not convert toammonium sulfateunder the conditions of this method.