Kjeldahl method

The Kjeldahl nitrogen determination, sometimes referred to in the clinical field as Kjeldahlometrie, is a 1883 developed quantitative, commonly used method of determination of nitrogen content. The Danish chemist Johan Kjeldahl they developed at the Research Centre of the Danish brewery Carlsberg. They quickly replaced the previously widespread nitrogen determination according to Will- Varrentrapp. Nitrogen can be determined by the method of Kjeldahl in many nitrogenous substances, which manifests itself in their wide range of applications:

• Food industry
• Environmental analysis
• Pharmaceutical and chemical industry
• Agricultural Analysis (manure, soil samples )

In the classic approach to include an accurately weighed amount of sample (0.5 to 3 g, according to their nitrogen content ) with sulfuric acid in a Kjeldahl flask of. The organic components of the sample are removed and converted the nitrogen in ammonium sulfate. The addition of a strong base liberates ammonia from the digestion solution, which is collected in acid and determined by titration. This is specified as Total Kjeldahl Nitrogen.

There are also methods for micro- samples of 5 to 15 mg.

In addition, the entire determination using fully automatic laboratory equipment - even with multiple samples simultaneously - automate, so that the analysis can be carried out without Present staff overnight.

Information

The digestion the sample is boiled with an excess of sulfuric acid in an open flask. Here, the carbon in the organic material to carbon dioxide (CO2) is oxidized and reduced sulfuric acid to sulfur dioxide ( SO2):

For improved conversion of the reaction is a catalyst consisting of Hg, Se, Cu - or Ti - compounds. The decomposition temperature in an open flask is limited by the boiling point of the sulfuric acid. In order to achieve a higher temperature, a semi-volatile compound may be added to effect a boiling point elevation, in the Kjeldahl digestion sodium or potassium sulphate (Na2SO4 or K2SO4 ) can be used.

Not all substances are digested directly. If the nitrogen in nitro, nitroso, azo, or before, so first, a reduction must be performed ( for example, with zinc alloy or Devardscher ).

The sulfuric acid digestion can be used in addition to the determination of nitrogen and for the determination of phosphorus, arsenic, and of metals in an organic material.

Steam distillation

After digestion, the nitrogen is present as ammonium sulfate ( NH4) 2SO4 before dissolved in sulfuric acid. With the addition of a strong base (eg NaOH), the sulfuric acid is neutralized and expelled ammonia, which can be initiated by steam distillation quantitatively in an acid template.

To catch can in principle be used, any acid. If a strong acid is used, it must be accurately measured since the remaining after the introduction of ammonia acid is titrated back with a base. If, however, a weak acid used, such as boric acid with a pKa of 9.25, the resulting while fielding strong base may be titrated with an acid without collecting excess acid is being included. This has the advantage that the weak acid must not be accurately measured. Boric acid does not react directly as a proton donor, but as OH - acceptor in the sense of a Lewis acid:

Titration

For the direct titration of an indicator mixture of methyl borate and methylene blue ( Tashiro ) is used which changes color in an acidic. The spent Maßlösungsvolumen can be converted into the amount of nitrogen of the sample.

Protein content

The nitrogen content obtained by this method is related to the protein content of a biological sample. For most foods one can assume that the proteins form the major component of the Kjeldahl nitrogen. When converting nitrogen content to protein content must be taken into account that the individual amino acids have a different nitrogen content and thus a far different from the average composition of the proteins leads to a different conversion factor. This factor must therefore be previously determined for different protein sources using another method, such as the cleavage of the proteins, followed by determination of the individual amino acids. The nitrogen contents as found in the protein amount to an average of 16 %. The analytically determined nitrogen content of a sample to be multiplied by a factor of 6.25 in order to calculate the protein content. In wheat flour, semolina and haze of the factor is 5.7.

Further deviations may occur when other nitrogen sources are present in addition to the proteins. For example, the melamine scandal in China in 2008 shows the limitations of this method: stretching of dairy products in order to simulate a higher protein content in commonly used measuring methods.