Kjeldahl nitrogen analysis is a useful tool for determining the nitrogen content of organic and inorganic substances. This test is the reference method for determining the protein content of food.
In general, the sample is digested using concentrated acids, catalysts and heat to convert many forms of nitrogen into ammonium sulfate. The ammonium is converted to ammonia by an excess of sodium hydroxide, The ammonia is distilled from the solution into a boric acid absorber solution. Ammonia is titrated with sulfuric acid to provide the analytical result. If nitrogen is present in trace quantites, the final result may be analyzed by an ammonium specific electrode.
Samples are weighed into a kjeldahl flask. A volume of sulfuric acid and a portion of catalyst (Sodium Sulfate and Copper Sulfate) are added to the preparation. The flask is then heated to high temperature on a hot plate until no evidence of carbon remains in the flask (transparent light green solution). The preparation is allowed to cool and a volume of concentrated sodium hydroxide solution is added to the flask. The flask is then immediately connected to a distillation assembly to distill the ammonia. Ammonia is distilled into a boric acid absorbing solution.
Distilled solutions are titrated with dilute sulfuric acid solutions. If the titration is small, the sample is automatically analyzed by an ion specific electrode (ISE).
Galbraith Laboratories employs a general method for the digestion and titration of samples for Kjeldahl Nitrogen. A second procedure is written for measurement of ammonium using ISE. The methods are indicated below. There is no method summary available for Nitrogen by ISE.
The following interfere with this test: Excess nitrate nitrogen, very large quantities of salt or inorganic solids. This test does not give quantitative results for nitrogen in N=N linkages and N-O linkages. Likewise, this method does not give results for nitrogen in the form of azide, azine, azo, hydrazine, nitrate, nitrite, nitrile, nitro, nitroso, oxime and semi-carbazone.
The quantitation limit depends on the mass of sample taken for the analysis. The range is % level down to approximately 10 ppm. Results that are below the quantitation limit are reported as a less than value, i.e. <0.5%.
Results are reported in % or ppm (wt/wt) unless another unit is requested. When protein content is requested, a factor of 6.25 is applied to the result, unless another factor is specified.