Kjeldahl determination

Commercially produced calcium metal is analyzed for metallic impurities by emission spectroscopy. Carbon content is determined by combustion, whereas nitrogen is measured by Kjeldahl determination. 

This is the basis for a common method for the determination of ammonia in soil.1 Soil is suspended in water and placed in a Kjeldahl flask. The suspension is made basic by the addition of a strong (5-50%) sodium hydroxide solution, and the flask is immediately attached to a steam distillation setup. Steam distillation of the suspension carries the released ammonia to an Erlenmeyer flask, catching the distillate in a standardized acid solution that is subsequently back titrated via acid-base titration. The amount of ammonia in soil can be calculated from the end point of the titration. This procedure is similar to a standard Kjeldahl determination and can be carried out using the same equipment, although no digestion is needed. 

Equipment for automated Kjeldahl determinations of organic nitrogen in water and solid samples is supplied by Tecator Ltd. Their Kjeltec system 1 streamlines the Kjeldahl procedure resulting in higher speed and accuracy compared to classical Kjeldahl measurements. 

With Dumas and Combustion methods, there are good agreements with the Kjeldahl determination however, there are dangers of interference if the product is high in carbohydrates or sodium chloride. 

Accurate Kjeldahl determinations both on total serum and on serum fractions obtained by salting out show, according to Rein (Rl), less spreading than clinical electrophoresis in its present form. This indicates that the accuracy and reproducibility of the electrophoretic results must be increased in order to obtain greater clinical significance. 

Figure 5-3 Absolute error in the micro-Kjeldahl determination of nitrogen. Each dot represents the error associated with a single determination.

The chloride is a source of interferences in TOC determination since it scavenges the free radicals that are the principal agents of oxidation. In addition, the oxidation of chloride to chlorine can produce detector failure. Therefore, some of these methods are not advisable to determine TOC in seawater. The photodecomposition method is not suitable for refractory nonpurgeable organic compounds. Dry combustion has also been used to determine NPOC in seawater [130]. Dry combustion loses even moderately volatile material as it first requires evaporation off all the water. Fry et al. [130] relied on the natural marine sulfate salts to act as an oxidant in a fashion similar to a Kjeldahl determination. However, due to the large sample volume required and the potential for contamination or loss in manipulations, this approach seems unlikely to catch on. 

Difficulties in the Kjeldahl determination of nitrogen (c), center chiefly on the problem of digestion. Variable factors known to influence the completeness (and speed) of conversion of protein nitrogen into ammonia by sulfuric acid digestion include the following sources of error ... 

Our aim has been that wherever the existing official or recognized method of assay is a gravimetric determination, an aqueous titration of any type, or a kjeldahl determination we have tried to develop a nonaqueous titrimetric procedure because of its greater speed and accuracy, and, in some instances, specificity. 

Some workers prefer to distil the ammonia into a 4 per cent solution of boric acid, in which it may then be titrated directly with standard acid using methyl red as indicator. This is the method used in the B,P, for Kjeldahl determinations and has the obvious advantage that only one standard solution is used. However, in our experience this method sometimes leads to unsatisfactory end-points and we prefer to recommend the reliable back-titration procedure. 

Calcium cyclamate, Ci2H2406N2S2Ca,2H20, Mol. Wt. 432-6. This material is officially assayed in the U.S.P, by a Kjeldahl determination of nitrogen but it may be determined through its organic sulphur content,... 

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