Kjeldahl Method for Nitrogen Determination

The Kjeldahl method for nitrogen determination is a good example of a back titration. The sample (for example, a food product) is oxidized by concentrated sulfuric acid to remove carbonaceous matter. Excess sodium hydroxide solution is then added, and the ammonia released is carefully distilled off into a known volume of standard acid, such as 0.1 M boric acid. The excess acid is then titrated with standard alkali. 

The classical method for nitrogen determination is the Kjeldahl procedure. The method is very precise and well... 

The Kjeldahl method for the determination of nitrogen is the one generally used in the analysis of food-products and other complex mixtures. The substance to be analyzed is heated with concentrated sulphuric acid until it is completely oxidized. The nitrogen present is converted into ammonia, which unites with the excess of sulphuric acid to form ammonium sulphate. The amount of ammonia formed is determined by treating the mixture of the acid and ammonium salt with an excess of sodium... 

Kjeldahl method An analytical method for the determination of nitrogen particularly in organic materials. The N is converted to NH with cone. H2SO4 and catalysts. After neutralization the NH j is distilled ofT and estimated by titration after absorption. 

Ammonia may be estimated by dissolving the gas in a known volume of standard acid and then back-titrating the excess acid. In a method widely used for the determination of basic nitrogen in organic substances (the Kjeldahl method), the nitrogenous material is converted into ammonium sulphate by heating with concentrated sulphuric acid. The ammonia is then driven off by the action of alkali and absorbed in standard acid. 

The original method for the determination of nitrogen by sulphuric acid-digestion was published by Kjeldahl in 1883 and fully described by Burns (1984). Many modifications have since been made with various catalysts and acid mixtures. 

Protein solubilities of soy flour and extrudates in the following solvent systems were determined by the micro-Kjeldahl method ( 9). A portion (0.1 g) of finely-ground sample (No. 60 sieve) was extracted with 9.9 ml of solvent for 1 hr at room temperature followed by centrifugation and filtration. An aliquot of the supernatant was used for nitrogen determination. 

Johan Kjeldahl first described his method for the determination of nitrogen in proteinaceous materials in 1883, when he was director of the Chemical Laboratory at the Carlsberg Foundation. At that time there was a pressing need for a more convenient and reliable procedure. Wanklyn s albuminoid ammonia method gave low and erratic results. Kjeldahl developed a succession of modifications that resulted in a technique very different from Wanklyn s - one which is still substantially the same today.65 66... 

Kjeldahl method for determination of nitrogen in organic compounds The... 

Kjeldahl method A titration method for the determination of nitrogen in organic compounds in which the nitrogen is converted to ammonia, which is then distilled and determined by a neutralization titration. 

The commonly used methods for the determination of total nitrogen involve variations of the Dumas or Kjeldahl techniques. The Dumas approach involves oxidation of the nitrogen to elemental or oxide form by combustion of the sample, reduction of nitrogen oxides, and determination of the elemental nitrogen. The Dumas oxidation is usually represented by the following example for acetanilide ... 

In the early studies of ammonium fixation there was much uncertainty as to the amount of fixed ammonium present in untreated soils. The capacity of the soil for fixing ammonium could be determined readily by the methods already mentioned, but such techniques furnished no information on the amount of fixed ammonium present initially. Since surface soils usually fixed less ammonium than did subsoils, it seemed likely that this was because the surface soils already contained fixed ammonium, and also potassium. Determination of naturally-occurring fixed ammonium was difficult because it was not known if the Kjeldahl method for total nitrogen actually accounted for all of the fixed... 

Proteins in foods can be measured by NIR reflectance spectrometry with no sample preparation. This has replaced the standard Kjeldahl protein nitrogen determination, which required extensive sample preparation to convert protein nitrogen to ammonia, distillation of the released ammonia, and subsequent titration of the ammonia. The replacement of the Kjeldahl method for routine analysis by NIR has permitted online measurement of protein in food and beverage products. The Kjeldahl method is required for assaying the materials used to calibrate the NIR and for method validation. 

Excess salt was added to aqueous formamlde, the mixture was shaken vigorously for at least 24 hours and then filtered through a fritted glass filter. The nitrogen content was determined by the Kjeldahl method, potassium was determined by Che Perrin method and phosphorus was determined gravimetrically by the quinoline molybdate method. 

The quantitative elemental analysis of organic compounds, perfected by Liebig for carbon and hydrogen in 1831 and by Dumas for nitrogen in 1834, has been described in Chapter 8. An alternative method for the determination of nitrogen was devised in 1883 by Johan Kjeldahl (1849-1900) in which the element is converted to ammonia, which is then determined by titration. The standard... 

As ammonium ions give a reduction wave in tetra-alkyl-ammonium hydroxide solutions, a method for the determination of nitrogen was devised. The method consists of mineralization by the conventional Kjeldahl method used in the semimicro or micro modification. Because waves at very negative potentials (e.g. of ammonium ions) are usually less reproducible than waves at more positive potentials, the accuracy of this method is somewhat lower than that for ordinary polarographic analysis. Other types of polarographic determinations of ammonium ions could be applied, e.g. using the anodic waves corresponding to the formation of salts with mercury or the condensation with phthalic dialdehyde (p. 142). 

Elkei O. 1976. An automated method for the determination of low-level Kjeldahl nitrogen in water and wastewater. Anal. Chim. Acta 86 63-68. 

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