Kjeldahl procedure

In a back titration, an excess amount of standardized reagent is reacted with an unknown amount of a component of interest. When the reaction is complete, the remaining unused reagent is titrated and the amount of component of interest is determined by difference. In the Kjeldahl procedures described next, ammonia is distilled into an add of known concentration. When all of the ammonia has been distilled, the remaining unreacted add is titrated. The... 

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. 

A dry food sample weighing 312.4 mg was digested by the Kjeldahl procedure (Section 7-2) to convert all nitrogen into NH4. The digestion solution was diluted to 1.00 L, and 20.0 mL were transferred to a 100-mL volumetric flask. The 20.0-mL aliquot was... 

Since late in the 19th century, the classical Kjeldahl method has been recognized and accepted universally as the authoritative method of analysis for determining the protein content in a wide variety of ingredients and finished products. As a result of technical innovations there are currently available and in use semiautomated or fully automated protein analysis systems that are based on the classical Kjeldahl procedure. 

The total nitrogen of orange juices was found to increase with the maturity of the fruit and ranged between. 068 to. 120 g per 100 ml (30). The actual protein values obtained by Clements (31) were about 20 percent of the acetone powder. Nearly 30 percent of the alcohol-insoluble solids of juice and about 20 percent of that of vesicular pulp were found to be protein as determined by the Kjeldahl procedure (32). These values are the actual protein that was precipitated by alcohol and are only a fraction of the total protein values usually reported for orange juice (8). The main source of proteins in citrus juice is probably in the form of enzymes and the plastids. At least 47 different enzymes have been reported to occur in citrus fruits (33). Citrus fruits also contain several phenolic amines (34), some of which such as syn-epherine, may have physiological importance (35). 

Nitrogen determination by perchloric acid digestion of coal without catalysts is reportedly safe and much more rapid than the conventional Kjeldahl procedure. Nitrogen is determined in the digest by the usual ammonia evolution and titration. However, extreme care should be taken whenever perchloric acid is used. Nuclear reactions have been applied successfully to the determination of nitrogen in coal. 

When a calibration hierarchy starts with the definition of an SI unit followed by a primary reference measurement procedure and descends through commutable calibrators and specific and selective measurement procedures full calibration hierarchy above the traceability to an SI unit is automatically ensured. The other four types of hierarchy do not preclude the use of a bona fide SI unit in the expression of a measurement result for a differential or rational quantity, but it is mandatory to specify the top measurement procedure and/or calibrator in the designation for the measurand. For example, amount-of-substance concentration of nitrogen(N) in human plasma by Kjeldahl procedure no. 3 (referring to the laboratory s list of procedures). The result in millimole per litre, however, is not unequivocally comparable with that of another Kjeldahl procedure, because the kinds-of-quanti-ty are differently specified, but the unit is unchanged. 

The data in the table demonstrate that about half of the water-soluble ammonium ion content is converted into some new form of nitrogen, the nature of which has yet to be determined. Concurrent total nitrogen analyses by the use of a modified Kjeldahl procedure (0) capable of cleaving heterocyclic structures confirmed that no detectable amount of nitrogen had been lost from the phenolysis reaction system. 

Urine and fecal samples were collected for 7 consecutive days every 4 weeks. Fecal samples were analyzed for nitrogen, fat, and zinc content. Fecal samples were weighed, lyophilized, digested with nitric acid, diluted to volume with deionized water, and analyzed by the atomic absorption spectrophotometer, model 303 or 306 (Perkin Elmer, Norwalk, Connecticut). Food samples were weighed, wet digested with nitric acid, and diluted to volume with deionized water, then analyzed for zinc level (6). Nitrogen levels of dried samples (food or feces) were determined by Kjeldahl procedure (.]). The fat content of dried samples (food or feces) was ascertained by ether extraction of the lipids ( ). ... 

The usual Kjeldahl procedure works for a large variety of nitrogen compounds such as amines, amino acids, and alkaloids, but fails for nitrates, nitrites, azo compounds, cyanides, and derivatives of hydrazine. For nitrates a method involving the addition of salicylic acid may be used. Azo compounds give quantitative results if first reduced by refluxing with tin(II) chloride and hydrochloric acid. Hydrazine derivatives are decomposed by heating with formaldehyde, zinc dust, and hydrochloric acid, followed by tin(II) chloride, prior to the usual Kjeldahl digestion. ... 

A sample of wheat weighing 1.2461 g was analyzed for protein nitrogen by the Kjeldahl procedure, with excess H3BO3 in the distillate receiving flask. Calculate the percentage of nitrogen if 13.19 ml of 0.0962 M HCl was required to the bromocresol green end point. Answer 1.426%. 

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

The determination of total serum protein is an important clinical measurement used in diagnosing liver malfunctions. Although the Kjeldahl method is capable of high precision and accuracy, it is too slow and cumbersome to be used routinely in determining total serum protein. The Kjeldahl procedure, however, has historically been the reference method against which other methods are compared. Methods commonly used include the Biuret method and the Lowry... 

A 0.9092-g sample of a wheat flour was analyzed by the Kjeldahl procedure. The ammonia formed was distilled into 50.00 mL of 0.05063 M HCl a 7.46-mL back-titration with 0.04917 M NaOH was required. Calculate the percentage of protein in the flour. 

Numerous variations to the classical Kjeldahl procedure, such as various digestion catalysts and various methods of determinations of ammonia, have been proposed and are summarized in the reviews of Ma et al. (1982). To determine nitrogen in compounds containing N—N, N=N, NO, and NO2, modifications such as zinc iron reduction must be made to the usual procedure (Styermark, 1961). Certain forms of nitrogen such as triazoles cannot be determined by the Kjeldahl method. 

A Kjeldahl procedure which reduces the azide ion to ammonia followed by colorimetric determination of the ammonia has been described [25],... 

Total nitrogen of the AIS was determined by a semi-micro Kjeldahl procedure on a 0.50 g sample and the protein content calculated using the factor 6.25. The ash content was determined on a 0.5 g sample of the AIS ignited In a muffle furnace at 550 C for 16 hours. 

In the classical Kjeldahl method, the proteins are digested (wet oxidized) in sulfuric acid with a catalyst (mercury and selenium tablets now succeed by the much safer potassium and copper sulfate tablets). An acid solution of ammonium sulfate is formed which is then diluted in water. The solution is made alkaline with sodium hydroxide and heated to distil off ammonia into excess standard acid sulfuric acid. The excess acid is back-titrated with standard sodium hydroxide to determine the amount of ammonia. It is more usual now to use boric acid in which to collect the ammonia and titrate with standard hydrochloric acid. The Kjeldahl procedure has been partially automated in systems such as the Kjeltec Analyzer. Total protein can be calculated as nitrogen content x 6.38. 

Titrimetric methods Titrimetric assays have been developed for acesulfame-K (titrated with sodium methoxide in benzene), aspartame, sodium cyclamate, and sodium saccharin (titrated with perchloric acid), and for saccharin (acid form) with potassium hydroxide as titrant. Precipitation, chelatometric, and redox titrations are proposed for the determination of cyclamate. The oldest methods for saccharin involve its determination by means of a Kjeldahl procedure. 

For 15 N-measurements about 2 mg of pure uric acid were degraded by a modified Kjeldahl procedure and molecular nitrogen liberated for analysis of its isotope content by Li 0 Br (Ross and Martin, 1970) For 13 C-analysis uric acid was combusted according to Winkler and Schmidt (1980). Uric acid concentrations were measured enzymatically (Zailner, 1963). 

All compounds or fractions which were not already in the form of ammonia were digested by the micro-Kjeldahl procedure and steam-distilled to collect the ammonia. 

Smart M.M., Reid F.A., and Jones J.R. 1981. A comparison of a persulfate digestion and the Kjeldahl procedure for determination of total nitrogen in freshwater samples. Water Res. 15 919-921. 

---