Method Kjeldahl

Compounds which contain N can usually be oxidised by heating with cone. H2S04, the C being converted into C02, and the N to (NH4)2S04. NH3 can then be liberated from this solution and estimated. 

0-5—5 gms. of the substance are weighed into a long-necked round-bottomed Kjeldahl flask of 300—500 c.cs. capacity, and 20—30 c.cs. of pure cone. added 

0 2 gm. Se are then added, and the contents of the flask heated over a sand bath or small Bunsen flame until almost boiling. 

The neck of the flask can be partially closed with a loose-fitting, short-stemmed, glass funnel. On further heating the dark-coloured mass grows lighter until finally it is colourless, or only a faint straw colour. 

The contents of the flask are then washed into the distilling flask of an ammonia estimation apparatus (Fig. 77) with successive small amounts of distilled water. The NH3 is then determined in the usual manner. 

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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 two chief methods for estimating nitrogen in organic compounds are (i) the Dumas method, which can be applied to all organic compounds (ii) the Kjeldahl method, which is of more restricted application, but which is frequently used in biochemical and physiological work. Its limitations are indicated in the description of the method (p. 492). 

Devarda s Method. Nitrogen in nitrates or nitric acid also may be deterrnined by the Kjeldahl method or by Devarda s method. The latter is both convenient and accurate when no organic nitrogen is present. The nitrate is reduced by Devarda s alloy to ammonia in an alkaline solution. The ammonia is distilled and titrated with standard acid. 

Commercially produced barium is analyzed for metallic impurities by means of emission spectroscopy. Carbon content can be deterrnined by combustion, and nitrogen by the Kjeldahl method. Total barium can best be deterrnined gravimetricaHy by precipitation as the sulfate. 

C. Adams G. Spaulding, Determination of Organic Nitrogen by Kjeldahl Method Without Distillation , AnalChem 27, 1003—4 (1955)... 

Protein content was determined by a semi-automated micro-Kjeldahl method [4], The conversion factor used was 6.25. 

Kjeldahl method Combustion elemental analysis Volumetry (titrimetry) Chromatography Gravimetry... 

Assays. Nitrogen assays to determine 1-amidoethylene unit content were done by Kjeldahl method. Limiting viscosity numbers were determined from 4 or more viscosity measurements made on a Cannon-Fenske capillary viscometer at 30°C. Data was extrapolated to 0 g/dL polymer concentration using the Huggins equation(44) for nonionic polymers and the Fuoss equation(45) for polyelectrolytes. Equipment. Viscosities were measured using Cannon-Fenske capillary viscometers and a Brookfield LV Microvis, cone and plate viscometer with a CP-40, 0.8° cone. Capillary viscometers received 10 mL of a sample for testing while the cone and plate viscometer received 0.50 mL. 

The micro Kjeldahl method involves reducing all of the organic nitrogen to ammonia, then distilling the ammonia into an absorbing solution and determining it colorimetrically [11,12,22],... 

The quantity of protein present is normally measured by the Kjeldahl method. As wheat endosperm protein is around 17.5% nitrogen a factor of 5.7 is normally used to convert Kjeldahl nitrogen measurements into protein. Tkachuk suggested that 5.7 should be used for whole wheat but 5.6 should be used for flour.24... 

The Kjeldahl method is not a rapid means of analysis but it does have the advantage of being absolute. It is a sobering thought that a batch of bread can be made in less time than it takes to check the protein content of the flour by Kjeldahl. 

Solyom has conducted an intercalibration of methods used for the determination of phosphorus in sludges [37]. The methods used to determine phosphorus were that of Koroleff [83] in which the sample is digested with potassium peroxydisulphate and phosphate determined spectro-photometrically. Alternatively a reducing Kjeldahl digestion was used followed by determination of phosphate using molybdate and ascorbic acid. The former method gives somewhat low results. The reducing Kjeldahl method is therefore recommended. 

A titrimetric method that has been used for many years for the determination of nitrogen or protein in a sample is the Kjeldahl method. Examples of samples include grain, protein supplements for animal feed, fertilizers, and food products. It is a method that often makes use of the back titration concept mentioned above. We will now describe this technique in detail. 

Experiment 12 Determination of Protein in Macaroni by the Kjeldahl Method... 

A flour sample was analyzed for nitrogen content by the Kjeldahl method. If 0.9819 g of the flour was used, and 35.10 mL of 0.1009 NHC1 was used to titrate the boric acid solution in the receiving flask, what is the percent nitrogen in the sample ... 

The Kjeldahl method measures the nitrogen content of a compound and may be used to determine the protein content of a sample provided that the proportion of nitrogen in the protein is known. Protein determinations are complicated by the presence of nitrogen from non-protein sources. The simplest way of eliminating this source of error is to precipitate the proteins using a suitable method and to determine the nitrogen content of the precipitate. 

Although the Kjeldahl method is tedious and cumbersome it can be extremely accurate if the proportion of nitrogen in the protein sample is known and the complete recovery of nitrogen can be assured. This can be checked by recovery experiments particularly for the digestion stage of the process. 

Total nitrogen determinations in barley and malt gave slightly higher results by the Dumas than by the Kjeldahl method. The Dumas method was adopted as the reference method by the Analysis Committee of the Institute of Brewing27. Comparison of the results obtained by the Kjeldahl method with those of automatic N analyzers is mentioned in Section III.A.2 above. 

For n = 15 cereal samples from barley, maize, rye, triticale, and wheat, the nitrogen contents, y, have been determined by the Kjeldahl method values are between 0.92 and 2.15 mass% of dry sample. From the same samples near infrared (NIR) reflectance spectra have been measured in the range 1100 to 2298 nm in 2nm intervals each spectrum consists of 600 data points. NIR spectroscopy can be performed much easier and faster than wet-chemistry analyses therefore, a mathematical model that relates NIR data to the nitrogen content may be useful. Instead of the original absorbance data, the first derivative data have been used to derive a regression equation of the form... 

The determination of nitrogen in petroleum products is performed regularly by the Kjeldahl method (ASTM D3228), the Dumas method, and the microcoulomet-ric (ASTM D3431) method. The chemiluminescence method is the most recent technique applied to nibogen analysis for petroleum and is used to determine the amount of chemically bound nitrogen in liquid samples. 

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