Determination of Total Nitrogen

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. 

Add additional 0.5 to 1.0 g alundum granules to cooled digestion flask. Add a sufficient volume of concentrated NaOH solution, slowly down the side of the flask. More than 45 ml of 45% NaOH is recommended to neutralize 20 ml of concentrated H2S04 standard, and to render the mixture strongly alkaline. Immediately connect digestion flask to the distillation apparatus, mix completely, and distill at a 7.5-min boil rate until 150 ml distillate is collected in titration beaker. 

Optionally, 2 to 3 drops of tributyl citrate may also be added to reduce foaming. 

Titrate excess standard acid in distillate with 0.1 N NaOH standard solution. Correct for blank determination on reagents. 

Calculate % crude protein, defined as 6.25 x % nitrogen, or 5.7 x % nitrogen for wheat grains. 

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Koroleff F (1969) Determination of total nitrogen in natural waters by means of digestion. International Council for the Exploration of the Sea, Hydrography Committee, CM/C 8, p 4... 

Nelson and Sommers [44] have described a Kjeldahl digestion procedure for the determination of total nitrogen in soils in which the sample is digested with sulphuric acid using a selenium catalyst. The digest is steam distilled with sodium hydroxide and ammonia titrated with 3.5mM sulphuric acid. Various other workers have discussed the application of Kjeldahl digestion to the determination of total nitrogen in soils [45-47]. 

Wong and Kemp [52] have described a rapid automated procedure involving combustion at 2000°C for the determination of total nitrogen in lake sediments. 

Potassium persulphate [53], hydrogen peroxide [51], and peroxydisulphate in alkaline medium [59], have all been used to digest river sediments prior to the determination of total nitrogen. Muhlhauser et al. [51] digested lOmg... 

Smart et al. [53] also used persulphate digestion in the determination of total nitrogen in sediments. 

The Standing Committee of Analysts (UK) have described a method for the determination of total nitrogen and phosphorus in sewage sludges [56]. 

Solyom [37] has carried out intercalibration studies on methods for the determination of total nitrogen in sludges. Nitrogen was determined by oxidation in alkaline solution with potassium peroxydisulphate, followed... 

The automated spectrophotometric procedure [47] for the determination of total nitrogen in soils discussed in section 12.5.1.2 has also been applied to the determination of 0-0.75% phosphorus in soils. The method is based on the formation of the yellow molybdate-vanadate complex which has an absorption maximum at 420nm. 

The determination of total nitrogen by oxidative combustion with chemiluminescence detection is based on the following reactions ... 

The manual procedure for determination of total nitrogen by chemiluminescence involves preparing the standards from a stock solution, diluting the samples, injecting the standards into the combustion furnace for analysis, and calculation of the nitrogen content of the samples. The calculation is based on the signal from the diluted sample compared to the standard curve and the dilution factor for the sample. 

B1.1 The Colorimetric Detection and Quantitation of Total Protein B1.2 Determination of Total Nitrogen... 

D Elia, C. F., Steudler, P. A., and Corwin, N. (1977) Determination of total nitrogen in aqueous samples using persulfate digestion. Limnol. Oceanogr. 22,760-764. 

Suzuki, Y., and Sugimura, Y. (1985) A catalytic oxidation method for the determination of total nitrogen dissolved in seawater. Mar. Chem. 16, 83-97. 

Henriksen, A. 1970. Determination of total nitrogen, phosphorus and iron in fresh water by photooxidation with ultraviolet radiation. Analyst 95 601-608. 

Rossi, G. and M. Savarese. 1997. On-line determination of total nitrogen in natural sea water samples by alkaline persulphate oxidation. Mar. Pollut. Bull. 35 174—175. 

Yang, C., R. Wu, P. Zhu, and X. Yang. 2007. Determination of total nitrogen in water using high temperature oxidation and chemiluminescence method. Fenxi Huaxue 35 529-531. 

Zhang, F. and J. He. 2006. Determination of total nitrogen in water by microwave digestion UV spectrophotometry. Fenxi Ceshi Xuebao 25 112-114. 

Determination of total nitrogen content of the gastric juice, introduced by Wolff and Junghans (W22) and used later by others (D4), is inadequate for quantitation of gastric juice mucosubstances because proteins, peptides, and amino acids contribute to this measurement. Other authors precipitated gastric juice with methyl alcohol or acetone and determined the amount of alkali bound by the precipitate (M4-M6). These methods determined only the buffer capacity of the precipitate,... 

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 ... 

The use of microwaves in the acceleration of digestion and colour development in the determination of total Kjeldahl nitrogen in soil has been examined [1]. Visual method based on the colour development of indothymol blue formed between ammonia and thymol has recently been developed for the determination of ammonia-nitrogen in environmental waters [2]. UV photo-oxidation of reduced forms of nitrogen to nitrate by peroxodisulfate can make a basis for the determination of total nitrogen in urban and industrial waste waters [3]. 

Jl. Jacobs, S., The determination of total nitrogen in small quantities of serum by the indanetrione hydrate. Protides Biol. Fluids, Proc. CoUoq. 10, 332-334 (1963). 

The determination of total nitrogen is not as simple as it is often thought to be. It is subject to many difficulties, any of which may lead to low results. For example, an... 

J.W.B. Stewart, J. Ruzicka, H. Bergamin-Filho, E.A.G. Zagatto, Flow injection analysis. Part III. Comparison of continuous flow spectrophotometry and potentiomety for the rapid determination of total nitrogen content in plant digests, Anal. Chim. Acta 81... 

Sample dissolution and analyte conversion to a more easily detected species, e.g., the sequential injection determination of total nitrogen in wastewaters involving Kjeldahl oxidation and nitrate formation [141]. The spectrophotometric flow injection determination of organic phosphorus in natural waters [142], relying on the release of the different forms of phosphorus as orthophosphate, is another good example. 

Another possibility is to take advantage of the individual treatment of every assayed sample. For sample lots with high variability in, e.g., pH or ionic strength, the required adjustments can be made in accordance with a prior assay performed in-line. This approach was proposed for the spectrophotometric determination of total nitrogen in Kjeldahl digests of plant leaves [397], As the method was very susceptible to sample acidity, which varied from sample to sample, individual sample pH adjustment was needed. To this end, an alkaline solution was added to every sample and the amount added was determined according to a pH-orientated feedback mechanism. This innovation was further applied to the spectrophotometric determination of aluminium in plant digests [398] and iron in estuarine waters [399], characterised by pronounced variations in... 

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