Mineralization Kjeldahl

There are three main reasons for digesting soils in hot acid - to determine the organic carbon content, to extract mineral elements for their total content, and to determine total nitrogen by the Kjeldahl digestion. 

After diluting the Kjeldahl digest to give a 50% aqueous solution, one observes a residue of silica and a fine white mineral deposit at the bottom of the digestion tube. There may also be some cloudiness in the solution, which should be left to settle for 48-72 hours. Using a disposable pasteur pipette, transfer sufficient of the upper clear solution to almost fill the autoanalyser sample cup. 

Kjeldahl nitrogen is the sum of nitrogenous organic compounds, free ammonia, and ammonium. In the presence of H2S04, K2S04, and selenium catalyzer organic nitrogen is converted to ammonium. Free ammonia is also converted to ammonium. After the addition of a base, the ammonia is distilled from an alkaline medium and absorbed in boric acid. The ammonia may be determined colorimetrically at 655 nm or by titration with a standard mineral acid... 

International Standard Organization. 1984. Water quality. Determination of Kjeldahl nitrogen. Method after mineralization with selenium. ISO 5663. International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland. 

Protein Determine the percent of nitrogen as directed in the Nitrogen Determination (Kjeldahl Method), Appendix IIIC, or by the Protein Nitrogen Combustion Method, AOAC 992.23 or AOCS Ba 4e-93. The percent protein equals percent N x 6.25 and is calculated to exclude added vitamins, minerals, amino acids, and food additives. 

The dissolution with a tissue solubiliser (e.g. TAAH) or a mineral acid is often sufficient for flame and GF-AAS techniques for a number of elements. Weigh 0.1 g of the sample in a Kjeldahl flask. Add 3 mL of suprapure cone. HNO3 (65% v/v) or a tissue solubiliser, e.g. TMAH solution. Gently warm till the sample is dissolved. Dilute with deionised water or a suitable solvent. The solution can be used for a number of elements including Cu. Zn, Mn, Hg. Cr. Cd, Pb. 

Crude protein Methods based on mineralization of nitrogen, such as the Kjeldahl method (i.e AFNOR NF VI8-100, 1977) or the Dumas method (AFNOR NF VI8-120, 1997) for the more recent data. The crude protein content is obtained by multiplying total nitrogen by 6.25. 

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). 

The mycelium was washed with Isotonic saline, resuspended In mineral salts medium with glucose (1% w/v) and mixed with a suspension of activated support. The mixture was stirred at 20 0 for 20 h and free mycelial forms were separated. The amount of bound dry weight per gram of carrier (the efficiency of Immobilization) was determined on the basis of nitrogen quantity by Kjeldahl s method. 

Kroon H. 1993. Determination of nitrogen in water comparison of a continuous-flow method with online UV digestion with the original Kjeldahl method. Anal. Chim. Acta 276 287-293. Oms M.T., Cerda A., and Cerda V. 2003. Sequential injection system for online analysis of total nitrogen with UV-mineralization. Talanta 59 319-326. 

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