Digestion Kjeldahl

Oxidative degradation of [B qH q] and [B22H22] C to boric acid is extremely difficult and requires Kjeldahl digestion or neutral permanganate. The heat of reaction obtained from the permanganate degradation leads to a calculated heat of formation for [B qH q] (aq) of 92.5 21.1 kJ/mol (22.1 5.0 kcal/mol) (99). The oxidative coupling of both [B qH q] and has been studied ia some detail (100). 

Another approach to the organic nitrogen problem is to use persulfate wet oxidation to convert the nitrogen to nitrate or nitrite, in place of the reduction to ammonia [13,14,24,25]. Results are fully comparable with those from the micro Kjeldahl digestion but the technique is far simpler. The precision should also be higher, since the final step in the measurement, the colorimetric determination of nitrite, is much more precise than any of the ammonia methods. 

Kjeldahl digestion, 22 88 Kjeldahl nitrogen determinations, for paper, 18 99... 

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]. 

The Kjeldahl digestion-titration technique [44] discussed in section 12.5.1.1 for the determination of total nitrogen in soils has been applied to the determination of total nitrogen in sediments. 

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. 

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. 

The Kjeldahl digestion can convert up to 20% of any nitrate-N to ammo-nium-N. This would usually only increase the ammonium-N by an insignificant amount and may therefore be ignored. 

Macro-Kjeldahl digestion unit - with adjustable heating. 

Nitrogen (i) by Kjeldahl digest (ii) elemental analyser, or (iii) dichromate oxidation. 

Calcium cyanamide and urea must be subjected to a Kjeldahl digest to convert —NH to NH4+. Various modifications to procedures are necessary when several different compounds are present in the same fertilizer sample. 

Kjeldahl digestion flasks, 50 ml, scratched with a graduation line at 25 ml... 

Eaithfull, N.T. (1969) Multiple Kjeldahl digestion unit. Laboratory Practice 18(12), 1302. 

Kjeldahl digestion of fresh material (with addition of a strong reducing agent if high nitrate is expected). Alkaline steam distillation with final measurement by titration. 

Figure 7-2 (a) Kjeldahl digestion flask with long neck to minimize loss from spattering. 

Kane, P.F. 1987. Comparison of HgO and CuS04/Ti02 as catalysts in manual Kjeldahl digestion for determination of crude protein in animal feed Collaborative study. J. Assoc. Offic. Anal. Chem. 70 907-911. 

Crude protein and total lysine plus furosine Total nitrogen was determined by macro Kjeldahl digestion and protein estimated as N x 6.25. Total lysine values were obtained from conventional amino acid analyses carried out on 500-mg samples following digestion with 800 ml of 6M HC1 under reflux by use of a Biotronic LC 6000 or Kontron Liquimat III amino acid analyzer. Furosine was determined in the same way using 300 ml 7.8 M HC1 as described in ( 6 ) with an amino acid analyzer ( 7 ). 

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