Nitrogen determination, Kjeldahl

As microwave sample preparation has evolved, standard microwave procedures have been developed and approved by numerous standard methods organisations (ASTM, AOAC International, EPA, etc.), see ref. [64]. Examples are standard test methods for carbon black/ash content (ASTM Method D 1506-97), lead analysis in direct paint samples (ASTM Method E 1645-94), etc. Table 8.15 shows some microwave ashing references (detection weight). A French AFNOR method utilises the atmospheric pressure single-mode microwave method as an alternative sample preparation procedure for Kjeldahl nitrogen determination [84], The performance of a microwave-assisted decomposition for rapid determination of glass fibre content in plastics for QC has been described [85]. 

Association Franjaise de Normalisation, Kjeldahl Nitrogen Determination with Microwave Sample Preparation, Rept NF V03-100 (1992). 

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

Possible hazards introduced by variations in experimental techniques in Kjeldahl nitrogen determination were discussed [1]. Modem variations involving use of improved catalysts and hydrogen peroxide to increase reaction rates, and of automated methods, have considerably improved safety aspects [2], An anecdote is given of the classic technique when sodium hydroxide was to be added to the sulphuric acid digestion and was allowed to trickle down the wall of the flask. It layered over the sulphuric acid. Gentle mixing then provoked rapid reaction and a steam explosion [3],... 

In the Kjeldahl nitrogen determination (Reactions 7-3 through 7-5), the final product is a solution of NHJ ions in HCI solution. It is necessary to titrate the HCI without titrating the NH ions. 

On average, the Kjeldahl nitrogen determination method takes about 2 to 3 hr to complete... 

Figure 6-1 Three-dimensional plot showing absolute error in Kjeldahl nitrogen determination for four different analysts. Note that the results of analyst 1 are both precise and accurate. The results of analyst 3 are precise, but the absolute error is large. The results of analysts 2 and 4 are both imprecise and inaccurate.

Kjeldahl is pronounced Kyell dahl. Hundreds of thousands of Kjeldahl nitrogen determinations are performed each year, primarily to provide a measure of the protein content of meats, grains, and animal feeds. 

The Kjeldahl nitrogen determination cannot be applied to leptazol. Low results (down to only 50 per cent) are obtained, presumably due to a loss of gaseous nitrogen when the tetrazole ring is broken open in the early stages of digestion. 

A check on the result is easily made by Kjeldahl nitrogen determination on the recovered saccharin as indicated above this is often desirable when the saccharin content is higher than the expected amount. 

A6.2.4 Boiling Chips—These consist of broken pieces of porcelain drying plates or broken alundum thimbles that are dropped into the flask before starting a distillation. Hengar granules of the plain type, as used in Kjeldahl nitrogen determinations, are also used in the same way (Note A6.1). 

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