Catalyst Kjeldahl

The performance of the combustion method compares favorably with that of the mercury catalyst Kjeldahl method for determination of crude protein in feeds. Standard deviations by the combustion method covered the full range of possibilities compared to the Kjeldahl method values were either equivalent, better than, or not as good as for the Kjeldahl method. For the 0.5-mm pairs, values ranged from 0.09% to 0.58% protein for the Kjeldahl method and from 0.14% to 0.33% protein for the combustion method, and (SR) values ranged from 0.23% to 0.86% protein (Kjeldahl) and from 0.30% to 0.61% protein (combustion). 

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. 

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]. 

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],... 

The original method for the determination of nitrogen by sulphuric acid-digestion was published by Kjeldahl in 1883 and fully described by Burns (1984). Many modifications have since been made with various catalysts and acid mixtures. 

Mix 100 - 250 mg sample exactly weighed with 1.5 g catalyst A, and then add 3 ml of concentrated sulfuric acid B. Heat the mixture at the temperature of boiling sulfuric acid (about 180 °C) for 2 h. Take care that the acid condenses in the middle of the neck of the Kjeldahl flask. 

Kjeldahl Method. A method of detg the N in an organic compd by digesting the substance with coned sulfuric acid in the presence of catalysts such as selenium, Devarda s alloy, etc. This treatment transforms the N into Amm sulfate. By adding an excess of caustic and distg the liberated ammonia into a measured quantity of standard sulfuric acid (which is later titrated), it is possible to determine the amount of N in the substance... 

KJELDAHL TEST. An analytical method for determination of nitrogen in certain organic compounds. It involves addition of a small amount of anhydrous potassium sulfate to the test compound, followed by healing the mixture with concentrated sulfuric acid, often with a catalyst such ns copper sulfate. As a result ammonia is formed. Alter alkaly/ing the mixture with sodium hydroxide, the ammonia is separated by distillation, collected in standard acid, and the nitrogen determined by back-iilruiion. 

This method was developed to replace the hazardous mercury catalyst required in the original mercuric oxide Kjeldahl method. It has been evaluated through an interlaboratory comparison of catalysts and has been adopted as the official replacement for the mercuric-oxide catalyzed Kjeldahl method. An inter-laboratory evaluation (Berner, 1990) indicated that this method (which uses the copper/titanium catalyst mixture) produces results more closely in agreement with the mercuric oxide catalyst method than methods using a copper sulfate catalyst. As a result of this study, mercuric... 

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. 

In the Kjeldahl method, pulverized coal is boiled coal with concentrated sulfuric acid containing potassium sulfate and a suitable catalyst to reduce the time for digestion. The catalyst is usually a mercury salt, selenium itself, or a selenium compound, or a mixture of the two. Selenium is regarded as being particularly advantageous. 

Nitrogen determination by perchloric acid digestion of coal without catalysts is reportedly safe and much more rapid than the conventional Kjeldahl procedure. Nitrogen is determined in the digest by the usual ammonia evolution and titration. However, extreme care should be taken whenever perchloric acid is used. Nuclear reactions have been applied successfully to the determination of nitrogen in coal. 

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