Determination of Nitrogen in Milk by Direct Nesslerization of the Digested Sample

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Abstract
The method consists of digestion of approx. 0.025 g. milk (0.12-0.15 mg. N) with 0.4 g. K2SO4, 0.5 ml. H2SO4 in Micro Kjeldahl flasks. Digestion is completed by addition of 30% H2O2 (3 drops). Total digestion time is 6 min. after samples clear. To the digested sample, after cooling, is added 2 ml. of a 2% gum acacia soln. and soln. made to vol. (50 ml.) with NH3-free H2O. A 10-ml. aliquot is placed in test tubes in a thermostat (23[degree]C) and 2 ml. of Nessler''s reagent added. After 10 min. color development, % transmission is measured with a spectrophotometer at a wave length of 420 m[mu]. using a prepared blank (K2Cr2O7) as reference. Milligrams of N are detd. from a standard reference curve prepared with known amts. of N. The effects of the variables involved in the test have been investigated and their tolerances established. The method yields results which are about 2.5% lower than the Macro Kjeldahl-Gunning-Arnold method but the results are reproducible and sufficiently accurate for all but the most exacting work with considerable saving in time and reagents when compared to other methods currently in use.
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