QUALITATIVE AND QUANTITATIVE ANALYSIS 69 



for about 2 hours after the liquid becomes clear. When com- 

 pleted, cool, add water, pumice stone, and strong alkali carrying 

 potassium or sodium sulfide (0.4 per cent) and distill as in the 

 official Kjeldahl method. 



DETERMINATION OF AMINO NITROGEN 



Amino acids and other amino compounds can be determined 

 gravimetrically or colorimetrically, either by the Van Slyke 

 method,^ which is based upon the interaction of nitrous acid with 

 the amiino group, giving gaseous nitrogen; by the Sorensen for- 

 malin titration method, ^ and by the Folin colorimetric method.^ 



QUANTITATIVE DETERMINATION OF CARBOHYDRATES 



1. Free Celluloses. — True celluloses are characterized by the 

 fact that they are insoluble in dilute acids (2 to 4 per cent), but 

 they are soluble in concentrated acids, such as 42 per cent hydro- 

 chloric or 72 per cent sulphuric. The quantitative determination 

 of pure cellulose in soil or in culture is based upon its solu- 

 bility in ammoniacal copper solution, from which it is reprecipi- 

 tated by alcohol. 



Preparation of Schweitzer^ s Reagent. — Two hundred grams of 

 copper sulphate are dissolved in hot water and precipitated with a 

 calculated amount of ammonia (95 cubic centimeters of ammonia, 

 specific gravity 0.90). The excess of ammonia is then neutral- 

 ized with sulphuric acid. The precipitate is washed by decanta- 

 tion in a large bottle three or four times and is then transferred 

 to a Biichner funnel and filtered through hardened filter paper 

 by the use of suction. With the aid of a porcelain spoon the 

 excess of water is pressed out from the copper hydroxide. It is 

 then removed in the form of a hardened paste from the filter 

 paper and introduced into a bottle containing ammonia water 

 and shaken in a shaking machine for 4 to 5 hours. An undissolved 

 part of the copper hydroxide should remain at the bottom of the 

 flask. The Schweitzer's reagent prepared in this way should 

 contain 1.5 grams of copper per 100 cubic centimeters of solution. 



1 Van Slyke, D. D., J. Biol. Chem., 10: 15-55, 1911, 12: 275-284, 1912; 

 16: 539-547, 1913. 



2 Sorensen, S. P. L., Biochem. Z., 7: 45, 1907-1908. 



3 Folin, 0., J. Biol. Chem., 51: 377, 1922. 



