264 Chemicai, Tests and Analyses 



for a day or two at ordinary temperatures. The filtered solution is 

 employed as indicator. 



DETERMINATION 



Place the substance to be analyzed in a digestion flask, employ- 

 ing from 0.7 to 3.5 grams, according to its proportion of nitrogen. 

 Add 10 grams of powdered potassium sulphate and from 15 to 25 

 c.c. (ordinarily about 20 c.c.) of sulphuric acid. Conduct the diges- 

 tion by starting with a temperature below boiling point and increas- 

 ing the heat gradually until frothing ceases. Digest for a time 

 after the mixture is colorless, or nearly so, or until oxidation is com- 

 plete. Do not add either potassium permanganate or potassium 

 sulphid. Dilute, neutralize, distil and titrate with standard alkali. 

 In neutralizing, it is convenient to add a few drops of phenolphtha- 

 lein indicator, by which one can tell, when the acid is completely 

 neutralized, remembering that the pink color, which indicates an 

 alkaline reaction, is destroyed by a considerable excess of strong 

 fixed alkali. 



CASEIN AND ALBUMIN 



"(a) Casein. — The determination should be made when the 

 milk is fresh, or nearly so. When it is not practicable to make this 

 determination within twenty-four hours, add one part of formal- 

 dehyde to twenty-five hundred parts of milk and keep in a cool 

 place. Place about 10 grams of milk in a beaker with about 90 c.c. 

 of water at 40 degrees to 42 degrees C, and add at once 1.5 c.c. of 

 a 10 per cent acetic acid solution. Stir with a glass rod and let 

 stand from three to five minutes longer. Then decant or filter, wash 

 two or three times with cold water by decantation and transfer pre- 

 cipitate completely to filter. Wash once or twice on filter. The 

 filtrate should be clear, or nearly so. If it be not clear when it first 

 runs through, it can generally be made so by two or three repeated 

 filtrations, after which the washing of the precipitate can be com- 

 pleted. Determine nitrogen in the washed precipitate and filter by 

 the Gunning method. To calculate the equivalent amount of casein 

 from the nitrogen multiply by 6.38. 



In working with milk which has been kept with preservatives, 

 the acetic acid should be added in small proportions, a few drops 

 at a time, with stirring, and the addition continued until the liquid 

 above the precipitate becomes clear or very nearly so. 



