346 CHEMICAL TESTS AND ANALYSES 



increasing the heat gradually until frothing ceases. Digest for a 

 time after the mixture is colorless, or nearly so, or until oxidation 

 is complete. Do not add either potassium permanganate or 

 potassium sulphid. Dilute, neutralize, distil and titrate with stand- 

 ard alkali. In neutralizing, it is convenient to add a few drops 

 of phenolphthalein 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. 



(b) Albumin. Exactly neutralize with caustic alkali the fil- 

 trate obtained in the preceding operation (a), add 0.3 c.c. of a 10 

 per cent solution of acetic acid and heat the liquid to the tempera- 

 ture of boiling water until the albumin is completely precipitated, 

 collect the precipitate on a filter, wash and determine the nitrogen 

 therein. Nitrogen multiplied by 6.38 equals albumin/' or 



