118 METHODS OF ANALYSIS. 



glass rod and let stand from three to five minutes longer. Then decant on 

 filter, wash two or three times with cold water by decantation, and transfer 

 precipitate completely to filter. Wash once or twice on filter. The filtrate 

 should be clear, or very 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 completed. Determine nitrogen in the'washed 

 precipitate and filter paper by the Kjeldahl or Gunning method ("I. Ferti- 

 lizers," p. 5). To calculate the equivalent amount of casein from the nitro- 

 gen 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. PROVISIONAL. 



Exactly neutralize with caustic alkali the filtrate obtained in the preceding 

 operation (a), add 0.3 cc of a 10 per cent solution of acetic acid and heat the 

 liquid to the temperature of boiling water until the albumin is completely pre- 

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

 therein. Nitrogen multiplied by 6.38 equals albumin. 



(c) CASEIN. OPTIONAL OFFICIAL. 



To 10 cc of milk add 50 cc of distilled water at 40 C., then add 2 cc of alum 

 solution saturated at 40 C. or higher. Allow precipitate to settle, transfer to a 

 filter, and wash. Treat the precipitate and filter paper by the Kjeldahl or 

 Gunning method ("I. Fertilizers," p. 5). 



(d) ALBUMIN. OPTIONAL PROVISIONAL. 



To the filtrate obtained from the casein determination (c) add 0.3 cc of a 10 

 per cent acetic-acid solution, boil the liquid until the albumin is completely 

 precipitated and proceed as in the provisional method for albumin (b). 



5. Milk Sugar. (Lactose.) 

 (a) OPTICAL METHOD. OFFICIAL. 



(1) PREPARATION OF REAGENTS. 



(a) Acid mercuric nitrate. Dissolve mercury in double its weight of nitric 

 acid, specific gravity 1.42, and dilute with an equal volume of water. One 

 cubic centimeter of this reagent is sufficient for the quantities of milk mentioned 

 below. Larger quantities may be used without affecting the results of 

 polarization. 



(&) Mercuric iodid with acetic acid. Mix 33.2 grams of potassium lodid, 

 13.5 grams of mercuric chlorid, 20 cc of glacial acetic acid, and 640 cc of water. 



(2) DETERMINATION. 



The milk should be at a constant temperature, and its specific gravity deter- 

 mined with a delicate hydrometer. When greater accuracy is required, a 

 pycnometer is used. 



The quantities of the milk measured for polarization vary with the specific 

 gravity of the milk as well as with the polariscope used. The quantity to be 

 measured in any case will be found in the following table : 



