56 METHODS OF QUANTITATIVE ANAIvYSIS. 



The polarization may be made in a Landolt's half 

 shadow instrument. The observation tube should first be 

 filled with distilled water, put in place in the polariscope 

 and a series of readings taken to establish the zero point : 

 that is, the point at which both halves of the field are 

 equally illumined. When this point is fixed exactly, the 

 tube is filled with the filtrate to be examined, and the 

 polariscope reading is made. The field is adjusted until 

 the light on both sides is again of equal intensity. Sev- 

 eral readings should be made in order to insure greater 

 accuracy. 



VIII. DETERMINATION OF CITRIC ACID. 



In the ordinary commercial analyses the estimation 

 of this acid may be omitted, since, from the amount of 

 it present, no conclusions as to the quality of the milk 

 may be drawn. To isolate the acid, a large quantity 

 (about 20 liters) of the milk is used. This is first skimmed 

 and then allowed to coagulate. The serum is clarified 

 by heating with acetic acid, and then filtered. To the 

 filtrate lead acetate solution is added. The resulting pre- 

 cipitate is filtered off and washed and suspended in 

 water and decomposed with hydrogen sulphide. The^lead 

 sulphide formed is filtered ofi and the filtrate evaporated. 

 The residue is cooled, and extracted with ether. The 

 ether solution is then placed in a small weighed dish and 

 evaporated to dryness on a water bath. The residual 

 citric acid is finally dried over concentrated sulphuric 

 acid in a desiccator and weighed. (Vaudin's Method.) 



The volumetric method for citric acid, proposed by A. 

 Scheibe, depends upon the precipitation of the acid by 

 means of alcoholic ammonia solution, and the titration of 

 the triammonium citrate with chromic acid. The latter 

 is reduced to chromium sesquioxide and the citric acid 



*One litre of cow milk contains o.g to i.o grams of citric acid in 

 the form of calcium citrate. Henkel, Jahresber, u die Fortsch, 

 der Thierchem., 1888, 94; 1891, 129; Schiebe, ibid 1891, 130.— 

 Translators. 



