CH. V.] QUANTITATIVE ESTIMATION. 125 



155. To 10 cc. of the solution add 20 cc. of strong alcohol, 

 shake vigorously and filter. To a portion of the filtrate add iodine 

 solution. No colour is obtained, showing that the whole of the 

 glycogen is precipitated. Dissolve the precipitate in a little hot 

 water : note that it is opalescent. Add three drops of strong sulphuric 

 acid and boil for about three minutes : the opalescence disappears. 

 Neutralise with sodium hydroxide and apply Fehling's test. A 

 marked reduction occurs, due to the conversion of the glycogen into 

 glucose by the boiling acid. 



D. The Quantitative Estimation of the Carbohydrates. 



A very large number of methods have been introduced 

 for the estimation of glucose, etc., and considerable experi- 

 ence is required to select the method best suited for a given 

 purpose. 



The following scheme indicates the principle of the more 

 important methods, only a few of which are described 

 below or in other sections of this book. 



A. Direct Volumetric Methods. 



1. Fehling's. A standard solution of copper sulphate in 

 Rochelle salt and caustic soda is boiled, and the sugar solution is 

 run in from a burette until the copper is reduced, as judged by the 

 disappearance of the blue colour. The end point is difficult to see 

 owing to the red precipitate that forms. 



2. Ling's modification. An external indicator containing 

 ferrous thiocyanate is employed. This is oxidised to red ferric 

 thiocyanate by cupric salts, but not by cuprous. 



3. Pavy's. Strong ammonia is added to Fehling's solution. 

 A measured amount of this is boiled and the sugar solution run 

 in from a burette. The cuprous oxide formed is kept in solution by 

 the ammonia (see Ex. 97, note 5), forming a colourless compound. 

 The end point is thus much easier to see. The practical difficulty 



of the method is to regulate the heating and also the rate at which 



