VOLUMETRIC ESTIMATIONS 95 



in a mixture of 10 c.c. water and 2-5 grams of strong hydro- 

 chloric acid. The solution is decolorized immediately before 

 use by adding a few pieces of granulated zinc. A dozen drops 

 of the reagent are placed separately on a glazed white porcelain 

 plate and a drop of the titration mixture is, from time to time, 

 added to one of the drops ; when no pink colour is produced, 

 the titration is complete. 



3. Harrison's indicator is made by adding a little starch 

 paste to loo c.c. of 10 per cent solution of potassium iodide; 

 as this solution will not keep more than a few hours, it must 

 be freshly prepared. One c.c. of the indicator is acidified by 

 the addition of 10 drops of acetic acid and a little of the 

 titration mixture is added. The presence of unreduced copper 

 is indicated by the appearance of a red or blue colour ; the 

 absence of any colour marks the end of the reaction. 



EXAMPLE. Amount of sugar solution required to de- 

 colorize I o c.c. of Fehling's : 



ist reading, 

 and 



11*6 c.c mean. . 



Now since 



10 c.c. Fehling's = -05 gram glucose 



.'. ii'6 c.c. of the solution contained -05 gram glucose. 



05 x 100 



.'. IOOC.C. -^g- 



= 4-31 per cent. 



Estimation of Galactose and Mannose. 



The procedure is exactly the same as for glucose : 

 10 c.c. Fehling's ='05 1 1 gram galactose = '437 g ram mannose. 



Estimation of Cane Sugar. 



Cane sugar does not reduce Fehling's solution ; it is there- 

 fore necessary to invert it in order to make the estimation. To 

 do this, take a known volume of the sugar solution and add a 

 sufficiency of strong hydrochloric acid to make it about a I o 

 per cent solution of the acid ; heat on a water bath for about 

 a quarter of an hour, at 70 C. Then neutralize with sodium 

 carbonate, make up to a known volume and titrate. 



