130 THE CARBOHYDRATES 



in a mixture of lO 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 100 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 10 c.c. of Fehling's : — 



117 c.c I st reading. 



11-5 c.c. ..... 2nd 



II-6C.C Mean. 



Now since 



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

 .-. 1 1 -6 c.c. of the solution contained -05 gram glucose. 



•05 X 100 



= 4-31 per cent. 



100 c.c. 



Estimation of Galactose and Mannose. 

 The procedure is exactly the same as for glucose : — 

 10 c.c. Fehling's = -0511 gram galactose = -04307 gram 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 10 

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

 a quarter of an hour, at 70° C* Then cool, neutralize with 

 sodium carbonate and make up to a known volume and titrate. 



♦Alternatively, add citric acid crystals to bring up to a 10 per cent 

 solution and boil for ten minutes. 



