78 THE CARBOHYDRATES 



observer's colour-sense be not very critical, an error is easily 

 made, hence v^arious methods have been suggested to determine 

 accurately the end point. 



1. Eilter off a small quantity of the solution, acidify it 

 with acetic acid and add a little potassium ferrocyanide ; the 

 presence of unreduced copper is indicated by the formation 

 of a brown coloration or precipitate of copper ferrocyanide. 



2. Ling's reagent consists of i gram of ferrous ammonium 

 sulphate and i'5 gram of ammonium sulphocyanide dissolved 

 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 ist riding. 



11*5 c.c. 



2nd 



Now since 10 c.c. of Fehling's are completely reduced by -05 

 gram of glucose, 



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

 .-. 100 c.c. ,, ,, - 05 X 100 ,, 



II-6 

 = 4"3I per cent. 



EstijnatioJi of Galactose and Mannose. 



The procedure is exactly the same as for glucose : — 



I c.c. I'ehling's_; '0511 gram galactose ^^},^^ gram mannose. 



