8o THE CARBOHYDRATES 



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

 made, hence various methods have been suggested to determine 

 accurately the end point. 



1. Filter 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 1-5 gram of ammonium sulphocyanide dissolved 

 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 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 : — 



ii'7 c.c 1st reading. 



11*5 c.c. ...... and „ 



11*6 c.c. 



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

 gram of glucose, 



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

 .♦. 100 c.c. „ „ -05 X 100 „ 



n-6 

 =4-31 per cent. 



Estimation of Galactose and Mannose. 



The procedure is exactly the same as for glucose : — 



I c.c. Fehling's .H3 •051 1 gram galactose • ^ 4307 gram mannose. 



