224 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(2) Starch and Potassium Iodide. 



This indicator was suggested in 1903 by Harrison (see p. 614). Its 

 use depends upon the liberation of iodine by cupric salts and it will show 

 the presence of copper sulphate in <x dilution of i in 20,000. 



A drop of the titration solution is added to i c.c. of the indicator 

 acidified with 10 drops of acetic acid. A red or blue colour is shown if 

 cupric salt be present ; no colour is given when reduction is complete. 



(3) Ferrous Thiocyanate. 



A more suitable and convenient indicator and one easy to prepare 

 is that described by Ling and Rendle in 1905 (see p. 614). It consists 

 of an acid solution of ammonium thiocyanate and ferrous ammonium 

 sulphate. On treatment with a cupric salt, the ferrous salt is oxidised 

 to ferric which reacts with the thiocyanate giving the bright-red ferric 

 thiocyanate. 



This indicator is recommended in making the sugar estimations in 

 malting processes. 



In practice, the titration is carried out in a small flask in prefer- 

 ence to a basin, and on nearing the end point at which the blue 

 colour is discharged drops are taken out and placed against one of a 

 series of drops of indicator upon a glass plate on a white surface. The 

 reduction of the Fehling's solution is complete when a red coloration is 

 no longer produced. 



(4) Reduced Phenolphthalein. 



Carletti suggested this indicator in 1913 (see p. 614). 



If cupric salt be still present in the titration solution when a drop is added 

 to a drop of the reagent + 2-3 drops of 10 per cent, potassium cyanide 

 solution, a red colour is produced. 



(/) Calculation of the Result. 



Knowing the dilution, the amount of sugar in the original solution 

 can be calculated. 



10 c.c. Fehling's solution = 0*05 gm. glucose, 



.*. 10*2 c.c. diluted sugar solution = 0*05 gm. glucose. 



Now 100 c.c. diluted sugar solution contain 30 c.c. original sugar solution, 



.'. 10-2 c.c. diluted sugar solution contain 3 X I0 2 c.c. original sugar solution, 



original solution = 0-05 gm. glucose, 



100 x loo x "05 

 .*. 100 c.c. original sugar solution = : 



= 1-6 per cent. 



The values of Fehling's solution for other monosaccharides are 

 almost the same as glucose, thus 10 c.c. = '05 gm. glucose - -051 1 gm. 

 galactose = '05144 gm. fructose = '0431 gm. mannose. 



