ESTIMATION OF CARBOHYDRATES 



ESTIMATION OF DISACCHARIDES. 



A. Cane Sugar. 



Cane sugar is estimated by taking the reducing power of the solu- 

 tion after hydrolysis by acid ; fructose and glucose have very nearly 

 the same reducing power. 



A known volume of the cane sugar solution (say 40 c.c.) is hydro- 

 lysed by warming on the water-bath for 5-10 minutes with 5 c.c. 

 of dilute hydrochloric acid. The solution is cooled, neutralised with 

 5 c.c. of caustic soda, and made up to a definite volume in a measuring 

 flask (say loo c.c), rinsing out the flask with the water necessary to 

 make up the 100 c.c. The amount of reducing sugar is then estimated 

 by Fehling's, Pavy's or any of the other methods previously described. 

 The sugar solution should be of a strength so that 10 c.c. = 10 c.c. 

 Fehling's or 50 c.c. Pavy's solution, i.e. contain about the equivalent of 

 5 per cent, of glucose. 



The percentage of cane sugar is calculated from the equation : 



C 12 H 22 O n + H .0 = 2C 6 H 12 8 



342 360 



0*047 '5 = I0 c - c - Fehling's solution. 



B. Lactose and Maltose. 



Lactose and maltose are estimated in the same way as glucose by 

 Fehling's or other methods, but their reducing power is less than that 

 of glucose. Consequently, since Fehling's solution is a standard 

 for glucose, a factor has to be employed in order to obtain the equi- 

 valent value for these disaccharides. The factor is obtained by de- 

 termining the reducing power before and after hydrolysing them into 

 monosaccharides by acid, thus : 



A definite volume of the lactose solution is taken and diluted, 

 as previously done for glucose, with water so that 10 c.c. reduce 

 10 c.c. Fehling's solution (or 50 c.c. Pavy solution). The value is de- 

 termined exactly. 



Exactly the same quantity of the lactose solution is hydrolysed 

 by boiling for 3-4 hours in a flask with one-tenth of its volume of 

 dilute sulphuric acid ; water must be added to replace that lost on 

 boiling, or the hydrolysis is carried out by heating under a reflux 



