VOLUMETRIC ESTIMATION. 163 



Volumetric Method. Instead of weighing the copper 

 reduced, the determination may be made volumetrically. The 

 estimation is carried out as follows : Place the solution obtained 

 by removing the proteins by Methods 1, 3, or 4 (given above) 

 in a burette graduated to y 1 ^ c.c. Measure into a small flask 

 10 c.c. of Fehling's solution accurately (or 5 c.c. of each of the 

 copper and alkaline tartrate solutions), dilute with 30 c.c. of 

 water, and bring to the" boil by means of a small flame. 

 Run in the sugar solution, adding 2 c.c. at a time, and boiling 

 between each addition. When the blue colour of the liquid has 

 nearly disappeared the sugar solution should be added in smaller 

 amounts, but the titration should not be unduly prolonged. 

 The end of the reaction is reached when, on removing the flame, 

 and allowing the cuprous oxide to settle, the supernatant liquid 

 appears colourless or faintly yellow when viewed against a white 

 surface. To make sure that the copper is all reduced a few 

 drops of the liquid may be filtered through a small filter into 

 acetic acid, and potassium ferrocyanide added. If copper be 

 still present, a brown coloration will be observed. It is advis- 

 able to repeat the titration, using 0-2 c.c. less of the milk-sugar 

 solution, which may all be added at once, and the boiling con- 

 tinued for four minutes ; a small excess of copper should be 

 present, and this is reduced by small additions of the sugar 

 solution. Should no copper be present, the experiment must 

 be repeated, using a still smaller amount of liquid. Ten c.c. 

 of Fehling's solution is reduced by 0-0676 gramme of hydrated 

 milk-sugar ; this quantity is, therefore, contained in the volume 

 of sugar solution used for titration. 



For example, 10-260 grammes of milk were, after removal of 

 proteins, made up to 100 c.c. Ten c.c. of Fehling's solution 

 required 13-0 and 13-1 c.c., mean 13-05 c.c. ; therefore, 



13-05 c.c. contain 0-0676 gramme milk-sugar. 



and 100 c.c. contain 0-518 



10-260 grammes milk contain 0-518 



= 5-05 per cent, hydrated milk-sugar. 

 = 4-80 per cent, anhydrous milk-sugar. 



It is advisable to titrate a solution of pure milk-sugar con- 

 taining about 0-5 gramme per 100 c.c. to obtain the exact value 

 of 10 c.c. of Fehling's solution. 



Ling and Rendle's Method. Ling and Rendle give the 

 following method for the volumetric determination of reducing 

 sugars : 



Fehling's solution is prepared thus : 



Solution No. 1. 69-278 grammes of crystallised copper sul- 

 phate are dissolved in water and made up to 1 litre. 



Sobition No. 2. 346 grammes of crystallised Rochelle salt 



