VOLUMETRIC ESTIMATIONS 85 



2, The Pavy solution must be boiling throughout the 

 titration, else air will enter the flask, owing to the lowered 

 temperature, and the solution of cuprous oxide will be oxi- 

 dized. 



III. ESTIMATION BY MEANS OF BENEDICT'S SOLUTION. 



In this method the difificulty of the red precipitate of 

 cuprous oxide obscuring the end point is overcome by carry- 

 ing out the reduction in the presence of potassium thiocyanate 

 whereby the cuprous oxide is converted into an insoluble 

 white compound, and thus the disappearance of the last trace 

 of blue colour from the solution is easy to observe. 



The solution is prepared as follows : — 



200 grams sodium citrate, 



200 grams crystallized sodium carbonate or 75 grams of 



the anhydrous salt, 

 125 grams potassium thiocyanate 



are dissolved in water, made up roughly to 800 c,c., and 

 filtered. 



Eighteen grams of pure crystallized copper sulphate dis- 

 solved in 100 c.c. of water are poured slowly with constant 

 stirring into the above solution. Five c.c. of a 5 per cent 

 solution of potassium ferrocyanide are now added as a further 

 precaution against the formation of cuprous oxide, and the 

 whole is then carefully made up to 1000 c.c. 



The above solution, which will keep indefinitely without 

 any special precautions, is of such a strength that 



25 c.c. = 0-05 gram glucose. 



The titration is performed as follows : — 



Twenty-five c.c. of Benedict's solution are placed in a 4 

 oz. flask with 3 or 4 grams of anhydrous sodium carbonate 

 and a few lumps of broken porcelain to prevent bumping ; the 

 mixture is kept boiling vigorously while the sugar solution is 

 run in until the blue colour just disappears. The sugar solu- 

 tion may be run in rapidly at first, but towards the end it 

 should be run in drop by drop. 



