226 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(iii) Gerrard's Method. 



In 1892 Gerrard found that potassium cyanide was an effective agent for 

 dissolving cuprous oxide and prevented its precipitation from Fehling's solution 

 when reduced by glucose. This observation led to a simple method for 

 estimating glucose. It was improved by Allen and described by him as the 

 best method. The method has an advantage over Pavy's method in the 

 absence of the ammonia vapour and in that the reoxidation of the cuprous 

 oxide is slower. 



On adding potassium cyanide to Fehling's solution it is decolorised, the 

 colourless double salt of copper and potassium cyanide being formed : 

 CuS0 4 + 4 KCN = CuCN 2 , 2 KCN + K 2 SO 4 . 



If excess of Fehling's solution above that capable of being decolorised 

 be added the blue colour remains, and when boiled with glucose this amount 

 is reduced without the precipitation of cuprous oxide. 



Allen described the following procedure : 



10 c.c. of Fehling's solution are diluted with 40 c.c. of water and heated 

 to boiling in a porcelain basin. An approximately 5 per cent, solution of 

 potassium cyanide is run into the boiling liquid from a burette until it is just 

 decolorised, excess being carefully avoided. Another 10 c.c. of Fehling's 

 solution are added and the sugar solution of about 0-5 per cent, strength run 

 in slowly until the blue colour vanishes. Only the last portion of the Fehling 

 solution is reduced by the glucose so that as in Fehling's method 10 c.c. = 

 0*05 gm. glucose. 



(iv) S. G. Benedict's Method. 



In 1907 S. G. Benedict introduced yet another method for the 

 direct volumetric estimation of glucose on account of the difficulties 

 and inconveniences of the other methods. In 1910 he published im- 

 provements in his method and it seems as if this will be the one most 

 generally used. 



If potassium thiocyanate be added to Fehling's solution, the 

 cuprous oxide is not precipitated on reduction, but if carbonate be 

 used instead of caustic alkali, a white precipitate of cuprous thiocyan- 

 ate is formed. The method depends upon the precipitation of the re- 

 duced copper as cuprous thiocyanate and decolorisation of the solution. 



Procedure. 



25 c.c. of the reagent (see p. 613) are measured with a pipette into 

 a porcelain basin, 25-30 cm. in diameter ; 10-20 gm. of cryst. sodium 

 carbonate (or 5-10 gm. anhydrous sodium carbonate) and a small 

 quantity of pumice, or a piece of porous earthenware, are added. The 

 solution is boiled vigorously over a free flame and the sugar solution is 

 run in rapidly till a heavy white precipitate is produced and the blue 

 colour begins perceptibly to diminish. The sugar solution is then 

 run in more slowly with constant vigorous boiling of the reagent until 

 the blue colour has entirely disappeared. An interval of 30 seconds 

 between the additions of sugar solution (drop by drop) towards the 

 end should be given, and water may be added to replace that lost by 

 evaporation. The sugar solution should be of 0-5-1 per cent, as in 

 Fehling's method. 



The calculation of the result is from 



25 c.c. reagent ~ 0^05 gm. glucose or -053 gm. fructose. 



