232 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



The Lehmann-Maquenne Method. 



The principle of this method depends upon the liberation of iodine 

 on adding potassium iodide to copper sulphate, and the estimation of the 

 iodine by means of sodium thiosulphate : 



2CuSO 4 + 4 KI = Cu 2 I 2 + I a + 2K 2 S0 4 

 I 2 + 2Na 2 S 2 O 3 = Na^Og + 2NaI. 



Lehmann's procedure was to make up the Fehling solution to a definite 

 volume after its reduction by glucose, and to take out an aliquot part for the 

 titration after the cuprous oxide had settled. Riegler filtered off the cuprous 

 oxide and titrated the filtrate and washings. Maquenne and Schoorl titrate 

 the cupric salt without removing the cuprous oxide. 



The conditions for the reduction of Fehling's solution by glucose 

 and by lactose have been standardised by Peters (1912) and by Cole, 1 who 

 described also a simple asbestos filter made out of a calcium chloride tube 

 and a simple way of boiling the Fehling solution for exactly 2 minutes. 



The procedure of Maquenne is the following : 



10 c.c. of each of the two Fehling's solutions are measured into a 250 c.c. 

 conical flask and 10-40 c.c. of the sugar solution 2 are added; the total 

 volume of the solution for the reduction must be 60 c.c. and water is added 

 if necessary. The solution is boiled for exactly 2 minutes, and cooled under 

 running water. 20 c.c. of 20 per cent, potassium iodide and 20 c.c. of 

 25 per cent, sulphuric acid are added and the solution is titrated with *iN 

 thiosulphate solution (24*83 gm. per litre), using i c.c. of i per cent, starch 

 solution as indicator. 



A blank determination of Fehling's solution gives the amount of 

 thiosulphate solution (27-8 c.c.) required for 20 c.c. of Fehling's solution and 

 from this figure is deducted the number of c.c. required in the titration. 



The following table gives the corresponding amounts of glucose : 



Thiosulphate. Glucose. Thiosulphate. Glucose. Thiosulphate. Glucose. 

 c.c. ing. c.c. mg. c.c. mg. 



1 3'i " 357 21 70-7 



2 6'2 12 3Q-I 22 74-5 



3 9'3 13 42'5 23 78-5 



4 12-5 14 45-9 24 82-5 



5 157 15 49'3 25 86-5 



6 19-0 16 52*8 26 90-6 



7 22-3 17 563 27 94-8 

 25-6 18 59-8 28 97-9 



9 28-9 19 63-3 29 loo'o 

 10 32-3 20 66 -9 30 



Bang's First Method. 



In this method the reduction of the copper solution is effected in 

 potassium carbonate solution to which potassium thiocyanate is added. 

 Cuprous thiocyanate is formed and remains in solution. Excess of cupric 

 salt is estimated by hydroxylamine solution. Bang has published a card giving 

 the reagents, the procedure, and a table of corresponding values of glucose 

 and hydroxylamine. This method has been superseded by his other method 

 (p. 230). 



1 Tables for converting the copper figures into glucose and lactose are given in this 

 paper, Biochemical Journal, 1914, 8, 134. 



9 The sugar solution added must contain less than -i gm. glucose. 



