228 PRACTICAL ORGANIC AND JBIO-CHEMISTRY 



2 minutes were insufficient, whereas 30 minutes sufficed if the heating were 

 carried out in a boiling water-bath and not over a flame. He has also shown 

 that the estimation was equally accurate when the precipitate was weighed as 

 cuprous oxide, cupric oxide or metallic copper. He cons : dered that the 

 estimation as cuprous oxide was the most accurate for the estimation of small 

 quantities of glucose. 



Pfliiger has published a table of the corresponding quantities of cuprous 

 oxide and glucose. 1 



The corresponding quantity of glucose can be obtained from the table on 

 p. 603, if the amount of Cu2O be multiplied by -8883 or 1-1117 respectively 

 to give the corresponding amount of Cu or CuO. 



Davis and Daish's preference for the estimation as cupric oxide in the 

 case of plant extracts is probably to be accounted for by incomplete re- 

 moval of other compounds carried down with the cuprous oxide and re- 

 moved by heating. 



II. (b) Volumetric Estimation of the Precipitated Cuprous Oxide. 



(i) Mohr-Bertrand Method. 



The volumetric estimation of the cuprous oxide formed by the reduction of 

 alkaline cupric sulphate solution seems to have been first carried out in 1873 

 by Mohr, who based his process upon one of Schwarz's methods (1852), namely 

 that of dissolving the cuprous oxide in an acid solution of ferric chloride and 

 estimating the amount of ferrous chloride so formed. Mohr dissolved the 

 cuprous oxide in an acid solution of ferric sulphate and titrated the ferrous 

 salt with permanganate. 



This process has been recommended by several workers, amongst whom 

 may be mentioned Sonntag, Wood and Berry, and Bertrand. The last 

 author has made very thorough experiments with this method and has 

 published tables giving the amounts of glucose, lactose and maltose corre- 

 sponding to the amount of reduced copper. The method is now generally 

 referred to as Bertrand's method. The solutions required are given on 

 p. 613. 



Procedure. 



20 c.c. of the sugar solution, which should contain about *i per cent, 

 and preferably a little less, are placed in a conical flask of 125-150 c.c. 

 capacity ; 20 c.c. of the copper solution A and 20 c.c. of the alkaline solution 

 B are added, and the mixture heated to boiling over a flame and kept gently 

 boiling for exactly 3 minutes. If a smaller volume of sugar solution be used, 

 water is added so that the total volume is 60 c.c. The flask is removed from 



1 PflUger's Archiv, 1903, 96, 105. 



