GRAVIMETRIC ESTIMATIONS 89 



converting a given weight of cuprous oxide into glucose, cane 

 sugar (after inversion), lactose, or maltose : — 



Supposing the weight of cuprous oxide obtained by the 

 oxidation of a lactose solution to be -185, then the weight of 

 lactose corresponding to this would be — 



•185 X -6843 = -126 gram. 



The chief source of error in this method lies in the possi- 

 bility of the cuprous oxide containing impurities, such as silica 

 or alumina, derived from the alkali, in which case, of course, 

 its weight would be too high ; to overcome this objection 

 several methods have been devised, such as reducing the 

 cuprous oxide to metallic copper, or depositing the copper 

 electrolytically and weighing this ; or else oxidizing the cu- 

 prous oxide to cupric oxide and weighing again. 



The factors for converting metallic copper and cupric oxide 

 into sugars are also given in the above table. 



Estimation of Glucose as Osazone. 



The following method of estimating glucose as osazone in 

 the products of the action of malt upon starch is recommended 

 by Davis and Ling : * Twenty c.c. of solution containing 2-3 

 grams of starch products per 100 c.c. are mixed with i c.c. of 

 phenylhydrazine and 1-5 c.c. of 50 per cent acetic acid. After 

 heating for an hour f over a water bath, the liquid, which has 

 by this time evaporated to a small bulk, is filtered through 

 a tared Gooch crucible, and the crystalline osazone is washed 

 with 20-30 c.c. of boiling water, so that the total filtrate does 

 not exceed 50 c.c. ; the precipitate is then dried in a steam 

 oven and weighed ; under these conditions, O'l gram of 

 glucose gives 0-0505 gram of glucosazone. 



* Davis and Ling: "Journ. Ciiem. Soc, Lond.," 1904, 85, 24. 

 t The heating should not be continued for more than one hour. 



