CARBOHYDRATES 25 



ii. Reduction Tests. To their aldehyde or ketone structure many 

 sugars owe the property of readily reducing alkaline solutions of the 

 oxides of metals like copper, bismuth and mercury; they also possess 

 the property of reducing ammoniacal silver solutions with the separa- 

 tion of metallic silver. Upon this property of reduction the most 

 widely used tests for sugars are based. When whitish-blue cupric 

 hydroxide in suspension in an alkaline liquid is heated it is converted 

 into insoluble black cupric oxide, but if a reducing agent like certain 

 sugars be present the cupric hydroxide is reduced to insoluble yellow 

 cuprous hydroxide, which in turn, on further heating, may be converted 

 into brownish-red or red cuprous oxide. These changes are indicated 

 as follows: 



OH 

 / 

 Cu -+Cu = O+H 2 O. 



\ Cupric oxide 



\ (black). 



OH 



Cupric hydroxide 

 (whitish-blue). 



OH 



/ 

 Cu 



\ 

 OH 



- 2Cu-OH+H 2 O+O. 



Cuprous hydroxide 

 (yellow). 



OH 



/ 

 Cu 



\ 

 OH 



Cu 

 Cu OH \ 



-> 0+H 2 0. 

 Cu OH | / 



Cu 



Cuprous hydroxide Cuprous oxide 

 (yellow). (brownish-red). 



The chemical equations here discussed are exemplified in Trommer's 

 and Fehling's tests. 



(a) Trommer's Test. To 5 c.c. of sugar solution in a test-tube add one-half its 

 volume of KOH or NaOH. Mix thoroughly and add, drop by drop, a very dilute 

 solution of copper sulphate. Continue the addition until there is a slight permanent 

 precipitate of cupric hydroxide and in consequence the solution is slightly turbid. 

 Heat, and the cupric hydroxide is reduced to yellow cuprous hydroxide or to brown- 

 ish-red cuprous oxide. 



If the solution of copper sulphate used is too strong a small brownish-red pre- 

 cipitate produced in a weak sugar solution may be entirely masked. On the other 



