200 QUANTITATIVE ANALYSIS. 



2. GRAVIMETRIC METHOD. 



Thirty cubic centimeters of Fehling's solution are diluted 

 with 50 cc. of water and heated to boiling in a porcelain dish. 

 Twenty cubic centimeters of the diluted sugar solution are 

 added, -the solution is kept gently boiling for five minutes, 

 and then it is diluted with about 120 cc. of water which has 

 been previously boiled. The fluid must remain blue. Filter 

 through a dried and weighed filter (Schleicher and Schiill, 

 No. 590, about 9 cm., or the ash-free so-called baryta-filter- 

 paper of Dreverhoff in Dresden), wash with hot water until 

 a portion of the wash-water is no longer made turbid with 

 hydrochloric acid and barium chloride, then with absolute 

 alcohol and ether, dry at 110-115 and weigh. The differ- 

 ence in the two weights is the cuprous oxide. To calculate 

 the amount of sugar from the cuprous oxide multiply by 



10 



n 5(y?8 



35.8 



More exact than the above method, but also more difficult 

 to carry out, is the modification according to Allihn. In this 

 the cuprous oxide is collected on an asbestos filter, reduced, 

 by heating in a current of hydrogen, to metallic copper and 

 weighed as such. 



Since the reducing power of the sugar towards copper 

 oxide is somewhat variable, according to the concentration 

 of the sugar solution, it is not permissible, in very exact deter- 

 minations, to calculate the amount of sugar from the quan- 

 tity of copper obtained. In this case we must use an empir- 

 ically determined table (pages 202, 203), which gives directly 

 the quantity of sugar corresponding to the weight of the 

 copper found. 



Of course this table may also be used when the cuprous 

 oxide itself has been weighed. It is only necessary to calcu- 

 late this cuprous oxide as copper by multiplying by 318 and 



