DETERMINATION OF LACTOSE. S3 



obtained. For conducting the reduction according to this 

 method of Soxhlet's, loo c. c. of the filtrate is added to 50 

 c. c. of Fehhng's solution, prepared by the formula /given 

 above, and boiled for about ten minutes. At the end of 

 this time the reduction is complete and the cuprous oxide 

 which separates out is filtered ofT. Owing to the diffi- 

 culty of filtering out this precipitate completely, several 

 processes of filtration have been devised (Soxhlet, Al- 

 lihn, PfeifTer). The filtration may be carried on easily 

 and successfully by using a hardened filter (Schleicher 

 and Schull). The filter is fitted tightly into the funnel 

 and saturated with hot water and the hot supernatant 

 liquid cautiously decanted, care being taken to retain the 

 precipitate in the beaker. This precipitate is washed 

 thoroughly with hot water by decantation. The soluble 

 matter in the paper is completely removed by washing 

 with water and finally the precipitate is transferred to the 

 filter and again washed to remove the last traces of sol- 

 uble copper. By careful manipulation the precipitated 

 cuprous oxide in the majority of cases will be completely 

 retained by the filter. If, however, any of the precipitate 

 should run through it is best to begin the determination 

 again. It is important in every case to remove all traces 

 of the excess of Fehling's solution from the filter before 

 transferring the precipitate. The washing of the cuprous 

 oxide in the beaker should be continued until the wash- 

 ings show no alkaline reaction or give, when acidulated, 

 no test for copper with potassium ferrocyanide. Soxhlet 

 and Allihn suggest the use of a bulbed tube provided with 

 an asbestos filter for carrying on these filtrations. 



The precipitate on the filter is dried at 100 degrees and 

 ignited in a porcelain, crucible previously dried and 

 weighed. Continue the ignition until the filter is entirely 

 incinerated. The fiame is then removed and the copper 

 oxide covered with a thick layer of powdered sulphur. 



