12 Operations used in Quantitative Analysis [12 



paper is very even in structure, and has been washed free from 

 most of its mineral matter by acids. A 9-cm. filter paper 

 when burned should not leave a milligram of ash. 



(b) The precipitate should be allowed to subside, and the 

 clear liquid poured down a glass rod into the filter (see 



% i3)- 



(c) The tip of the filter funnel should touch the edge of 

 the beaker which is to receive the filtrate. This will prevent 

 splashing. 



(d) The precipitate should be washed, when possible, by 

 decantation. When as much of the liquid as can be poured out 



without removing the sedi- 

 ment has been transferred to 

 the filter, hot water is poured 

 on to the precipitate which i's 

 still in the beaker, and it is 

 allowed to settle. The clear 

 liquid is again poured off as 

 before, and the operation is 

 repeated until the filtrate is 

 pure water. Finally, the pre- 

 cipitate is transferred to the 

 filter, the last portions being 

 washed on to the paper by the 

 wash-bottle spray. Should 

 any portions adhere to the side of the beaker, they may be 

 removed by rubbing with a glass rod tipped with a piece of 

 india-rubber tubing. 



(e) Never use a glass rod tipped with india-rubber ex- 

 cept for the operation above mentioned. If such a rod be 

 placed in a liquid during precipitation, some of the pre- 

 cipitate may get in between the rod and the rubber and thus 

 be lost. 



Fig. 



-Filtration. 



