METHODS FOR Q UANTITA TIVE DETEEMINA TIONS. 251 



weighed and stands on a piece of glazed, colored paper in order to 

 collect any particle of the dried precipitate which may happen to fall 

 beside the crucible. The filter, from which the precipitate has been 

 removed as completely as possible, by slightly rubbing it, is now 

 folded, placed upon the lid of the crucible, which rests on a triangle 

 over a gas-burner, and completely incinerated. The remaining filter- 

 ash, with particles of the precipitate mixed with it, is transferred to 

 the crucible, which is now placed over the burner and heated until 

 all water (or possibly other substances) is completely expelled. After 

 cooling, the crucible is weighed, the weight of the empty crucible and 

 that of the filter-ash (the latter having been previously determined 

 by burning a few filters of the same kind) deducted, and thus the 

 quantity of the precipitate determined. 



As platinum crucibles and many precipitates, after ignition, absorb 

 moisture from the air, it is well to allow the heated crucible to cool 

 in a desiccator. This is a closed vessel in which the contained air is 

 kept dry by means of concentrated sulphuric acid. Fig. 29 shows a 

 convenient form of desiccator. 



The empty crucibles should be weighed under the same conditions 

 i. e., after having been heated and cooled in a desiccator. 



PIG. 29. FIG. 30. 



Desiccator. Watch-glasses for weighing filters. 



Some precipitates (as, for instance, potassium platinic chloride), 

 cannot be ignited without suffering partial or complete decomposition. 

 It is for this reason that some precipitates are collected upon filters 

 which have been previously dried at 100 C. (212 F.) and weighed 

 carefully. The precipitate is then collected upon the weighed filter, 

 well washed, dried at 100 C. (212 F.) and weighed. 



The weighing of dried filters is best accomplished by placing them 



