34, 35] Estimation of Potash 25 



cent, solution of platinum chloride, and evaporate on the 

 water bath. Evaporation must be continued until, on removing 

 the beaker from the water bath, the liquid sets to a pasty con- 

 dition. If it has evaporated to dryness a few drops of water 

 should be added, and evaporation continued until the pasty 

 condition is reached. The beaker is then allowed to cool, 

 and strong alcohol is added and gently swilled around the 

 beaker until the liquid is of uniform colour. 



34. Filtration and Washing. — Two pieces of filter 

 paper are taken and placed one in each pan of the balance, 

 and the heavier one trimmed with a sharp pair of scissors until 

 the two are identical in weight. They are then folded together 

 and placed in a funnel. The alcoholic liquor is poured through 

 them, and the crystalline precipitate washed with alcohol by 

 decantation until the filtrate is colourless. The precipitate is 

 then transferred to the filter, and washed with alcohol until the 

 filter papers appear quite white and the filtrate is no longer 

 yellow. In this operation the flaky crystals of the precipitate 

 should not be broken up. Further, the india-rubber tip to the 

 glass rod must not be used, as the alcohol is apt to render it 

 sticky. The filter papers are dried in the steam oven, cooled 

 in a desiccator, and separated. The blank filter is put on the 

 pan with the weights, and the one containing the K 2 PtCl 6 on 

 the other, and the difference is weighed. 



By this method the two filters are both treated in the same 

 way, and will therefore lose weight equally. The error which 

 would creep in if the filter paper were simply weighed and its 

 weight subtracted from the total weight of paper and K 2 PtCl 6 

 is thus eliminated. 



35. Another method of arriving at the weight of the pre- 

 cipitate — which, however, takes a slightly longer time — is as 

 follows : 



Filter off, and wash on an ordinary filter paper. Dry at 1 oo°, 



