ESTIMATION OF POTASH BY PLATINUM METHOD 567 



calls attention to the difficulties attending the ordinary method of 

 determining potash in the presence of the sulfates of the alkalies 

 and alkaline earths. 47 He modifies the method by precipitating 

 the potash directly without previous precipitation with barium 

 chlorid. This, however, is not a new variation, as Regel sup- 

 poses, since it is the basis of the one proposed by Moore and 

 practiced with such success in this country for many years. The 

 precipitate of the platinum potash salt is washed in the usual man- 

 ner. If sodium sulfate is present some of this is contained in the 

 precipitate. In this case the precipitate is washed into a small 

 beaker, the parts remaining on the filter being dissolved in hot 

 water and powdered magnesium is added in excess. It is to be 

 noticed that the magnesium, even in almost completely neutral 

 solutions, reduces the double salt. Upon its introduction there 

 is a notable evolution of heat and a decomposition of the neutral 

 salt with a separation of black, finely divided metallic platinum 

 The precipitate of platinum is separated by filtration and any ex- 

 cess of magnesium removed by washing with dilute hydrochloric 

 acid. The separated platinum is dried, ignited, and weighed. 

 If magnesium sulfate or calcium sulfate is present in the original 

 platinum precipitate, the method of separation is exactly the same 

 as that just described except that five per cent, nitric acid is 

 used instead of hydrochloric acid for the solution of any excess 

 of sulfates. In this way calcium sulfate in the quantities in which 

 it is present in potash salts is completely removed. 



The reasons for using nitric acid are the following: 



If the mixture of metallic platinum and the residual salts is 

 treated with hydrochloric acid, the filtrate is clear. If, however, 

 an attempt be made to wash out the hydrochloric acid with water 

 there is obtained a deep black tinted filtrate containing dissolved 

 platinum from which metallic platinum separates slowly. This 

 phenomenon only occurs if the platinum salts are mixed with con- 

 siderable quantities of potassium chlorid or calcium sulfate, a 

 fact for which up to the present time no satisfactory explanation 

 has been made. Since the separation of the platinum from the 



47 Chemiker-Zeitung, 1906, 30 : 684. 



