262 Prof. R. Bun sen on Rhodium. 



out with water, contains the rhodium almost entirely free from 

 all impurities. If the solution be warmed on the water-bath in 

 a loosely stoppered flask, a fresh precipitate is formed, which is 

 yellowish white and consists mainly of the above-mentioned 

 rhodium- salt mixed with a greater or less quantity of the corre- 

 sponding iridium salt. If, after filtering off this precipitate, the 

 filtrate be evaporated to a small volume, two precipitates are 

 formed : one is flocculent and of a yellowish white, which con- 

 tains almost exclusively iridium with small traces of rhodium; 

 the other consists of heavy crystalline scales which speedily fall 

 to the bottom. This latter maybe readily separated by washing 

 and decantation ; it weighed 16 grms. Whether this salt, which 

 in general behaves like an iridium compound, contains a new 

 metal I shall afterwards attempt to decide. The precipitates 

 produced by sulphite of soda in the cold and on warming weighed, 

 excluding that 16 grms., altogether 99 '5 grms. In the mother- 

 liquor, after adding hydrochloric acid and partial evaporation 

 with zinc, traces only of platinum-metals could be detected. 



The complete separation of iridium from rhodium is readily 

 effected by merely treating the yellow precipitates with concen- 

 trated sulphuric acid. The precipitate is added in small por- 

 tions to concentrated sulphuric acid, heated in a platinum cru- 

 cible until sulphurous acid is disengaged, and the crucible heated 

 in a sand-bath until all free sulphuric acid is volatilized and bi- 

 sulphate of soda has been formed. On boiling out the contents 

 with water, iridium is dissolved out as a sulphate with a deep 

 chrome-green colour, while sulphate of rhodium is left in combi- 

 nation with sulphate of soda as a flesh-coloured double salt. It 

 rapidly settles as a heavy powder, which must be boiled out with 

 water and aqua regia, and washed by decantation. When I come to 

 the determination of the atomic weight of rhodium, I shall dwell 

 more fully on the constitution of this remarkable double salt, 

 which is insoluble in hydrochloric or nitric acid or in aqua regia, 

 can be heated to 250° without any change, and on ignition de- 

 composes into metallic rhodium and sulphate of soda. The 

 total quantity of this double salt obtained from a kilogramme 

 of material amounted to 33*2 grms. 



The first precipitates obtained by sulphite of soda in the cold 

 yielded the rhodium almost entirely pure when treated in this 

 way ; the subsequent precipitates are very rich in iridium, and 

 yield rhodium which is far purer than that prepared by previous 

 processes, but not quite free from iridium. Hence these products 

 obtained by sulphuric acid, which are not yet quite pure, and 

 are recognized by their brownish tint, are collected, the rhodium 

 separated by ignition, and the metal obtained heated once more 

 with chloride of barium in a current of chlorine to purify it a 



