PLATINUM RESIDUES. 123 



trated, is, whilst still hot, mixed with a saturated solu- 

 tion of chloride of potassium, when, on cooling, a great 

 portion of the iridium separates in the form of crystal- 

 line black chloride of iridium and potassium, which is 

 filtered off and several times washed with solution of 

 chloride of potassium. 



The remaining solution is mixed with crystallized 

 carbonate of soda in excess, evaporated to dryness, the 

 mass gently ignited in a crucible, and when cold 

 washed with hot water, which usually acquires a yellow 

 color, owing to the presence of an alkaline chromate. 



The black powder which is left undissolved, consists 

 of a compound of sesquioxide of iridium with soda, 

 contaminated with sesquioxide of iron. It is reduced 

 by being gently heated in a current of hydrogen. 

 Water then extracts caustic soda, and the iron is 

 removed by digestion with hydrochloric acid. On 

 digesting it, after this treatment, with some very dilute 

 nitro-hydrochloric acid, a small amount of platinum is 

 usually extracted, which may then be precipitated 

 with chloride of ammonium. 



The iridium in the first solution maybe precipitated 

 by chloride of ammonium instead of chloride of potas- 

 sium. When the black perchloride of iridium and am- 

 monium is washed with a solution of chloride of ammo- 

 nium, and then digested with a solution of cyanide of 

 potassium, it is completely dissolved if it is free from 

 platinum, as a protochloride. If it contains platinum, 

 there remains undissolved a light brown residue, soluble 

 in hot water, which crystallizes in dark yellow octa- 

 hedrons. This salt has the composition expressed by 

 the formula (NH 4 , K) Cl + Pt C1 2 . 



Or the chloride of iridium and ammonium or potas- 

 sium is melted in a porcelain crucible with one and a 

 half times its weight of cyanide of potassium, the mass 

 dissolved in a little water, filtered, an excess of acetic 

 acid added to decompose the free cyanide of potassium, 



