W. Gibbs on the Platinum metah. 347 



chemical purity may be effected by either of theTollowina: pro- 

 cesses. The iridium is in the first place to be brought into the 

 form of bichlorid by means of a current of chlorine or by nitric 

 rtcid, and the two metals are then to be precipitated together as 

 PtCl^.KCl and IrCl^KCJ, by the addition of a concentrated 

 solution of chlorid of potassium. The color of the mixed salts 

 varies from orange to almost black, according to the quantity of 

 HKlium present. The tnass of crystals is to be rubbed fine in 

 an unglazed porcelain morlar and boiling water added in the 

 proportion of three volumes of water to one of salt. A dilute 

 solution of nitrite of potash is then to be added, until the liquid 

 beeomes deep olive-green, carbonote of potash being thrown in 

 from time to time in quantity sufficient to prevent the solution 

 from becoming strongly acid. The iridium is instantly rediu*ed 

 to sesquichlorid, while"^ the platinum salt remains as a reddish 

 orange powder. The deep olive-green solution is to be poured off 

 and tlie undissolved mass treated a second time with hot water 

 and nitrite. This process must be repeated as long as the liquid 

 remains olive-green. The mixed solutions on cooling, or after 

 evaporation, deposit a beautiful mass of crystals of the double 

 chlorid of potassium and iridium, Ir,Cl,,3KCl-f 6H0. By 

 re-solution and repeated crystallization the iridium salt may be 

 obtained perfectly free from platinum. _ Instead of nitrite of pot- 

 ash nitrite of soda may be employed in the above mentioned 

 P^^^^ess; the iridium and sodium salt has the formula Ir^Cl,, 

 <5^^aCl-f-24:HO, and crystallizes well. The undissolved mass and 

 the mother liquors from the iridium salt contain a large quantity of 

 Phatinum with a comparatively small quantity of iridium. When 

 the absolute quantity of platinum salt is not very large it may 

 te dissolved in boiling water, a small quantity of an alkaline 

 nitrite added, and the solution allowed to crystallize ; the result- 

 ing chlorplatinate of potassium" contains only a trace of iridium. 

 '^'be process just mentioned gives satisfactory results when 

 carefully executed but requires attention to two points. In the 

 ^'■■^t place the alkaline nitrite must be added m quantity just 

 sufficient to reduce the iridium from bichlorid to sesquichlorid, 

 b«t not so as to prodiice further chemical changes by tlie forma- 

 ^'?n of the double nitrites of iridium and potassium or sodium. 

 ^^ ith a very little experience this is easily managed. In conse- 

 qi^ence of the facility with which the double nitrite of indium 

 Y«l sodium is decomposed by boiling with chlorhydricacid into 

 ^'^e double chlorid IrCl,,NaCl, it is better to use nitrite of soda 

 iti the above process, because, in case an excess of nitrite is used, 

 f'e mixed solution of double chlorid and double nitrite can easily 

 DC brought to the form of double chlorid, Ir^Cl^jSNaCl, by 

 boiling with chlorhydric acid, neutralizing with carbonate of 



