876 PRINCIPLES OF CHEMISTRY^ 



Metallic platinum in 'a fused state has a specific gravity of 21 ; ft 

 is grey; softer than iron but harder than copper, exceedingly ductile, 

 and therefore easily drawn into wire and rolled into thin sheets, and 

 may be hammered into crucibles and drawn into thin tubes, &c. In 

 the state in which it is obtained by the ignition of its compounds* 

 it forms a spongy mass, known as spongy platinum, or else as powder 

 (platinum black). 6 In either case it is dull grey, and is characterised, 

 as we already know, by the faculty of absorbing hydrogen and other 

 gases. Platinum is not acted on by hydrochloric, hydriodic, nitric, and 

 sulphuric acids, or a mixture of hydrofluoric and nitric acids. Aqua 

 regia, and any liquid containing chlorine or able to evolve chlorine or 

 bromine, dissolves platinum. Alkalis are decomposed by platinum at 

 a red heat, owing to the faculty of the platinum oxide, Pt0 2 , formed to 

 combine with alkaline bases, inasmuch as it has a feebly-developed acid 

 character (see Note 8). Sulphur, phosphorus (the phosphide, PtP 2f 



if the insoluble residue of the platinum metals (Ir, Ru, Os) obtained, after treating with 

 'aqua regia, be fused with a mixture of 1 part of KNOj and 6 parts of KgCOj (in a gold 

 crucible), and then treated with water, it gives a solution containing the Ru (and a 

 'portion o the Ir), but which throws it all down when -saturated with chlorine and 

 boiled ; (4) that if indium be fused with a mixture of EHO and KN0 3 , it gives a soluble 

 'potassium salt, IrK 2 4 (the solution is blue), which, when saturated with chlorine, gives 

 IrCI 4 , which is precipitated by NH 4 C1 (the precipitate is- black), forming a double salt, 

 leaving metallic Ir after ignition; (5) that rhodium mixed with NaCl and ignited in a 

 current of chlorine gives a soluble double salt (from which sal-ammoniac separates Pfc 

 and Ir>, which gives (according to Jb'rgensen) a difficultly soluble purpureO'Salt (Chapter 

 XXH, Note 85), Rh 2 Cl 3 ,5NHa, when treated with NH 5 ; in this form the Rh may be 

 easily purified and obtained in a metallic form by igniting in hydrogen ; and (6) that 

 palladium, dissolved in aqua regia and dried (NH^Cl ^hrows down any Pt), gives soluble 

 PdCl 2 , which forms an easily crystallisable yellow salt, PdCl 2 NH 3 , with ammonia ; this- 

 salt (Wilm) may be easily purified by crystallisation, and gives metallic Pd when 

 ignited. These reactions illustrate the method of separating the platinum metals from 

 each other. 



6 We have already become acquainted with the effect of finely-divided platinum on 

 many gaseous substances. It is best seen in the so-called platinum black, which is a 

 coal-black powder left by the action of sulphuric acid, on the alloy of zinc and platinum, 

 or which is precipitated by metallic zinc from a dilute solution of platinum. In any 

 case, finely-divided platinum absorbs gases more powerfully and rapidly the more 

 finely divided and porous it is. Sulphurous anhydride, hydrogen, alcohol, and many 

 organic substances in the presence of such platinum are easily oxidised by the oxygen of 

 the air, although they do not combine with it directly. The absorption of oxygen is as 

 much as several hundred volumes per one volume of platinum, acd the oxidising power 

 of such absorbed oxygen is taken advantage of not only in the laboratory but even in 

 manufacturing processes. Asbestos or charcoal, soaked in a solution of platinic chloride 

 and ignited, is very useful for this purpose, because by this means it becomes coated with 

 pjatintim black. If 60 grams of PtCl 4 be dissolved in 60 c.c. of water, and 70 c.c tj of a 

 strong (40 p.c.) solution of formic aldehyde added, the mixture cooled, and then a 

 flolu'tion of 60 grams of NaHO in 60 grams of water added, the platinum is pre- 

 cipitated. After washing with water the precipitate passes into solution and forms * 

 black liquid containing soluble colloidal platinum (Loew, 1890). If the precipitated 

 platinum be allowed to absorb oxygen on the filter, the temperature rises 40, and a 

 very porous platinum black is obtained which vigorously facilitates oxidation. 



