PLATINUM. 367 



Platinum, Pt =193.3. Platinum, like gold, is found in nature in 

 the free state, the chief supply being derived from the Ural mountains, 

 where it is associated with a number of metals (iridium, ruthenium, 

 osmium, palladium, rhodium) resembling platinum in their properties. 



While the solubility of platinum in molten lead is sometimes used for its 

 separation by the cupellation process (see refining of gold) the abstraction of 

 platinum is usually accomplished by the wet process. The material contain- 

 ing it is treated with nitre-hydrochloric acid under slight pressure, when pla- 

 tinic chloride is formed. The solution is evaporated to dryness and the mass 

 heated to a temperature of 125 C. in order to decompose the higher chlorides 

 of iridium and palladium, which metals, if present, would otherwise accompany 

 the platinum. After dissolving the residue in water, ammonium chloride is 

 added, which precipitates platinum as ammonium platinic chloride, PtCl 4 . 

 2NH 4 C1. The washed precipitate when heated to redness is completely decom- 

 posed, metallic platinum being left as a gray, spongy mass, which may be fused 

 by means of the oxy-hydrogen flame or in an electric furnace. 



Platinum is of great importance and value on account of its high 

 fusing-point and its resistance to the action of most chemical agents, 

 for which reason it is used in the manufacture of vessels serving in 

 chemical operations. While sulphuric, nitric, hydrochloric, and hydro- 

 fluoric acids have no action on platinum it is readily attacked by 

 chlorine, and at a red heat by caustic alkalies, sulphur, and phosphorus. 



Platinum is of a silver-white color with a tinge of blue ; it is very malleable 

 and ductile ; its rate of expansion by heat is low, about that of glass. This 

 property is of value in the use of the metal for the pins of artificial teeth, and 

 as a base for continuous gum work. Addition of iridium renders platinum 

 harder, more rigid and more elastic, all of which properties platinum confers 

 upon silver and gold. 



The property of platinum to condense oxygen upon its surface and to dis- 

 solve hydrogen is made very conspicuous in platinum sponge and platinum black. 

 The former is made by heating precipitated ammonium chloroplatinate, by 

 which a gray mass of finely divided platinum is left; the latter is obtained as 

 a black powder by adding zinc to chloroplatinic acid. Either of these forms 

 or platinum, when thrown into a mixture of oxygen and hydrogen, causes 

 instant explosion. This is an example of catalytic action, in which the speed 

 of a chemical change is enormously increased. The gases condensed in the 

 pores of the finely divided metal unite rapidly with production of sufficient 

 heat to cause the rest of the gases to unite with explosion. 



Chloroplatinic acid, H 2 PtCl 6 .6H 2 O (often called Platinic chloride), 

 is obtained as reddish-brown deliquescent crystals when platinum is 

 dissolved in aqua regia and the solution evaporated. It serves as a 

 valuable reagent for potassium and ammonium, as explained in con- 

 nection with the analytical reactions of these bodies. In the acid and 



