THE PLATINUM METALS 877 



Is formed), arsenic and silicon all act more or less rapidly on platinum, 

 under the influence of heat. Many of the metals form alloys with it. 

 Even charcoal combines with platinum when it is ignited with it, and 

 therefore carbonaceous matter cannot be subjected to prolonged and 

 powerful ignition in platinum vessels. Hence a platinum crucible soon 

 becomes dull on the surface in a.smoky flame. Platinum also forms 

 alloys with zinc, lead, tin, copper, gold, and silver. 7 Although mercury 

 does not directly dissolve- platinum, still it forms a solution or amalgam 

 with spongy platinum in the presence of sodium amalgam ; a similar 

 amalgam is also formed by the action of sodium amalgam on a solution 

 of platinum chloride, and is used for physical experiments. 



There are two kinds of platinum compounds, PtX 4 and PtX 2 . 

 The former are produced by an excess of halogen in the cold, and the 

 latter by the aid of heat or by the splitting up of the former. The 

 starting-point for the platinum compounds is platinum tetrachloride, 

 platinic chloride, PtCl 4 , obtained by dissolving platinum in aqua 

 regia. 7bis The solution crystallises in the cold, in a desiccator, in the 

 form of reddish-brown deliquescent crystals which contain hydrochloric 

 acid, PtCl 4 ,2HCl,6H 2 0, and behave like a true acid whose salts cor- 

 respond to the formula K 2 I >t Cl 6 -- ammonium platinochloride, for 

 example. 7 tri The hydrochloric acid is liberated from these crystals by 

 gently heating or evaporating the solution to dryness ; or, better still, 

 after treatment with silver nitrate a reddish-brown mass remains 

 behind, which dissolves in water, and forms a yellowish-red solution 

 which on cooling deposits crystals of the composition PtCl 4 ,8H 2 0. 

 The tendency of PtCl 4 to combine with hydrochloric acid and water 

 that is, to form higher crystalline compound^ is evident in the 

 platinum compounds, and must be taken into account in explaining 

 the properties of platinum and the formation of many other of its 

 complex compounds. Dilute solutions of platinic chloride are yellow, 

 and are completely reduced by hydrogen, sulphurous anhydride, and 

 many reducing agents, which first convert the platiuic chloride into 



* It is necessary to remark that platinum when alloyed wifh silver, or as amalgam, 

 is soluble in nitric acid, and in this respect it differs from gold, so that it is possible, 

 by alloying gold with silver, and acting on the alloy with nitric acid, to recognise 

 the presence of platinum in the gold, because nitric acid does not act on gold alloyed 

 with silver. 



7 t>ta Ptd 4 is also formed by the action of a mixture of HC1 vapour and air, and by 

 the action of gaseous chlorine upon platinum. 



7 lrl Pigeon (1891) obtained fine yellow crystals of PtH 3 Cl 6 ,4H 2 by addingstrong sul- 

 phuric acid to a strong solution of PtH 2 Cl 6 ,6H 2 If crystals of H 2 PtCl 6) 6H 2 be 

 melted in vacuo (60) in the presence of anhydrous potash, a red-brown solid hydrate is 

 obtained containing less water and HC1, which parts with the remainder at 200, leaving 

 anhydrous PtCl 4 . The latter does not disengage chlorine before 220, and is perfectly 

 eoluble in water. 



