Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/202

192 PLATINUM evaporated with one of chloride of potassium to a sufficiently small volume, deposits rose-coloured crystals of a double salt PtCl., + 2KCl = PtCl 4 K... From a solution of this double salt platinous hydrate, Pt("OH) 2 , is obtained, by boiling it with the by boiling chloroplatinic acid solution with excess of caustic soda and then acidifying with acetic acid, as an almost white precipitate, Pt(OH) 4 + 2H,0, which loses its 2H 2 at 100 C. and becomes brown ; at a certain higher temperature it loses all its water and assumes the form of the black anhydride Pt0 2 . Both oxides are bases in so far as their hydrates combine with a limited number of acids; towards strong bases they behave as feeble acids. Only a few of the salts of the acid Pt0 2 have been investigated. Either oxide when heated to redness breaks up into oxygen and metal. Platin- Ammonium Compounds. In this very numerous family of bodies a compound radical containing platinum and some ammonia residue plays the part of a basilous metal. The first member was discovered by Magnus in 1828. By adding ammonia to a hydrochloric solution of platinous chloride, he obtained a green precipitate of the composition PtCl 2 .N 2 H fi , which soon became known as "Magnus s green salt," and served as a starting point for subsequent investigations. Platinocyanides. These were discovered by L. Gmelin, who obtained the potassium salt Pt(NC) 4 K 2 by fusing the metal with prussiate of potash. Martius s method is more convenient : chloroplatinate of ammonia is heated in a strong mixed solution of caustic potash and cyanide of potassium as long as ammonia is going off. The solution on cooling deposits crystals contain ing SH.,0 of water, which appear yellow in transmitted and blue in reflected light. From the potash salt numerous other platiuocyanides can be made by double decompositions ; and a very interesting series is derived from these by the addition of chlorine or bromine. All these bodies are distinguished by their magnificent fluorescence. The Polyxene Metals Generally. The metals all exist in the three forms of "black," "sponge," and compact regulus. The colours of the compact metals are shades of white, except in the case of osmium, which forms blue crystals. Platinum, palladium, and rhodium are ductile ; the rest break under the hammer. 1 In regard to specific gravity they arrange themselves into two groups as shown by the following table, which at the same time gives the atomic weights (those of Pt and Ir according to Seubert) and the formula?, of the most stable chlorides : The order of fusibility is as follows : Pd, Pt, Ir, Rh, Ru, Os. Palladium almost fuses in the strongest heat of a wind furnace, but like the four metals following requires an oxyhydrogen flame for its real fusion ; osmium has never been fused at all ; but it volatilizes abundantly at the highest temperature producible by the oxyhydrogen blast. Action of Air. Platinum and palladium do not oxidize at any temperature ; rhodium also does not oxidize by itself, but when cupelled with lead it remains as monoxide RhO. Compact iridium does not oxidize appreciably even in the heat; but the finely-divided metal, at some temperature below 800 C., suffers gradual conversion into Ir.,0 :) , which when heated more strongly begins to dissociate at 800, and is completely reduced at 1000 C. Ruthenium draws a film of oxide in even cold air ; at a red heat it passes into Ru._,0 8 , which retains its oxygen at a white heat. Osmium (the finely-divided metal), when heated in air to about 400 C., takes fire and burns into vapour of tetroxide, Os0 4 . This and the analogous ruthenium compound are the only volatile oxides of the group. Water. None of our metals seem to decompose water or steam at any temperature. Hydrochloric Acid acts slowly on palladium in the presence of air; otherwise there is no action in any case. Hot Nitric Acid dissolves palladium as nitrate Pd(N0 8 ) 2 , and converts finely divided osmium into tetroxide vapour. Compact osmium, and platinum, iridium, and rhodium in any form, are not attacked by the acid. 1 It still remains to be seen linw far this latter statement holds for the absolutely pure metals. Mr Georee Matthey lias succeeded in producing iridium wire, which could be bent into a loop without breaking. Xame. Atomic Weight. = 16. Specific Gravity. Chlorides. Platinum . Pt =194-8 21-50 PtCl., ; PtCl 4 . Iridium Ir =193-0 22-38 Ir.Cl 6 . Osmium Os =195 22-48 (?) Palladium Pd = 106 6 11-4 PdCL. Rhodium.. Rh = 104 3 12-1 Rh^Clg. Ruthenium Ru=103 8 12-26 Ru,Cl 6 + a,-RCl. Aqua Regia, in the heat, dissolves palladium (very readily) and platinum (somewhat more slowly) as MeCLH a ; only the palladium compound is very unstable, being completely reduced to dichloride, PdCl 2 , by mere evaporation over a water-bath. Iridium black, or iridium alloyed with much platinum, dissolves slowly as IrClgHjj, readily reducible (by, for instance, addition of alcohol, or evaporation to dryness and heating of the residue to about 150 C.) to lr 2 Cl 6 . Compact iridium, like ruthenium or rhodium, is hardly attacked even by the hot acid ; rhodium exhibits the highest degree of stability. Native osmiridium is not touched by aqua regia. Osmium, in the heat at least, becomes tetroxide. Free Chlorine combines directly with all polyxene metals at suitable temperatures. As a disintegrator it is useful chiefly for the manipulation of osmiridium and other such platinum-ore components as refuse to dissolve in aqua regia. The action of the gas is greatly facilitated by the presence of fixed alkaline chloride. Polyxene Oxides and Salts. Monoxides have been produced from platinum, palladium, ruthenium, and osmium. PtO and PdO are decided, the other two are very feeble bases. Sesquioxides, Me 2 3 , have been got from rhodium, iridium, ruthe nium, and osmium. All are basic. Binoxidcs, Me0 2 , exist from all the metals except rhodium. Pd0 2 , like Pt0 2 (see above), is basic or feebly acid ; Ir0 2 is a feeble base ; Ru0 2 and Os0 2 are neutral. Tetroxides, Me0 4 , are formed by osmium and ruthenium only. Both Os0 4 and Ru0 4 are easily fusible and very volatile solids. Their vapours have a most powerful smell and are most dangerously poisonous. Trioxidcs and Hcptoxidcs do not exist as substances ; but the groups Ru0 3 , Os0 3 , and R 2 7 unite with alkalies into soluble salts analogous to chromates and permanganates in their constitution respectively. The oxides MeO, Me 2 g , Me0 2 are as a rule pre- parable by evaporating a solution of the respective chloride or potassio- &c. chloride to dryness with excess of carbonate of soda, heating the residue to dull redness, and removing the alkaline chloride and excess of carbonate by lixiviation with water. The oxides remain as very dark-coloured powders insoluble in acids. The corresponding hydrates are precipitated from the solutions of the chlorides or potassio- &c. chlorides, on addition of excess of caustic potash or soda and heating. These hydrates of the oxides are soluble in certain aqueous acids with formation of salts, and in this limited sense only the " oxides " can be said to be " bases." Salts. Of these the most characteristic and the best known are compounds of certain of their chlorides with alkaline chlorides. 1. The compounds MeCl fi R 2 (chloroplatinates and analogues), formed by all polyxene metals, except rhodium, are all crystalline salts, more or less soluble in water but as a rule insoluble or nearly so in alcohol. The acids MeCl 6 H 2 , in which Me is not platinum, exist only as unstable solution, which by the action of excess of caustic soda in the heat, if not by the action of a gentle heat alone, are all reduced to lower chlorides ; only the platinum compound possesses a higher degree of stability. 2. Chlorides, MeCl 2 , and potassio- &c. chlorides, MeCl 4 R 2 , exist only in the platinum and palladium series. 3. Hexachlorides, Me 2 Cl 6 , and compounds thereof with other chlorides are formed only by rhodium, iridium, and ruthenium. Preparation of the Rarer Polyxene Metals. For this the residues obtained in the industrial extraction of platinum from the ore form the natural raw material. These residues are two in number, (1) that part of the ore which resisted the action of aqua regia (we will call it the osmiridium residue), and (2) the filtrate from the chloroplatinate of ammonia. 1. Part of the osmiridium in the first residue consists of scales or grains so hard that they cannot be powdered even in a steel mortar. They must be reduced to a fine powder, which is best done by fusing them up with eight to ten parts of zinc and then driving off the "solvent" in a wind-furnace. The osmiridium remains as a dark friable mass, which is easily powdered and in corporated with the originally sifted-off part. The disintegration of the residue may then be effected, according to Wohler, by mixing it with its own weight of common salt and exposing the mixture to a current of chlorine at a dull red heat within a com bustion tube. If the chlorine is moist much of the osmium goes off as vapour of tetroxide, which must be collected in solution of caustic potash. After complete chlorination the contents of the tube are treated with water, when as a rule some undisintegrated osmiridium remains which is filtered off. The solution is mixed with nitric acid and distilled as long as any osmic tetroxide vapours are going off, which are readily recognized by their powerful pungent smell, and of course must be carefully collected in caustic potash ley. The residual liquor (which contains the iridium as IrCl.Nag) is

supersaturated with carbonate of soda, and evaporated to dryness,