Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/549

Rh PLATINUM GROUP.] H E M I S T li Y 537 ruthenium, rhodium, and palladium, and between osmium, iridium, and platinum, is similar to that which obtaius between iron, cobalt, and nickel ; they are indeed very closely related to these last elements, the resemblance in properties being greatest between iron, ruthenium, and osmium ; between cobalt, rhodium, and iridium ; and between nickel, palladium, and platinum. Palladium and platinum also manifest much analogy to silver and gold, and ruthenium and osmium exhibit properties which con nect them with the elements molybdenum and tungsten. They are white, or nearly white, hard metals, of high specific gravity, osmium being the heaviest of the elements ; they may be melted, but only at a very high temperature with the aid of the oxyhydrogen blowpipe, with the exception of osmium, which does not melt, but volatilizes, palladium being more fusible, and rhodium, ruthenium, and iridium much less fusible, than platinum. Palladium is soluble in concentrated nitric acid, and platinum in nitrous acid, as well as in aqua-regia ; the remaining metals are difficultly soluble or are insoluble in aqua-regia. They are all attacked on fusion with alkalies and potassic nitrate. Most of them possess in a high degree the property to which allusion has already been made of con densing gases on their surfaces, especially when in a finely divided state ; thus, finely divided platinum at ordinary tsmperatures absorbs about 240 times its volume of hydro gen, and finely divided palladium at 200 C. absorbs nearly 700 times its volume of hydrogen. When heated in an atmosphere of the gas platinum and palladium also occlude hydrogen when in a compact state. The latter metal, accord ing to Troost and Hautefeuille, forms a definite compound with hydrogen of the composition Pd 2 H, which is further capable of absorbing hydrogen in considerable quantity ; the density of the pure compound is 11 06, that of the melted metal from which it was prepared being 12, and, therefore, if it be supposed that the elements unite without condensation, that is to say, that the hydrogen has the same density as it would have if solidified in the free state, the density of solidified hydrogen is 62, or -slightly higher than that of lithium. Iron, cobalt, and nickel also possess the property of occluding hydrogen, although only in a slight degree. The following chlorides and oxides of the platinum metals are known : Eu,Cl 4 Os 2 Cl 4 Pd.Cl. Pt,Cl 4 Ru 2 3 03,0, Rh 2 2 Ir 2 3 PJ,0 Chlorides. Eu 2 Cl 6 OsgClg RhjCl, Ir.CL Oxides. Ru 2 3 os,o, Kh,0 KuCl 4 OsCl 4 IrCL PdCl 4 PtCl 4 Ru. 2 4 Os 2 4 Rl ,04 Ir 2 4 Pd,0 4 Pt,0 4 Ruthenium absorbs oxygen at a red heat, and is con verted into the oxide, Ru. 2 3, which is its most stable oxide, whereas the dioxide is the most stable oxide of osmium. Ruthenium tetroxide, RuO 4, is a golden-yellow crystalline substance, sparingly soluble in water, which melts at 58 C., and boils a little above 100* C. ; hydro chloric acid converts it into the chloride Ru 2 Cl 6, with evolution of chlorine. The solution of this chloride is decomposed by boiling, the hydroxide Ru. 2 (OH) 6 being pre cipitated : like ferric chloride, it is reduced to the chloride Ru 2 Cl 4 by hydrogen sulphide, the colour changing from orange-yellow to blue. Osmium, in the compact state, is slowly converted into- the tetroxide, OsO 4, when roasted in air, but the pulveru lent metal obtained by reducing this oxide by hydrogen oxidizes spontaneously in the air. Osmium tetroxide is a white, crystalline, extremely volatile substance; it melts considerably below ] 00 C., and boils at a temperature a little above its melting point. The vapour density deter mination shows that it is correctly represented by tha formula Os0 4. Its vapour has an intolerably pungent odour, and is excessively poisonous. It is dissolved by water, and is a powerful oxidizing agent, but does not evolve chlorine on treatment with hydrochloric acid ; most metals, even silver and mercury, excepting gold and the platinum metals, partially reduce its solution and cause the separation of osmium ; it is decomposed by ammonia, being converted into the hydroxide Os(OH) 4, with evolution of nitrogen: 3OsO 4 + 4NH 3 = 3Os(OH) 4 + 2X 2. A lower hydroxide of osmium, Os 2 (OH) 4, is known, which like ferrous hydroxide rapidly oxidizes on exposure to the air. The tetroxides of ruthenium and osmium are apparently perfectly neutral bodies, possessing neither basic nor acid properties ; the remaining oxides exhibit only feeble basic properties, and furnish unstable salts, of which very little is. known. The oxides of ruthenium are readily reduced when heated in hydrogen ; osmium tetroxide is reduced at a red heat, but the lower oxides of osmium are reduced by hydrogen at ordinary temperatures. Ruthenium and osmium furnish ruthenio- and osrnio- cyanides, such as K g Ru 2 (CN) 12 and K g Os 2 (CX) 19, iso- morphous with the corresponding ferrocyanides, from which the acids H 8 Ru 2 (CN) 12 and H 8 Os 2 (CX) 12 may be prepared ; the compounds analogous to the ferricyanides are not known. On heating the rnetals with potassium nitrate, compounds analogous in composition to potassium ferrate, but more stable, are formed; potassium rutheniate, K RuO 4&amp;gt; is apparently much less stable than potassium osmate, K 2 0s0 4, which may be obtained in large rose-coloured crystals ; their solutions are at once decomposed by acids, the former yielding the hydroxide Ru.,(OH) 6 with evolution of oxygen, and the latter the hydroxide Os(OH) 4 and osmium tetroxide. The chlorides of ruthenium and osmium form numerous double salts, but the most important are the ruthenio- and osmio-chlorides, such as K 9 RuCl 6 and K.,OsCl 6, which are isomorphous with the corresponding platiuochlorides. Iridium and rhodium are insoluble in aqua-regia, and even when in a finely divided state are only slowly oxidized at a red heat ; the most stable oxide of iridium is the dioxide, and the tendency of iridium is always to form compounds which correspond to this oxide in composition, but compounds corresponding to the oxide Rh 2 O 3 are more often produced from rhodium, this being apparently its most stable oxide. The oxides of rhodium are reduced when heated in an atmosphere of hydrogen, but the oxides of iridium are reduced at ordinary temperatures. The differ ence between the two metals is therefore similar to that between ruthenium and osmium, which they much resemble in their general behaviour, although neither furnishes a tetroxide, and rhodium forms only one stable chloride, the hydroxide Rh(OH) 4 being dissolved by hydrochloric acid with evolution of chlorine. Iridium tetrachloride forms double chlorides or iridiochlorides isomorphous with the corresponding platinochlorides. Like cobalt, rhodium and iridium form compounds which are not only analogous in composition to the ferricyanides, but also isomorphous with them. A number of rhodium salts corresponding to the oxide Rh. 2 O 3 are known, but they crystallize with difficulty ; the hydroxides of iridium are soluble in acids, but no definite oxygen salts of iridium have been obtained. The hydroxides Rh 2 (OH) 6 and Ir.,(OH) 6 manifest feeble acid V. 68