In the platinum metals the intermediate properties of feebly acid and feebly basic metals are developed with great clearness, so that there is not one sharply-defined acid anhydride among their oxides, although there is a great diversity in the grades of oxidation from the type RO4 to R2O. The feebleness of the chemical forces observed in the platinum metals is connected with the ready decomposability of their compounds, with the small atomic volume of the metals themselves, and with their large atomic weight. The oxides of platinum, iridium, and osmium can scarcely be termed either basic or acid; they are capable of combinations of both kinds, each of which is feeble. They are all intermediate oxides.
The atomic weights of platinum, iridium, and osmium are nearly 191 to 196, and of palladium, rhodium, and ruthenium, 104 to 106. Thus, strictly speaking, we have here two series of metals, which are, moreover, perfectly parallel to each other; three members in the first series, and three members in the second—namely, platinum presents an analogy to palladium, iridium to rhodium, and osmium to ruthenium. As a matter of fact, however, the whole group of the platinum metals is characterised by a number of common properties, both physical and chemical, and, moreover, there are several points of resemblance between the members of this group and those of the iron group (Chapter [XXII.]) The atomic volumes ([Table III.], column 18) of the elements of this group are nearly equal and very small. The iron metals have atomic volumes of nearly 7, whilst that of the metals allied to palladium is nearly 9, and of those adjacent to platinum (Pt, Ir, Os) nearly 9·4. This comparatively small atomic volume corresponds with the great infusibility and tenacity proper to all the iron and platinum metals, and to their small chemical energy, which stands out very clearly in the heavy platinum metals. All the platinum metals are very easily reduced by ignition and by the action of various reducing agents, in which process oxygen, or a haloid group, is disengaged from their compounds and the metal left behind. This is a property of the platinum metals which determines many of their reactions, and the circumstance of their always being found in nature in a native state. In Russia in the Urals (discovered in 1819) and in Brazil (1735) platinum is obtained from alluvial deposits, but in 1892 Professor Inostrantseff discovered a vein deposit of platinum in serpentine near Tagil in the Urals.[1] The facility with which they are reduced is so great that their chlorides are even decomposed by gaseous hydrogen, especially when shaken up and heated under a certain pressure. Hence it will be readily understood that such metals as zinc, iron, &c., separate them from solutions with great ease, which fact is taken advantage of in practice and in the chemical treatment of the platinum metals.[1 bis]
All the platinum metals, like those of the iron group, are grey, with a comparatively feeble metallic lustre, and are very infusible. In this respect they stand in the same order as the metals of the iron series; nickel is more fusible and whiter than cobalt and iron, so also palladium is whiter and more fusible than rhodium and ruthenium, and platinum is comparatively more fusible and whiter than iridium or osmium. The saline compounds of these metals are red or yellow, like those of the majority of the metals of the iron series, and like the latter, the different forms of oxidation present different colours. Moreover, certain complex compounds of the platinum metals, like certain complex compounds of the iron series, either have particular characteristic tints or else are colourless.
The platinum metals are found in nature associated together in alluvial deposits in a few localities, from which they are washed, owing to their very considerable density, which enables a stream of water to wash away the sand and clay with which they are mixed. Platinum deposits are chiefly known in the Urals, and also in Brazil and a few other localities. The platinum ore washed from these alluvial deposits presents the appearance of more or less coarse grains, and sometimes, as it were, of semi-fused nuggets.[2]
All the platinum metals give compounds with the halogens, and the highest haloid type of combination for all is RX4. For the majority of the platinum metals this type is exceedingly unstable; the lower compounds corresponding to the type RX2, which are formed by the separation of X2, are more stable. In the type RX2 the platinum metals form more stable salts, which offer no little resemblance to the kindred compounds of the iron series—for example, to nickelous chloride, NiCl2, cobaltous chloride, CoCl2, &c. This even expresses itself in a similarity of volume (platinous chloride, PtCl2, volume, 46; nickelous chloride, NiCl2 = 50), although in the type RX2 the true iron metals give very stable compounds, whilst the platinum metals frequently react after the manner of suboxides, decomposing into the metal and higher types, 2RX2 = R + RX4. This probably depends on the facility with which RX2 decomposes into R and X2, when X2 combines with the remaining portion of RX2.
As in the series iron, cobalt, nickel, nickel gives NiO and Ni2O3, whilst cobalt and iron give higher and varied forms of oxidation, so also among the platinum metals, platinum and palladium only give the forms RX2 and RX4, whilst rhodium and iridium form another and intermediate type, RX3, also met with in cobalt, corresponding with the oxide, having the composition R2O3, besides which they form an acid oxide, like ferric acid, which is also known in the form of salts, but is in every respect unstable. Osmium and ruthenium, like manganese, form still higher oxides, and in this respect exhibit the greatest diversity. They not only give RX2, RX3, RX4, and RX6, but also a still higher form of oxidation, RO4, which is not met with in any other series. This form is exceedingly characteristic, owing to the fact that the oxides, OsO4 and RuO4, are volatile and have feebly acid properties. In this respect they most resemble permanganic anhydride, which is also somewhat volatile.[3]
When dissolved in aqua regia (PtCl4 is formed) and liberated from the solution by sal-ammoniac ((NH4)2PtCl6 is formed) and reduced by ignition (which may be done by Zn and other reducing agents, direct from a solution of PtCl4) platinum[3 bis] forms a powdery mass, known as spongy platinum or platinum black. If this powder of platinum be heated and pressed, or hammered in a cylinder, the grains aggregate or forge together, and form a continuous, though of course not entirely homogeneous, mass. Platinum was formerly, and is even now, worked up in this manner. The platinum money formerly used in Russia was made in this way. Sainte-Claire Deville, in the fifties, for the first time melted platinum in considerable quantities by employing a special furnace made in the form of a small reverberatory furnace, and composed of two pieces of lime, on which the heat of the oxyhydrogen flame has no action. Into this furnace (shown in fig. 34, Vol. I. p. [175])—or, more strictly speaking, into the cavity made in the pieces of lime—the platinum is introduced, and two orifices are made in the lime; through one, the upper, or side orifice, is introduced an oxyhydrogen gas burner, in which either detonating gas or a mixture of oxygen and coal-gas is burnt, whilst the other orifice serves for the escape of the products of combustion and certain impurities which are more volatile than the platinum, and especially the oxidised compounds of osmium, ruthenium, and palladium, which are comparatively easily volatilised by heat. In this manner the platinum is converted into a continuous metallic form by means of fusion, and this method is now used for melting considerable masses of platinum[4] and its alloys with iridium.
To obtain pure platinum, the ore is treated with aqua regia in which only the osmium and iridium are insoluble. The solution contains the platinum metals in the form RCl4, and in the lower forms of chlorination, RCl3 and RCl2, because some of these metals—for instance, palladium and rhodium—form such unstable chlorides of the type RX4 that they partially decompose even when diluted with water, and pass into the stable lower type of combination; in addition to which the chlorine is very easily disengaged if it comes in contact with substances on which it can act. In this respect platinum resists the action of heat and reducing agents better than any of its companions—that is, it passes with greater difficulty from PtCl4 to the lower compound PtCl2. On this is based the method of preparation of more or less pure platinum. Lime or sodium hydroxide is added to the solution in aqua regia until neutralised, or only containing a very slight excess of alkali. It is best to first evaporate and slightly ignite the solution, in order to remove the excess of acid, and by heating it to partially convert the higher chlorides of the palladium, &c., into the lower. The addition of alkalis completes the reduction, because the chlorine held in the compounds RX4 acts on the alkali like free chlorine, converting it into a hypochlorite. Thus palladium chloride, PdCl4, for example, is converted into palladious chloride, PdCl2, by this means, according to the equation PdCl4 + 2NaHO = PdCl2 + NaCl + NaClO + H2O. In a similar manner iridic chloride, IrCl4, is converted into the trichloride, IrCl3, by this method. When this conversion takes place the platinum still remains in the form of platinic chloride, PtCl4. It is then possible to take advantage of a certain difference in the properties of the higher and lower chlorides of the platinum metals. Thus lime precipitates the lower chlorides of the members of the platinum metals occurring in solution without acting on the platinic chloride, PtCl4, and hence the addition of a large proportion of lime immediately precipitates the associated metals, leaving the platinum itself in solution in the form of a soluble double salt, PtCl4,CaCl2. A far better and more perfect separation is effected by means of ammonium chloride, which gives, with platinic chloride, an insoluble yellow precipitate, PtCl4,2NH4Cl, whilst it forms soluble double salts with the lower chlorides RCl2 and RCl3, so that ammonium chloride precipitates the platinum only from the solution obtained by the preceding method. These methods are employed for preparing the platinum which is used for the manufacture of platinum articles, because, having platinum in solution as calcium platinochloride, PtCaCl6, or as the insoluble ammonium platinochloride, Pt(NH4)2Cl6, the platinum compound in every case, after drying or ignition, loses all the chlorine from the platinic chloride and leaves finely-divided metallic platinum, which may be converted into homogeneous metal by compression and forging, or by fusion.[5]