PLATINUM METALS. Although generally found in nature in the metallic condition, native platinum is never chemi cally pure. With it in varying proportions five other metals are 1 generally associated, and to this group of six elements the name "platinum metals" has been given. The six metals in the order of their atomic numbers and weights are as f ollows:— Atom- Atom- Sym- Atom- Atom Metal bol ic No. ic Wt. Metal bol ic No. icWt.
Ruthenium Ru 44 IoI•7 Osmium Os 76 590.8Rhodium Rh 45 102.91 Iridium Ir 77 193.1 Palladium Pd 46 106.7 Platinum Pt 78 195•23Although certain properties are common to the whole group, yet very great chemical and physical differences are found between the various members. It will be noticed that the atomic numbers of the first three and second three members are consecutive and that a common difference of 32 separates them. This common difference in atomic number corresponds to similarity in chemical properties between members differing by this constant number ; at the same time similarities, more especially physical, are noticed amongst the members of each group.
One property common to the whole of the platinum metals is the high temperature of their melting points. The most easily fusible of them, palladium, only assumes the liquid condition at about 1,55o° C; platinum comes next with a melting point of about 1,750° C and the other metals fuse at temperatures inter mediate between these and the melting point of osmium, which is about 2,700° C. Another property common to this group of metals is the valuable one of resisting chemical action and especially oxidation (the particular cases of ruthenium and osmium as regards oxidation will be dealt with when those metals are considered in detail).
Another property common to all members of the group is their great catalytic action, that is, the power of bringing about chemi cal action between other substances without themselves under going any alteration. In small chemical operations platinum and palladium are extensively used, but for large-scale or commercial working these metals, on account of their high cost, are not suitable unless the wastage is very small or negligible. Thus platinum and palladium are most efficient catalysts in the hydro genation of oils and fats for the production of edible glycerides (q.v.) ; they are, however, seldom used for this purpose since the inevitable wastage would cause the process to be too costly.
Metallic nickel or certain of its compounds are used instead, with the result that the operation has to be carried out at a con siderably higher temperature, resulting in the destruction of some of the important vitamins which would have survived had palladium been used instead of nickel.
As regards density or specific gravity, the platinum metals resolve themselves naturally into two groups : the three members of lowest atomic weight have specific gravities of 11-12, i.e., somewhat higher than, but not far removed from, that of silver, 0–I I ; the three higher members, however, are much denser and their specific gravities range from 21.4 for platinum to 22.5 for osmium which is the heaviest massive substance known under terrestrial conditions. As the specific gravity of platinum is near to that of gold, 19.4, an alloy of platinum with some base metal is easily prepared possessing the same density as gold.
Being bad conductors of electricity, platinum and its allied metals in the form of wire easily become incandescent when an electric current is passed through them. Incandescent lamps with very luminous filaments for electric lighting could be made with out any danger of fusing of these metals. The first practical success was attained with filaments of metallic osmium, which was found to glow more vigorously and thus give better illumi nation than the other platinum metals. This fact is perpetuated in the name "Osram" which is still a trade-mark of certain lamp manufacturers, although the use of osmium for this purpose has long been discontinued. Although the platinum filaments for lamps was soon displaced by tantalum and tungsten, yet for a considerable time platinum entered into their construction, as some material had to be used, which could be fused into the glass without cracking, to carry the necessary leading-in wires. The only suitable substance then known was platinum, which has a coefficient of expansion almost the same as that of glass, and for a considerable time this was the only available material. This wasteful use of platinum has now been obviated by the production of alloys of much cheaper material having the same coefficient of expansion as ordinary glass.