Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/201

Rh PLATINUM 191 periments on annealed welded wire). Unit length of the (fused) metal expands by 000907 from to 100 0. (Fizean). The specific conductivity for heat at 12&quot; C. is 8 4, for electricity at C. 1(5 4 (silver = 100). The statement regarding electricity refers to the annealed metal. The fusing point, according to recent determina tion by Violle, is 1779 C. ; the same experimenter finds for the true specific heat 5Q/SY = 0317 + 00001 2t (centigrade scale). When platinum is heated beyond its fusing point, it soon begins to vola tilize. The fused metal, like silver, absorbs oxygen, and consequently &quot;spits &quot; on freezing. At a red heat the then viscid metal, as Graham has shown, &quot;occludes&quot; hydrogen gas; i.e., it dissolves the gas (just as, for instance, liquid water would), which explains the fact pre viously discovered by Deville that a platinum tube, although it may be perfectly gas-tight in the cold, at a red heat allows hydrogen (but not, for instance, oxygen, nitrogen, or carbonic acid) to pass through its walls. According to Graham the quantity of gas occluded is independent of the surface of the metal operated on, but proportional to its weight. No gas is taken up in the cold ; but the gas occluded at a red heat, though extractable at that temperature by means of an absolute vacuum as producible by a Sprengel pump (see MERCURIAL Am PUMP, vol. xvi. p. &quot; is retained on cooling and cannot be thus liberated at the ordinary temperature. The volume of hydrogen absorbed by unit-volume of metal at a red heat under one atmosphere s pressure was found, in the case of fused metal, to vary from 13 to 21 volume measured cold ; in the case of merely welded metal, from 2 &quot;34 to 3 8 volumes (compare Palladium below). Oxygen gas, though absorbed by the liquid, is not occluded by the solid metal at any temperature, but when brought in contact with it at moderate tem peratures sutlers considerable condensation at its surface. The thin condensed film of oxygen exhibits a high degree of chemical activity : a perfectly clean piece of platinum foil, when immersed in a mixture of hydrogen or ammonia or other combustible gas and air, begins to glow and starts a process of slow combustion or there may be an explosion. The spongy metal of course exhibits a very high degree of activity: a jet of hydrogen gas when made to strike against a layer of spongy platinum causes it to glow and takes fire. This is the principle of the (now defunct) Dobereiner lamp. But the most striking effects are produced by a peculiar kind of very finely divided platinum, which was discovered by Liebigand called by him platinum black on account of its resemblance to lamp-black. A particularly active &quot; black &quot; is produced by dropping platinum chloride solution into a boiling mixture of three volumes of glycerin and two of caustic potash of 1 08 specific gravity. Platinum black, according to Liebig, absorbs 800 times its volume of oxygen from the air, and in virtue thereof is a most active oxidizing agent, which, in general, acts &quot; catalytically &quot; because the black, after having given up its oxygen to the oxidizable substance present, at once takes up a fresh supply from the atmosphere. For examples see FERMENTATION, vol. ix. pp. 94-98. Platinum Alloys. Platinum alloys of almost any kind are easily produced syntheti cally ; and, as a rule, a temperature little if at all above the fusing point of the more fusible component suffices to start the union. We will begin with the cases in which the metal combines with another member of its own family. Iridium. In the heat of an oxyhydrogt-n name the two metals unite permanently in all pro portions. The alloy has pretty much the appearance of platinum, but it is less fusible, harder, more elastic, specifically heavier, and less readily attacked by aqua regia, all these qualities increasing as the percentage of iridium increases. The 19 per cent, alloy was produced for the first time by G. Matthey. It has the hardness and elasticity of soft steel (modulus of elasticity = 22, 000 for milli metre and kilogramme), and is hardly attacked by aqua regia. Alloys richer in iridium are difficult to work. The 10 per cent, alloy on the other hand still retains enough of the virtues referred to to be far superior to platinum itself perhaps we might say, to any other solid as a material for standard measures of length or weight. In 1870 Messrs Johnson, Matthey, & Co. exhibited a standard metre made of this alloy, and it gave such unqualified satisfaction that the international metric committee which sat in Paris some years ago adopted it for the construction of their standards. Rhodium. An alloy of 30 per cent, of this metal and 70 of platinum is absolutely proof against aqua regia, but is very expensive. Deville and Debray once elaborated an igneous process for producing, directly from the ore, a triple alloy of platinum, iri dium, and rhodium, which is quite workable and, besides being more highly infusible than platinum, is almost proof against aqua regia. Crucibles made of this alloy used to be sold in Paris and elsewhere at moderate prices ; but they are now no longer to be had. Gold. This metal unites with platinum in all proportions, forming greyish-yellow or greyish-white alloys. A graduated series of these alloys was recommended by Schertel and Ehrhard as a means for defining certain ranges of high temperatures. According to their experiments, while the fusing-point for gold was 1075 C., and for platinum 1775 J, it was 1130 for 10 per cent, of platinum, 1190 for 20, 1255 for 30, 1320 for 40, 1385 for 50, 1460 for 60, 1535 for 70, 1610 for 80, and 1690 for 90 per cent. Silver and platinum unite readily in any proportion, but the alloys are in general liable to &quot;liquation&quot; (see METALS, vol. xvi. p. 67). Now platinum is as proof against nitric acid as gold ; and yet these alloys cannot, like gold-silver, be parted by means of nitric acid ; because, if the alloy is rich enough in silver to be at all attacked by the acid, part at least of the platinum passes into solution along with the silver. But concentrated oil of vitriol effects a sharp separation ; the platinum remains. A considerable variety of alloys of platinum with other noble metals are used in mechanical dentistry. The following examples may be quoted : 667 per cent, of gold and 33 of platinum ; platinum 50, silver 25, palladium 25 ; platinum 417, gold 25, palladium 33 3. Of the great variety of alloys of platinum with base metals which have been recommended as substitutes for noble metals or other wise we select the following : Platinum. Silver. Copper. Tin. Brass. Nickel. 1 19

1

2 1

26

3 2 1 5

2 1 4 1

10

100 5 1 2

20

100 6 0-5

15

100 / 20

20

100 8 5 to 10

120 60 (1) Known to jewellers and dentists as hard platinum ; (!) a rose-coloured fine grained ductile alloy; (o) introduced bj Bolzani in Paris as an imitation gold ; (4 to 7) platinum bronzes, recommended (4) for knife and fork handles, (5) for bells, (6) for articles de luxe, (7) for telescopes ; (8) not subject to oxidation. Platinum Compounds. Platinum is not changed by air, water, or steam at any tempera ture. It is proof against the action of all ordinary single acids, including hydrofluoric, in the heat or cold. Aqua regia (a mixture of hydrochloric and nitric acids) dissolves it slowly as chloro- platinic acid PtClgH.,. The metal is not attacked by even very strong boiling caustic potash or soda ley, nor is it changed by fusion with carbonate of soda or potash. Carbonate of lithia, and the hydrates of potash, soda, and baryta, however, when fused in platinum vessels, attack them strongly, with formation of com pounds of Pt0 2 with the respective bases. According to recent experiments by the writer, none of these reactions go on in the absence of air ; hence, for instance, a fusion with caustic baryta or potash can safely be carried out in a platinum crucible if the latter is protected by an atmosphere of hydrogen or nitrogen. Fused hepar (alkaline sulphide) dissolves platinum at a red heat ; so does- fused cyanide of potassium, especially if mixed with caustic potash. Chloroplatinic Add. The solution of the metal in aqua regia is evaporated down repeatedly in a water bath with hydrochloric acid to destroy the excess of nitric acid and the very concentrated solution allowed to stand, when the acid gradually separates out in brown-red deliquescent crystals of the composition PtCl 6 H 2 + 6H.,0, which are abundantly soluble in water and also easily in even strong alcohol. The aqueous solution, if free of iridium and platinous chlorides, is of a rich but clear yellow colour free of any tinge of brown. The &quot;chloride of platinum&quot; solution of the analyst is an aqueous solution of this acid. When the solution is mixed with those of certain chlorides, the 2HC1 are displaced by their equivalent of metallic chloride, and metallic &quot;chloroplatin- ates&quot; are produced. Of these the potassium (ru &amp;gt; idium and caesium) and the ammonium salts are most easily prepared, by addition of the respective chlorides to a moderately strong solution of chloroplatinic acid ; they come down almost completely as pale yellow crystalline precipitates, little soluble in cold water and very nearly insoluble in alcohol. The sodium salt PtC] 6 Na 2 -t-6H.,O and the lithium salt PtCl ri Li.,-f 6H 2 are readily soluble in water and in aqueous alcohol (the Li L ,-compound dissolves even in ab solute alcohol) ; hence &quot; chloride of platinum &quot; is used for the separation of K, NH 4, Kb, Cs from Na and Li. On the other hand chloride of potassium or ammonium may serve as a precipitant for platinum, but in this case a large excess of a concentrated solution of the precipitant must be used to bring the solubility of the chloroplatinate precipitate to its minimum. Gold, copper, iron, and many other metals not belonging to the polyxene group, if present, remain dissolved. Real platinic chloride, PtCl 4, can be produced from the acid PtCl K H 2 only by precipitating from its solution the chlorine of the 2HCl&quot; by the exact equivalent of nitrate of silver. The filtrate when evaporated (cold) over vitriol deposits red crystals of the composition PtCl 4 -f 5H 2 0. When chloropla tinic acid is heated to 300 C. it loses its 2HC1 and half the chlorine of its PtCl 4 and platinous chloride, PtCl 2, remains as a dull green powder, insoluble in water but soluble in aqueous hydrochloric acid. Either chloride when heated to redness leaves spongy I metal. The hydrochloric solution of platinous chloride, when