1902 Encyclopedia > Platinum and the Platinum Metals

Platinum and the Platinum Metals




PLATINUM AND THE PLATINUM METALS. The metals platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os) are united into a family by a striking similarity in chemical characters and by their association in natural occurrence. A rather rare ore, called platinum ore or polyxene, is almost the only native material which is available for their extrac-tion ; it contains them all in the reguline form. Traces of platinum are found in almost all native gold.

As early as the first half of the 16th century it appears to have been noticed that the gold ore in the Spanish mines of Darien includes grains of a white metal, endowed with the qualities of a noble metal and yet distinctly different from silver; but the fact remained unknown in Europe because the Spanish Government, having found out that the new metal lent itself most admirably for the adulteration of gold, prohibited its exportation. Only from about the middle of last century did the metal begin to find its way to Europe and to become known there, at first as a curiosity, under its Spanish name of " platina del Pinto " (the little silver from the river Pinto). Its chemi-cal individuality and qualities were established by the successive labours of Schefier (1752), Marggraf (1757), Bergmann (1777), and others. An amateur, Count von Sickingen, it appears, was the first who succeeded in working the metal (1772); the first platinum crucible was pro-duced by Achard (1784). Achard's mode of rendering the native metal amenable to mechanical working was founded upon the fact that it forms a readily fusible alloy with arsenic, from which the latter can be driven off again by intense heating. This method was worked industrially for a time, but subsequently superseded by another superior process, which is usually credited to Wollaston, because it was he who, after having wrought it as a rich source of revenue for years, published it in 1828. But as early as 1800 Knight of London had published all that is essential in the process; and Messrs Johnson, Matthey, & Co. in-form the writer that Wollaston obtained the secrets of both the refining and the compressing of the spongy into com-pact metal from a relative of theirs, Thomas Cock, who, they are convinced, is the true inventor. Undisputed merits of Wollaston's are his discoveries of palladium (1803) and rhodium (1804). About the same time iridium and osmium were discovered by Smithson Tennant.

Platinum ore well deserves its cognomen of " polyxene," because it is a most complex mixture of mineralogical species, including (1) a number of heavy reguline species designated as platinum, osmiridium, iron-platinum, platin-iridium, iridium, palladium (also gold), and (2) a number of non-metallic species, notably chrome-iron ore, magnetic oxide of iron, zircone, corundum, and occasionally also diamond. The reguline components always form detached granules, which are generally small, but occasionally assume considerable dimensions. The Demidoff museum contains a native platinum lump weighing 21 pounds troy. The ore, as already stated, was discovered first in South America ; it is found there chiefly in the provinces of Choco and Barbacos, New Granada, and also in Brazil. It occurs besides in San Domingo, in California, at the Rogue river in Oregon, in Canada, and in the island of Borneo. But the richest deposits are those of the Ural Mountains; these were discovered about 1823, and have been wrought by the Russian Government since about 1828. Part at least of the Ural ore, as Daubre showed, was embedded origin-ally with chrome-iron in a serpentine derived from olivine. The very variable percentages of the several components range approximately as follows:—platinum, 60 to 87; other polyxene metals 3 to 7; gold up to 2 and more; iron 4 to 12; copper 0 to 4; non-metallic gangue 1 to 3.

Platinum, though a noble metal chemically, has too modest an appearance to lend itself much to the jeweller's purposes. The Russian Government used, for a while, to strike platinum coins, but soon came to give up the prac-tice on account of the immense fluctuations in the commer-cial value of the metal. Almost all the platinum produced now-a-days is made into chemical utensils. Platinum, in fact, is the metal of the chemist. " Without platinum crucibles, which share the infusibility of porcelain with the chemical inertness of gold ones the composition of most minerals could not have been ascertained " (Liebig), and chemistry generally could not have come up to its present level. In industrial chemistry platinum is used chiefly for the construction of those stills for the concentration of oil of vitriol which, although a single one costs a fortune, are cheaper in the long run than glass retorts.

The technical extraction of platinum from its ore is to the present day effected everywhere by some modification or other of the so-called "Wollaston" process. Heraeus of Hanau operates as follows. The ore is digested within glass retorts in aqua regia diluted with three times its weight of water, an over-pressure of some 12 inches of water being established within the retorts to accelerate the process, which always takes several days. The whole of the osmiridium, along wuth more or less of other polyxene metals, and the "sand" (corundum, chrome-iron, &c.) remain undissolved, as a heavy black deposit; the platinum, palladium, part of the rhodium, and more or less of the other three polyxene metals pass into solution, the platinum, iridium, and pal-ladium as tetrachlorides. From the clarified solution the whole (almost) of the platinum can be precipitated as PtCl6(NH4)2 by addition of a large excess of sal-ammoniac ; and this simple process used to be adopted formerly. But the precipitate then includes much chloro-iridiate of ammonium IrCl6(NHj2 and other impuri-ties. Heraeus, therefore, first evaporates to dryness and heats the residue to 125° C. for a sufficient time, to reduce the palladic and iridic chlorides to the lower stages of PdCl2 and Ir2Cl6, which form soluble double salts with sal-ammoniac. The heated residue is dissolved in water acidulated with hydrochloric acid, the solution, filtered, and mixed with hot concentrated solution of sal-ammoniac; when a (relatively) pure chloroplatinate comes down as a yellow precipitate (the iridium compound is dark-red), which is washed, first with saturated sal-ammoniac solution, then with dilute hydro-chloric acid. The precipitate needs only be exposed to a dull red heat to be converted into "spongy platinum," i.e., metallic platinum in the form of a grey porous mass. As platinum is infusible even at the highest temperature producible in a wind-furnace, the spongy metal cannot be fused together into a regulus like an ordinary metal; but it shares with wrought iron the rare quality of assuming a high degree of softness and viscosity at a strong red heat; and consequently the sponge, after a preliminary compression by purely mechanical means, needs only be exposed to a strong heat to " frit" into a coherent mass ; and this mass, by repeated forging at a white heat is readily made into a perfectly homogeneous compact bar, which, as the metal is very ductile, is easily rolled out into sheet or drawn into wire. In the former form more especially it goes into the workshop to be made into utensils.

This process of welding at the time of Aehard (who used it first) and of Knight was a necessary make-shift ; but it is singular that it was retained long after the invention of the oxyhydrogen blast (see vol. xviii. p. 105), by means of which platinum can be fused as easily as lead can in an ordinary fire. With the oxyhydrogen-blowpipe Hare, as early as 1847, fused 970 grammes (upwards of two pounds) of platinum into one regulus. Yet platinum manu-facturers did not utilize this obvious process until Deville and Debray, in 1859, again demonstrated its practicability. Their furnace is of the simplest description. Two flat pieces of quick-lime, scooped out so as to represent two cupels, are placed one upon the other so that they enclose a fiat space similar in form to two superimposed soup-plates. The lower cupel has a notch cut out of its side to serve as a spout for pouring out the liquefied metal, the upper and shallower one is pierced with a central slightly conical round hole through which the (platinum) nozzle of the blowpipe enters, so that the flame flattens itself out on the intro-duced metal. By means of this simple contrivance Deville and Debray had no difficulty in fusing as much as twelve kilogrammes of platinum into one regulus ; and Messrs Johnson, Matthey, & Co. of London now think nothing of fusing up as much as 1000 ounces of metal in one operation. A regulus made under Mr Geo. Matthey's superintendence for the metric commission in Paris in 1874 weighed one quarter of a ton.

The shaping of compact platinum is effected pretty much in the same way as that of gold or silver ; only the difficulties are less because platinum, unlike the two ordinary noble metals, is sus-ceptible of "welding"; i.e., two pieces of the metal, at a white heat, can be united into one by a stroke of the hammer. Soldering is rarely necessary; it used to be effected (and still is occasionally) by means of gold as a connecting medium and an ordinary blow-pipe. But platinum workers, following the lead of Messrs Johnson, Matthey, & Co., have long learned to unite two platinum seams by the '' autogenic " process—the local fusing of the two contiguous parts in the oxyhydrogen flame.

For the preparation of chemically pure platinum Schneider's process is the one most easily executed and explained. The commercial metal is dissolved in aqua regia and the excess of nitric acid removed by evaporation to a syrup in a water-bath. The residue is redissolved in water and boiled for a long time with a large excess of potash-free caustic soda. If care be taken to main-tain a strong alkaline reaction, all the foreign polyxene chlorides are reduced to lower forms than that of tetrachloride ; while only the platinum itself retains this state of combination. The hypo-chlorite formed is then reduced (to NaCl) by addition of a little alcohol to the boiling alkaline liquid, which is now allowed to cool and acidified strongly with hydrochloric acid so as to redissolve any hydrated platinic oxide which may have been precipitated by the first instalments of acid. The liquid at last is filtered, and precipitated by sal-ammoniac to obtain a pure chloroplatinate (PtCl6(NH4)2), which, on ignition, of course, yields an equally pure spongy metal.

Pure compact platinum is a tin-white metal about as soft as pure copper and nearly (but not quite) equal in plasticity to gold. The specific gravity of the fused metal is 21 '48 to 21 '50 at 17°'6 C. (Deville and Debray). The breaking strain is 34'1 kilos for hard-drawn and 23'5 kilos for annealed wires ; the modulus of elasticity 15,518 (kilogramme and millimetre as units ; by Werthum's experiments on annealed welded wire). Unit length of the (fused) metal expands by 0-000907 from 0° to 100° C. (Fizean). The specific conductivity for heat at 12" C. is 8 -4, for electricity at 0* C. 16'4 (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 SQ/5« = 0-0317 + 0-000012« (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 "spits " on freezing. At a red heat the then viscid metal, as Graham has shown, " occludes " 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 AIR PUMP, vol. xvi. p. :__), 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 0-13 to 0'21 volume measured cold ; in the case of merely welded metal, from 2'34 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 suffers 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 Liebig and called ' by him platinum black on account of its resemblance to lamp-black. A particularly active " black " 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 " catalytically" 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 oxyhydrogen flame 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°, 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 "liquation" (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 oi alloys of platinum with other noble metals are used in mechanical dentistry. The following examples may be quoted :—66 "7 per cent, of gold and 33 of platinum; platinum 50, silver 25, palladium 25 ; platinum 41 '7, 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
2 3 4 5 6 7 8 19 1 2 1 1
0-5
20 5 to 10 0 0 1 0 2 0 0 0 1
26 5 0 0 0 0 0 0 0 0 10 20 15 20 0 0 0 2 0 0 0 0 120 0
0
1
100 100 100 100 60

(1) Known to jewellers and dentists as hard platinum ; (2) a rose-coloured fine-grained ductile alloy ; (3) introduced by 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 PtCl6H2. 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 Pt02 wdth 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 Acid.—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 PtCl6H2 + 6H20, 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 "chloride of platinum" 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 " chloroplatin-ates" are produced. Of these the potassium (rubidium 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 PtCl6Na2 + 6H„0 and the lithium salt PtCl6Li,, + 6H„0 are readily soluble in water and in aqueous alcohol (the Li2-compound dissolves even in ab-solute alcohol) ; hence " chloride of platinum" is used for the separation of K, NH4, Eb, 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 plalinic chloride, PtCl4, can be produced from the acid PtClfiH2 only by precipitating from its solution the chlorine of the 2HC1 by the exact equivalent of nitrate of silver. The filtrate when evaporated (cold) over vitriol deposits red crystals of the composition PtCl4 + 5H20. When chloropla-tinic acid is heated to 300° C. it loses its 2HCT and half the chlorine of its PtCl4 and platinous chloride, PtCl2, remains as a dull green powder, insoluble in water but soluble in aqueous hydrochloric acid. Either chloride when heated to redness leaves spongy metal. The hydrochloric solution of platinous chloride, when evaporated with one of chloride of potassium to a sufficiently small volume, deposits rose-coloured crystals of a double salt PtCl2 + 2KCl = PtCl4K2. From a solution of this double salt platinous hydrate, Pt(OH)2, is obtained, by boiling it with the calculated quantity of caustic soda, as a black precipitate, which, when gently heated, becomes anhydrous. Platinic hydrate, Pt(0H)4, is obtained by boiling chloroplatinic acid solution with excess of caustic soda and then acidifying with acetic acid, as an almost white precipitate, Pt(OH)4 + 2H20, which loses its 2H20 at 100° C. and becomes brown ; at a certain higher temperature it loses all its water and assumes the form of the black anhydride Pt02. 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 Pt02 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 PtCl2.N2H6, 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)4K2 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 3H20 of water, which appear yellow in transmitted and blue in reflected light. From the potash salt numerous other platinocyanides 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. 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 formulae of the most stable chlorides :—

Name. Atomic Weight. 0 = 16. Specific Gravity. Chlorides.
Platinum
Palladium Pt =194-8 Ir =193-0 Os =195 Pd = 106-6 Eh = 104-3 Bu = 103-8 21-50
22-38
22-48
11-4
12-1
12-26 PtCl,; PtCl4. Ir2Cl6.
(?) PdCl2. Rh2Cl„. Ru2Cl6 + a:RCl.

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 lr203, 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 Ru203, 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, Os04. 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(N03)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.

Aqua Regia, in the heat, dissolves palladium (very readily) and platinum (somewhat more slowly) as MeCl6H2 ; only the palladium compound is very unstable, being completely reduced to dichloride, PdCl2, by mere evaporation over a water-bath. Iridium black, or iridium alloyed with much platinum, dissolves slowly as IrCl6H2, readily reducible (by, for instance, addition of alcohol, or evaporation to dryness and heating of the residue to about 150° C.) to lr2Cl6. 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, Me203, have been got from rhodium, iridium, ruthe-nium, and osmium. All are basic.

Binoxides, Me02, exist from all the metals except rhodium. Pd02, like Pt02 (see above), is basic or feebly acid ; Ir02 is a feeble base; Ru02 and Os02 are neutral.
Tetroxides, Me04, are formed by osmium and ruthenium only. Both Os04 and Ru04 are easily fusible and very volatile solids. Their vapours have a most powerful smell and are most dangerously poisonous.

Trioxides and Heptoxides do not exist as substances; but the groups Ru03, Os03, and R207 unite with alkalies into soluble salts analogous to chromates and permanganates in their constitution respectively. The oxides MeO, Me203, Me02 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 MeCl8R2 (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 MeCl6H2, 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, MeCl2, and potassio- &c. chlorides, MeCl4R2, exist only in the platinum and palladium series.

3. Hexachlorides, Me2Cl6, 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 nitrie 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 IrCleNa2) is supersaturated with carbonate of soda, and evaporated to dryness, the residue kept at a dull red heat and then lixiviated with water. Alkaliferous oxide of iridium, lr203, remains as a blue-black powder, _which needs only be heated in hydrogen to be reduced to metal, from which the alkali is now easily removed by washing with water. Such iridium is always contaminated with more or less osmium, ruthenium, rhodium, and platinum, to remove which the crude metal is fused up with ten parts of lead, and the alloy treated with dilute nitric acid to dissolve the bulk of the lead, when the polyxene metals remain in the shape of a black powder. From this the platinum is extracted by prolonged treatment with dilute aqua regia, and from the residue the rhodium by fusion with bisulphate of potash and subsequent treatment with water, which dissolves away the sulphate of rhodium formed. The residue now left is fused in a gold crucible with ten parts of caustic and three of nitrate of potash, when the ruthenium and osmium assume the form of soluble Me03K20 salts, wdiich are extracted with water and thus removed. What remains is an alkaliferous (blue) sesquioxide of iridium, which as a rule still retains some iron, ruthenium, and traces of gold and silica (G. Matthey). For the final purification of the metal and the recovering of the ruthenium and rhodium see G. Matthey's memoir (Chem. Soc. Journ., 1879, Abstr., p. 772) and chemical handbooks.

The osmium, as already stated, is obtained at an early stage of the process in the shape of a solution of its volatile tetroxide in caustic potash. This solution is mixed with a little alcohol to bring the osmium into the state of osmite, K20 + aOs02, which is insoluble in alcohol. This precipitate is digested in sal-ammoniac, to convert it into a yellow compound of the composition 2NH4C1 + Os02(NH3)2, which latter needs only be heated in hydro-gen to be converted into finely divided metallic osmium.

2. The second residue consists of a solution of a variety of polyxene chlorides in sal-ammoniac. This liquor is kept in contact with metallic iron, when the dissolved polyxene metals, and any gold or copper present, come down as a black heavy precipitate. This precipitate includes all the palladium and part of the rhodium as principal components. Bunsen has worked out an exhaustive method for the extracting of all its polyxene metals in pure forms; but it is too complicated to be reproduced here. The customary method for extracting the palladium is to treat the metallic preci-pitate with aqua regia, which dissolves the palladium and platinum along with some of the iridium and rhodium, to filter, evaporate the residue to a syrup (for bringing the palladium into the form of PdCl2), redissolve and precipitate the palladium by addition of the exact quantity of mercuric cyanide as cyanide Pd(NC)2. This cyanide needs only be ignited strongly to leave a residue of metal. But this metal includes at least part of the copper of the original material. To remove it and other impurities, the crude metal is dissolved in hydrochloric acid with the help of free chlorine, and the solution next evaporated to dryness to reduce the PdCl6H2 to PdCl2. The chloride is redissolved, the solution mixed with enough of ammonia to redissolve the precipitate first produced, and hydrochloric acid gas is now passed into the solution. Yellow palladiochloride of ammonium, PdCl4(NH4)2, is precipitated, while copper and iron remain dissolved. After removal of the mother liquor the double salt is ignited and thus converted into palladium-sponge, which is easily fused up in the oxyhydrogen flame and thus brought into the form of regulus.

Notes on Palladium, Osmium, and Osmiridium.

Palladium, a silver-white metal of great ductility, is much used, notwithstanding its high price, in mechanical dentistry and occasionally also for the graduated limbs of theodolites and other instruments, because, unlike silver, it remains bright in sulphur-etted hydrogen.

Of all the properties of this metal the most remarkable is its extra-ordinary power of " occluding " hydrogen. According to Graham (to whom we owe almost all our knowledge on the subject) the compact metal when immersed in cold hydrogen gas takes up none or at most very little of it; but at higher temperatures very con-siderable occlusions take place. A certain specimen of foil was found to occlude 526 volumes of the gas at 245° C, and 643 at 90° to 97° C., measured at 17°'5 to 18° and one atmosphere's pressure, per unit-volume of metal. The hydrogen, as in the case of platinum, is retained on cooling, and from the cold compound cannot be extracted by means of an absolute vacuum, which re-extracts the gas at a red heat.

Far more striking results can be obtained by using palladium as a negative pole in the electrolysis of (acidulated) water. The coefficient of occlusion then assumes very high values ; in Graham's hands it attained its maximum when the palladiunTwas produced electrolytically from a 1 '6 per cent, solution of its chloride, and thus hydrogenized while itself in the nascent state. The galvani-cally deposited sheet was found to contain 982 volumes of hydrogen (measured cold) per unit-volume of original metal, corresponding approximately to the formula Pd4H3 for the compound. When palladium unites with (nascent or free) hydrogen it suffers a very appreciable expansion which on the removal of the hydrogen is followed by a contraction beyond the original volume of the plain metal. This can bo most beautifully illustrated by electrolysing water in an apparatus in which the negative electrode consists of a long strip of palladium-foil of wdiich one side is covered over with varnish or electrolytically deposited platinum. The hydrogen goes in at the bare side of the electrode ; this side consequently expands more strongly than the other and the originally straight strip of metal becomes curved. When the current is reversed, hydrogen bubbles at once rise from what is now the negative pole, but the oxygen due at the palladium plate is for a time taken up by the hydrogen occluded there ; this hydrogen is gradually consumed, and as it diminishes the plate unbends more and more completely and at last gets bent over in the opposite sense. Palladium by being hydrogenized does not lose any of its metallic properties, but (in the case of complete saturation) its density sinks from 12'38 to 1179, its tenacity to 82 per cent, of its original value, its electric conductivity in the ratio of 8'1 to 5'9.

Graham views hydrogenized palladium as a true allov, containing its hydrogen in the form of a metal " hydrogenium. He found that certain palladium alloys take up hydrogen as readily (though less abundantly) as the pure metal does with corresponding expan-sion, but when dehydrogenized shrink back into exactly their ori-ginal volume. He calculated that the density of hydrogenium lies somewhere about the value 0733 (water = l),—which of course means only that the weight of the occluded hydrogen, measured by the weight of a volume of water equal to the expansion observed, is =0733. Dewar arrived at 0'620 as being probably nearer the truth, and for the specific heat of hydrogenium found values from 3 79 to 5-88.

Osmium. —According to Deville and Debray, powdery osmium is most readily obtained by mixing the vapour of the tetroxide with that gas (CO + C02) which is prepared by the decomposition of oxalic acid with oil of vitriol, and passing the mixture through a red-hot porcelain tube. The powdery metal readily fuses up with 3 or 4 parts of tin into a homogeneous alloy. When this alloy is treated with hydrochloric acid most of the tin dissolves, and the rest of it can be driven off by heating the residue in HC1 gas. There remains ultimately pure osmium in the form of blue crystals endowed with a grey to violet reflex, which are hard enough to scratch glass. Their specific gravity is 22'48, so that osmium, besides being the most infusible of metals, is the heaviest of all known bodies.

Osmiridium.—Native osmiridium forms crystalline plate-shaped grains, distinguished by an extraordinary degree of hardness, which certainly exceeds that of hard-tempered steel. Most of the grains are very minute; the larger ones are utilized for- making the so-called "diamond points " of gold pens. Osmiridium would lend itself for endless other applications if it Were possible to unite the native dust into large compact masses. From a series of articles in the Chemical News (Jan. 2, 9, and 16, 1885), by Nelson W. Perry, it would appear that this problem has been solved, in a sense. John Holland, an American pen-maker, starting from the long-known fact that platinum metals readily unite with phosphorus into relatively easily fusible alloys, succeeded in producing a phos-phorized osmiridium which can be cast (and pressed while liquid) into thin continuous slabs even harder than the native substance, and susceptible of being wrought into drills, knife-edges, &c.

Statistics.

The production of platinum-ore in Russia was 2327 kilogrammes in 1862, 492 in 1863, 397 in 1864, 2273 in 1865, 1768 in 1867, and 2050 in 1871,—a total in those six years of 9307. The average production of platinum metal, from 1828 to 1845, amounted to 2623'8 kilogrammes per annum. In 1870 it was only 2005'8 kilos, of which about 80 per cent, came from the Ural Mountains.2

The manufacture of platinum utensils is in the hands of a very few firms, of which that of Messrs Johnson, Matthey, & Co. of London is generally understood to be the most important. Even the total amount of metal which passes through these works in the aggregate is difficult of ascertainment, the more so as some of them at least are discounting large reserves of old metal, including more or less of the obsolete coins. According to an approximate estimate which a very competent authority has kindly furnished, the consumption during the last five years fell little short of 100,000 lb troy,3 of which from 75 to 80 per cent, are believed to have passed through the hands of London manufacturers.

The price of the metal during the last ten or twelve years has ranged from four to eight times that of silver. It is very high at present (1885) in consequence of the constantly increasing demand for platinum utensils. (W. D.)


Footnotes

It still remains to be seen how far this latter statement holds for the absolutely pure metals. Mr George Matthey has succeeded in producing iridium wire, which could be bent into a loop without breaking.

Jahresb. d. Chemie, 1868, p. 280; Ann. d. Chemie, vol. cxlvi. 265.

2 From Kamarsch and Heeren's Technisches Wbrterbuch.
3 Equal to 7464 kilogrammes per annum, which is 3'7 times the amount given above for 1870.




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