396 PLATINOUS AND PLATINIC OXIDES. oxygen condensed within the pores of the metal. Spongy platinum is obtained in a very active form by heating the ammonio-chloride of platinum very gently in a stream of coal-gas or hydrogen as long as any fumes of hydrochloric acid are evolved. If platinum be precipitated in the metallic state from a solution, it is obtained in the form of a sooty powder, called platinum-black, which possesses this power of promoting combination with oxygen in the highest perfection. This form of platinum may be obtained by dissolving the metal in aqua regia, which converts it into bichloride of platinum (PtCl2), evaporating the solution to dryness, and heating the residue on a sand-bath as long as it smells of chlorine. The chloride of platinum (PtCl) thus obtained is dissolved in a strong solution of potash and heated with alcohol, when the platinum-black is precipitated, and must be filtered off, washed, and dried at a gentle heat. Platinum in this form is capable of absorbing 800 times its volume of oxygen, which does not enter into combination with it, but is simply condensed into its pores, and is available for combination with other bodies. A jet of hydrogen allowed to pass on to a grain or two of this powder is kindled at once, and if a few particles of it be thrown into a mixture of hydrogen and oxygen, explosion immediately follows. A drop of alcohol is also inflamed when allowed to fall upon a little of the powder. Platinum-black loses its activity after having been heated to redness. Although platinum resists the action of hydrochloric and nitric acids, unless they are mixed, and is unaffected at the ordinary temperature by other chemical agents, it is easily attacked at high temperatures by phosphorus, arsenic, carbon, boron, silicon, and by a large number of the metals; the caustic alkalies and alkaline earths also corrode it, so that some discretion is necessary in the use of vessels made of this costly metal. When platinum is alloyed with 10 parts of silver, both metals may be dissolved by nitric acid. oxygen 298. Oxides of platinum.-Only one compound of platinum with is known in the separate state, the other having been obtained in combination with water. The protoxide, PtO (platinous oxide), is precipitated as a black hydrate by decomposing the protochloride with potash, and neutralising the solution with dilute sulphuric acid. It is a feeble base, and decomposes when heated, leaving metallic platinum. Binoxide of platinum, PtO, (platinic oxide), is also a weak base, but occasionally plays the part of an acid, whence it is sometimes termed platinic acid. The hydrated binoxide (PtO,. 2HO) is obtained by mixing solution of bichloride of platinum with excess of potash, heating until the precipitate of platino-chloride of potassium (KCl, PtCl) is redissolved, and adding acetic acid, which gives a brown precipitate of the hydrated binoxide; this may be freed from water by a moderate heat, and at a higher temperature loses its oxygen. Platinate of soda (NaO. 3PtO. 6Aq.) may be crystallised from a solution of the hydrated binoxide in soda. Platinate of lime is convenient for the separation of platinum from iridium, which is generally contained in the commercial metal; for this purpose the platinum is dissolved in nitro-hydrochloric acid, the solution evaporated till it solidifies on cooling, the mixed chlorides of iridium and platinum dissolved in water, and decomposed with an excess of lime without exposure to light; the platinum then passes into solution as platinate of lime, and the platinic acid may be separated from the filtered solution BICHLORIDE OF PLATINUM. 397 by exposure to light. Acids dissolve binoxide of platinum, forming salts of a brown colour which have not been crystallised. If binoxide of platinum be dissolved in diluted sulphuric acid and the solution mixed with excess of ammonia, a black precipitate of fulminating platinum is obtained, which detonates violently at about 400° F. This compound is said to have a composition corresponding to the formula NH, HO, PtO, + HO (hydrated platinate of ammonia); and might also be represented as NHPt. 4HO, or a combination of water with ammonia (NH), in which two equivalents of hydrogen are replaced by one equivalent of platinum (which, in the binoxide, PtO,, represents H.). Chlorides of platinum.-The bichloride, or platinic chloride (PtCl2), is the most useful salt of the metal, and may be prepared by dissolving scraps of platinum foil in a mixture of four measures of hydrochloric acid with one of nitric acid (100 grains of platinum require 3 measured ounces of hydrochloric acid), evaporating the liquid at a gentle heat to the consistence of a syrup, redissolving in dilute hydrochloric acid, and again evaporating to expel excess of nitric acid. The syrupy liquid solidifies on cooling to a red-brown mass, which is very deliquescent, and dissolves easily in water or alcohol to a red-brown solution. If the concentrated solution be allowed to cool before all the free hydrochloric acid has been expelled, long brown prismatic crystals of a combination of the bichloride with hydrochloric acid are obtained. The bichloride of platinum is remarkable for its disposition to form sparingly soluble double chlorides with the chlorides of the alkali metals and the hydrochlorates of organic bases, a property of great value to the chemist in effecting the detection and separation of these bodies. A good example of this has lately been afforded in the separation of potassium, rubidium, and cosium. The chlorides of these three metals having been separated from the various other salts contained in the mineral water in which they occur, are precipitated with bichloride of platinum, which forms combinations with all the three chlorides. The platino-chloride of potassium is more easily dissolved by boiling water than those of rubidium and cœesium, and is removed by boiling the mixed precipitate with small portions of water as long as the latter acquires a yellow colour. The remaining platino-chlorides of rubidium and cœsium are then heated in a current of hydrogen, which reduces the platinum to the metallic state, and the chlorides may then be extracted by water, in which they are very soluble. Platino-chloride of potassium (KCl, PtCl,) forms minute yellow octahedral crystals; those of rubidium and cosium have a similar composition and crystalline form. Platino-chloride of sodium differs from these in being very soluble in water and alcohol; it may be crystallised in long red prisms, having the composition (NaCl, PtCl2, 6Aq.) Ammonio-chloride of platinum (NH,. HCl, PtCl) has been already noticed as the form in which platinum is precipitated in order to separate it from other metals. It crystallises, like the potassium-salt, in yellow octahedra, which are very sparingly soluble in water and insoluble in alcohol. It is the form into which nitrogen is finally converted in analysis in order to determine its weight. When heated to redness, this salt leaves a residue of spongy platinum. The bichloride of platinum is sometimes used for browning gun-barrels, &c., under the name of muriate of platina. 398 PLATOSAMINE AND PLATINAMINE COMPOUNDS. Protochloride of platinum or platinous chloride (PtCl).—The bichloride of platinum may be heated to 450° F. without decomposition, but above that temperature it evolves chlorine, and is slowly converted into the protochloride, which is reduced, at a much higher temperature, to the metallic state. Platinous chloride forms a dingy green powder, which is insoluble in water and in nitric and sulphuric acids, but dissolves in hot hydrochloric acid, and in solution of platinic chloride, yielding in the former a bright red, in the latter a very dark brown-red solution. Its solution in hydrochloric acid is not precipitated by chloride of potassium, but a soluble double chloride (KCl, PtCl) may be crystallised from the liquid. If hydrochlorate of ammonia be added to the hydrochloric solution, a double salt of hydrochlorate of ammonia with protochloride of platinum (NH,. HCl. PtCl) may be obtained in yellow crystals by evaporation. If, instead of hydrochlorate of ammonia, free ammonia be added in excess to the boiling solution of protochloride of platinum in hydrochloric acid, brilliant green needles (green salt of Magnus) are deposited on cooling, which contain the elements of platinous chloride and ammonia (PtČI. NH ̧); but from the behaviour of this compound with chemical agents, its true formula would appear to be N,H,Pt. HCl, PtCl, in which the place of 1 equivalent of hydrogen in two of ammonia is occupied by platinum. By heating this salt with an excess of ammonia, the platinous chloride which it contains may be decomposed, and when the solution is allowed to cool, it deposits yellowish-white prismatic crystals of hydrochlorate of diplatosamine; N,H,Pt. HCl + Aq., the production of which may be represented by the equation N2H,Pt. HCl, PtCl + 2NH, = 2(N,H,Pt. HCI). By decomposing a solution of this salt with sulphate of silver, the sulphate of diplatosamine is obtained; N.H.Pt. HCl + AgO. SO, = N2H,Pt. HO. SO, + AgCl. When the solution of sulphate of diplatosamine is treated with hydrate of baryta, sulphate of baryta is precipitated, and a powerfully alkaline solution is obtained, which yields crystals of hydrate of diplatosamine N2H Pt. 2HO, a strong alkali which may be regarded as a compound of water with 2 equivalents of ammonia (N2H ̧) in which 1 equivalent of hydrogen is replaced by platinum. The hydrate of diplatosamine has a strong resemblance to the hydrated mineral alkalies, eagerly absorbing carbonic acid from the air, and expelling ammonia from its salts. When the hydrate of diplatosamine is heated to 230° F. it gives off water and ammonia, and becomes converted into a grey insoluble substance, which is hydrate of platosamine, NH,Pt. HO, and may be regarded as a compound of water with a single equivalent of ammonia (NH), in which one-third of the hydrogen is replaced by platinum. This substance is also a base, and forms salts, most of which are insoluble; the sulphate of platosamine, NH,Pt. HO. SO, + HO, may be regarded as sulphate of ammonia (NH,. HO. SO1), in which 1 equivalent of the hydrogen is replaced by platinum. The hydrochlorate of platosamine (NH,Pt. HCl) is isomeric with the green salt of Magnus, and may be obtained from that compound by dissolving it in a hot solution of sulphate of ammonia, from which it crystallises on cooling.* If the hydrochlorate of platosamine, suspended in boiling water, be treated with chlorine, it is converted into hydrochlorate of platinamine, NHPt. 2HCl, which may be represented as the hydrochlorate of an ammonia, in which 2 equivalents of hydrogen have been replaced by 1 equivalent of platinum in the condition in which it exists in the bichloride (PtCl2), where it is equivalent to H,. The conversion of the hydrochlorate of platosamine into hydrochlorate of platinamine may be represented by the equation, NH,Pt. HCl + Cl= NHPt. 2HCl. By boiling the hydrochlorate of platinamine with nitrate of silver, it is converted into nitrate of platinamine (NHPt. HO. NO), and when this is dissolved in boiling water and decomposed by ammonia, the hydrate of platinamine (NHPt, 4HO) is obtained in yellow prismatic crystals, having the same composition as that assigned to fulminating platinum. Several other platinum compounds derived from ammonia have been obtained, but cannot at present be so conveniently classified. The following table exhibits The salts of diplatosamine are distinguished from those of platosamine by the action of nitrous acid, which gives a fine blue or green precipitate or coloration with the former. For the cause of this change, and for many other interesting points in the history of these platinum compounds, the author sorrowfully refers to the elaborate and accurate memoir by his lamented colleague, the late Mr Hadow, written during his last illness, and published in the Journal of the Chemical Society for August 1866, in which month he died, having there given a summary of the results of persevering and sagacious experimental work, extending over several years. He too truly wrote, that he was "prevented from making any further experiments for some time to come." He died August 11, 1866, aged 35. the composition of those here enumerated, the platinum, as it exists in platinous chloride (PtCl), occupying the place of 1 equivalent of hydrogen, being represented by Pt', and the platinum, as it exists in platinic chloride (PtCl2), occupying the place of 2 equivalents of hydrogen, by Pt". Hydrate of platosamine, Hydrochlorate of platosamine, 'Hydrate of platinamine, Hydrochlorate of platinamine, Hydrate of diplatosamine, Hydrochlorate of diplatosamine, 2 NH,Pt'. HO NHPt”.4HO NHPt". 2HCI N2H,Pt'.2HO NH Pt'. HCI NH Pt'. HO.SO, Some of tire salts of diplatinamine (N2H Pt") have been obtained, this base being derived from two equivalents of ammonia in which H, have been replaced by Pt". The sulphides of platinum correspond in composition to the oxides and chlorides, and may be obtained by the action of hydrosulphuric acid upon the respective chlorides, as black precipitates. 299. Equivalent and atomic weights of platinum.-Platinic chloride, analysed in its pure combination with chloride of potassium, is found to contain 35.5 parts by weight (1 eq.) of chlorine combined with 49.3 parts of platinum, and this latter number would represent the equivalent weight of platinum if the platinic chloride contained a single equivalent of chlorine; but since the platinous chloride contains only half as much chlorine as the platinic chloride, it is usual to represent the former as containing 1 eq. and the latter 2 eqs. of chlorine combined with 1 eq. of platinum, so that the equivalent weight of platinum (the quantity combined with 71 parts or 2 eqs. of chlorine) would be 98'6, a view which is confirmed by the composition of the ammonia derivatives containing platinum. The specific heat of platinum shows that its atomic weight must be twice its equivalent weight, or 197-2, and since, in the platinic compounds, which may be regarded as the normal compounds of platinum, this weight of the metal occupies the place of 4 atoms of hydrogen, platinum is generally regarded as a tetratomic element (Ptiv 1972), and the atomic formula of its chief compounds are written thus: platinous oxide, Pto; platinic oxide, PtO,; platinous chloride, PtCl; platinic chloride, PtCl; hydrate of platosamine, N,H,Pt". H,O; hydrate of platinamine, NH¿Pť1ˇ, 4H ̧¤; hydrate of diplatosamine, NH, Pt". 2H2Ð. 300. PALLADIUM is found in small quantity associated with native gold and platinum. It presents a great general resemblance to platinum, but is distinguished from it by being far more easily oxidised, and by its special attraction for cyanogen, with which it forms an insoluble compound. This circumstance is taken advantage of in separating palladium from the platinum ores, for which purpose the solution from which the greater part of the platinum has been precipitated by hydrochlorate of ammonia (p. 394) is neutralised with carbonate of soda, and mixed with solution of cyanide of mercury (Hg, C,N), when a yellowish precipitate of cyanide of palladium is obtained, yielding spongy palladium when heated, which may be welded into a compact form in the same manner as platinum. When alloyed with native gold, palladium is separated by fusing the alloy with silver, and boiling it with nitric acid, which leaves the gold undissolved. The silver is precipitated from the solution as chloride by adding chloride of sodium, and metallic zinc is placed in the liquid, which precipitates the palladium, lead, and copper, as a black powder. This is dissolved in nitric acid, and the solution mixed with an excess of ammonia, which precipitates the oxide of lead, leaving the copper and palladium in solution. On adding hydrochloric acid in slight excess, a yellow precipitate of hydrochlorate of palladamine (NH,Pd. HCI) is obtained, which leaves metallic palladium when heated. Palladium is harder than platinum and much lighter (sp. gr. 11·5); it is malle able and ductile like that metal, and somewhat more fusible, though it cannot be melted in an ordinary furnace. It is unchangeable in air unless heated, when it becomes blue from superficial oxidation, but regains its whiteness when further heated, the oxide being decomposed. Unlike platinum, it may be dissolved by nitric acid, forming nitrate of palladium (PdO. NO), which is sometimes employed in analysis for precipitating iodine from the iodides, in the form of black iodide of palladium (PdI). Palladium is useful, on account of its hardness, lightness, and resistance to tarnish, in the construction of philosophical instruments; alloyed with twice its weight of silver, it is used for small weights. Of the oxides of palladium, two correspond with those of platinum, and a basic suboxide (Pd,O) has been obtained by gently heating the protoxide. Bichloride of palladium (PdCl2) is very unstable, being easily decomposed, even in solution, into the protochloride (PdCl) and free chlorine. Both the chlorides form double salts with the alkaline chlorides, those containing the palladious chloride (PdCl) having a dark green colour. A pulverulent carbide of palladium is formed when the metal is heated in the flame of a spirit-lamp. 301. RHODIUM, another of the metals associated with the ores of platinum, has acquired its name from the red colour of many of its salts (pódov, a rose). It is obtained from the solution of the ore in aqua regia by precipitating the platinum with hydrochlorate of ammonia, neutralising with carbonate of soda, adding cyanide of mercury to separate the palladium, and evaporating the filtered solution to dryness with excess of hydrochloric acid. On treating the residue with alcohol, the double chloride of rhodium and sodium is left undissolved as a red powder. By heating this in a tube through which hydrogen is passed, the rhodium is reduced to the metallic state, and the chloride of sodium may be washed out with water, leaving a grey powder of metallic rhodium, which is fused by the oxyhydrogen blowpipe with greater difficulty than platinum, and forms a very hard malleable metal not dissolved even by aqua regia, although this acid dissolves it in the ores of platinum, because it is alloyed with other metals. If platinum be alloyed with 30 per cent. of rhodium, however, it is not affected by aqua regia, which will probably render the alloy useful for chemical vessels. Rhodium may be brought into solution by fusing it with bisulphate of potash, when sulphurous acid escapes, and a double sulphate of rhodium and potash is formed, which gives a pink solution with water. Finely divided rhodium is oxidised when heated in air. It appears to form two oxides, the protoxide (RoO), which is very little known, and the sesquioxide (Ro2O3), obtained by fusing rhodium with carbonate of potash and nitre, and washing the fused mass with water, which leaves an insoluble compound of the sesquioxide with potash; on treating this with hydrochloric acid, the sesquioxide of rhodium is left. It is not decomposed by heat, and is insoluble in acids, though it is a basic oxide, and its salts, which have a red colour, are obtained by indirect methods. Sesquichloride of rhodium (Ro,Cl) has a brownish black colour, and does not crystallise. Its aqueous solution is red, and it forms crystallisable double salts with the alkaline chlorides, which have a fine red colour. The double chloride of rhodium and sodium, (3NaCl. Ro2Cl2).18Aq., is prepared by heating a mixture of pulverulent rhodium and chloride of sodium in a current of chlorine. It crystallises in red octahedra. On boiling a solution of sesquichloride of rhodium with ammonia in excess, a yellow ammoniated salt (Ro,C1.5NH2) may be crystallised out, from which metallic rhodium may be obtained by ignition. With sulphur, rhodium combines energetically at a high temperature; a protosulphide and a sesquisulphide have been obtained. 302. OSMIUM is characterised by its yielding a very volatile acid oxide (osmic acid, Os04), the vapours of which have a very irritating odour (óøμn, odour). It occurs in the ores of platinum in flat scales, consisting of an alloy of osmium, iridium, ruthenium, and rhodium. This alloy is also found associated with native gold, and being very heavy, it accumulates at the bottom of the crucible in which the gold is melted. The osmium alloy is extremely hard, and has been used to tip the points of gold pens. When a grain of it happens to be present in the gold which is being coined, it often seriously injures the die. When the platinum ore * Palladium, at a slightly elevated temperature, absorbs, mechanically, many times its volume of hydrogen. Hammered palladium foil condenses 640 times its volume of hydrogen, below 212° F., though it has not the power of absorbing oxygen or nitrogen. Foil made from fused palladium only absorbs 68 times its volume of hydrogen.-(Graham, Proc. Roy. Soc., June 1866). |