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CHAPTER XIV

THE TRANSITIONAL ELEMENTS OF THE SECOND

AND FOURTH LONG PERIOD

Ruthenium, Ru=101.7. Rhodium, Rh=103. Palladium, 106.
Osmium, Os=191.

Iridium, Ir=193. Platinum, 194.8. THESE elements, although constituting two transitional groups, are very closely related to each other. In nature they all occur associated together in what is commonly known as platinum ore, and they are on this account usually spoken of as the platinum metals.

Platinum ore, or native platinum, contains all these elements in the metallic state. It is found in small grains, sometimes in nuggets, in alluvial deposits and river sand, principally in Brazil, Borneo, California, Australia, and the Urals, Native platinum contains from 60 to 86 per cent. of platinum, the remainder consisting of the other five metals of the group, together with varying quan. tities of gold, copper, and iron. Amongst the grains of platinuni ore there are also found grains which consist essentially of an alloy of platinum and iridium (containing from 30 to 75 per cent. of iridium) known as platiniridium: and also particles of an alloy of osmium and iridium (called osmiri. dium), which contain from 30 to 40 per cent. of osmium, as well as small quantities of rhodium and ruthenium.

They are all white lustrous metals, having high melting-points. They are unacted upon by air or oxygen at ordinary temperatures; and, with the exception of osmium (which burns when strongly heated, forming the tetroxide), they are scarcely oxidised by oxygen at any temperature.

With the exception of palladium, which readily dissolves in hot nitric acid, these metals are unacted upon by ordinary acids. Aqua regia converts osmium into the tetroxide; it dissolves platinum with formation of the tetrachloride, and slowly acts upon ruthenium, but is without action upon rhodium and iridium.

The specific gravities of the metals of the first group, although very close to one another, are widely different from those of the second group; and it will be seen that the specific gravities fall, with increasing atomic weights, thus-

Ru, sp. gr.= 12, 26. Rh, sp. gr. = 12.1. Pd, sp. gr.=11.4.
Os, 1 =22.47. Ir,

= 22. 38. Pt, I = 21.5. The element osmium is the heaviest known substance.

The most easily fusible of these metals is palladium, which melts about the temperature of wrought iron. The melting-point of platinum is somewhat higher, but it may be boiled by the oxyhydrogen flame. Rhodium and

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iridium come next in order of fusibility, the latter metal being just fusible by the oxyhydrogen flame, while ruthenium has a still higher melting-point. Osmium has not been melted. When heated to the melting-point of iridium, osmium volatilises; and if air be present, it burns.

The following oxides of these metals are known

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Ruthenium, osmium, rhodium, and iridium form salts corresponding to the sesquioxide, such as ruthenious chloride, Ru,Cloi rhodium sulphate, Rh.(SO4)3; iridious chloride, Ir,Cla.

With the exception of rhodium, they all form chlorides, corresponding to the dioxides, thus-ruthenic chloride, RuCle; iridic chloride, IrCle; platinic chloride, PtCl4, while palladium and platinum yield palladous and platinous compounds, corresponding to their monoxides.

The tetroxides of ruthenium and osmium are remarkable in melting at an extremely low temperature (about 40°), and boiling about 100°. They yield intensely irritating vapours, which, in the case of osmium tetroxide, exerts a most injurious effect upon the eyes, and is extremely poisonous. (Osmium tetroxide is commonly known as osmic acid.) Osmium and ruthenium also exhibit a non-metallic character in forming compounds derived from the unknown ruthenic and osmic trioxides, such as potassium ruthenate, K RuO, and potassium osmate, K.,OsO4 (the corresponding ruthenic and osmic acids are unknown). Ruthenium also forms potassium per-ruthenate, KRuO4 (analogous to permanganate), although the corresponding acid and peroxide, Ru07, are unknown. The most important of these elements is platinum.

PLATINUM.

Symbol, Pt. Atomic weight = 194.8. • In order to separate platinum from the other metals with which the native platinum (see page 690) is mixed, the ore is digested in dilute aqua regia, under slightly increased pressure. The solution so obtained contains the higher chlorides of platinum, palladium, rhodium, and iridium (for although in the pure state the last two named metals are scarcelyattacked by aqua regia, when alloyed with much platinum they dissolve). The solution is evaporated to dryness, and heated to 125°, whereby the palladium and rhodium are obtained in the form of their lower chlorides, PdCl, and Rh,Clo (the latter of which, in the anhydrous condition, is insoluble in water). The residue is extracted with water, and to the clear solu

tion, acidified with hydrochloric acid, ammonium chloride is addec. The double chloride of platinum and ammonium (PtCl. 2NH.Ch separates out as yellow crystals, while the corresponding iridion salt, being more soluble, remains for the most part in solution, and may be obtained by concentrating the mother-liquor. The ammonium platinic chloride, on being ignited, loses ammonin chloride and chlorine, leaving the metal in the form of a black spongy mass known as spongy platinum, which is then melted by means of the oxyhydrogen flame in a lime crucible. The platinan so obtained usually contains small quantities of iridium and traces of associated metals.

Pure platinum is obtained by alloying commercial platinum with pure lead, and treating the alloy first with nitric acid, whid dissolves any copper and iron, a part of the palladium and rhodiun, and most of the lead ; and then with dilute aqua regia, which dissolves the whole of the platinum and the remaining lead, with traces of rhodium. From this solution the lead is precipitated as sulphate, and the platinum is then precipitated as the double chloride, by ammonium chloride. To remove traces of rhodium which are present, the dried double chloride is ignited with hydrogen potassium sulphate, whereby the rhodium is converted into a soluble double sulphate of rhodium and potassium, while the platinum is reduced to the condition of the spongy metal.

Properties.—Platinum is a lustrous, greyish-white, malleable, and ductile metal. At a red heat it may be welded with great ease. It is melted by the oxyhydrogen flame, and vessels platinum are readily made by fusing the metal together in this way. Heated platinum absorbs large quantities of hydrogen (see page 179); and when the metal is melted in the oxyhydrogen flame, it exhibits the phenomenon of “spitting," when it again solidifies (see Silver, page 562). Platinum does not combine with oxygen at any temperature, neither does the heated metal absorb this gas; but it has the property, when cold, of condensing oxygen upon its surface. A piece of clean platinum foil or wire, when introduced into a mixture of oxygen, and a readily inflammable gas or vapour (such as hydrogen, ether, alcohol, &c.), causes their combination ; and occasionally the metal becomes red hot, and ignites the mixture. This action is more rapid in the case of platinum sponge, when a larger surface is brought into play, and a fragment of this material introduced into a detonating mixture of oxygen and hydrogen at once determines its explosion.

Platinum is not acted upon by either nitric or hydrochloric acid. It is oxidised when fused with caustic alkalies, or with potassium nitrate, and is also attacked by fused alkaline cyanides. In the form of sponge, it is dissolved by boiling potassium cyanide, with the evolution of hydrogen and formation of a double cyanide.

Platinum readily combines with phosphorus, silicon, and carbon. The carbide of platinum is sormed when the metal is continuously heated by a smoky flame, or one in which combustion is incomplete, hence care is necessary in the use of platinum vessels.

Platinum Black is the name given to the finely divided metal obtained by precipitating platinum from its solutions by reducing agents or by metals. It is a soft, black powder, which is capable of absorbing, or condensing upon its surface, large quantities of oxygen. It therefore acts as a powerful oxidising agent.

Platinum Alloys.- Platinum readily alloys with many metals ; hence compounds of easily reducible metals should not be heated in vessels of platinum. The most important alloys are those with iridium. The addition of 2 per cent of iridium is found greatly to increase the hardness and raise the melting-point of platinum. An alloy containing to per cent. of iridium resists the corrosive action of chemical reagents to a greater extent than pure platinum (see Fluorine, page 348).

Oxides of Platinum.-Platinous oxide, P10, and platinic oxide, PtO2, are obtained in the form of dark grey or black powders by gently heating the corresponding hydroxides. When strongly heated they are converted into the metal.

Platinous Hydroxide, Pt(HO),, is obtained by the action of potassium hydroxide upon platinum dichloride. It is a black powder, which dissolves in the halogen acids, yielding platinous compounds.

Platinic Hydroxide, Pt(HO), is prepared by adding boiling potassium hydroxide to a solution of platinum tetrachloride, and treating the precipitate with acetic acid to remove the potash. When dried it forms a yellowish powder, which is soluble in acids to form platinic salts. Platinic hydroxide behaves both as a weak base and a feeble acid. With stronger bases it forms compounds known as platinates, which are yellow crystalline salts. The sodium salt has the composition Na,0,3P+0,6H.,O.

Platinum Dichloride (platinous chloride), PtCly, is produced when platinum tetrachloride is heated to about 250°. It forms a greenish powder, insoluble in water. It dissolves in hydrochloric

acid, giving a reddish-brown solution which is believed to contain the double compound PtC1.,,2HCl, or H,PtCly, to which the name chloro-platinous acid has been given. The compound has never been isolated, but a number of double salts of platinous chloride with other chlorides are known, which may be regarded as derivatives of this acid, and which are therefore termed chloroplatinites ; thus, potassium platinous chloride, 2KCI, PtCl., or potassium chloro-platinite, K,PtCl,, is obtained as fine red crystals, by adding potassium chloride to a solution of platinous chloride in hydrochloric acid. This salt is used in the platinotype photographic process.

Platinum Tetrachloride (platinic chloride), PtCl,, is obtained by dissolving the metal in aqua regia, and removing the excess of the acids by evaporating to dryness and gently heating the residue. From its aqueous solution, the salt deposits in large red crystals having the composition PtC14,5H,O, which are not deliquescent. When the salt is crystallised from a hydrochloric acid solution, or when the aqua regia solution is evaporated to expel the nitric acid, with frequent addition of hydrochloric acid, the double compound of platinic chloride and hydrochloric acid is formed, PtC1,,2HCl, which is deposited as reddish-brown deliquescent crystals, with 6H.O. To this substance (which is commonly called platinic chloride), the name chloro-platinic acid has been given, and the double salts of platinic chloride and various chlorides are regarded as salts of this acid. The most important of these chloro-platinates are those of the alkali metals, their different solubilities being made the basis for the separation of these metals.

Potassium Chloro-platinate (or potassium platinic chloride), 2KCI, PtCl4 or K,PtClg, is obtained as a yellow crystalline precipitate by adding potassium chloride to platinic chloride. It is soluble in 100 parts of water at the ordinary temperature to the extent of 1.1 part, and at 100°, 5.18 parts. It is insoluble in alcohol.

The rubidium and caesium compounds are very similar, but are still less soluble in water, 100 parts of water at 20° dissolving o.141 of the rubidiumn and 0.07 of the caesium salt.

Ammonium Chloro-platinate, 2NH,C1,PtCl,, closely resembles the potassium salt, being slightly less soluble, but more so than the rubidium compound.

Sodium Chloro-platinate, 2NaCI,P+C12,61,0, is a reddishyellow salt, readily soluble in both water and alcohol.

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