Iridium sulphate

Chloro - pentammino - iridium Chloride, [Ir(NH3)sCl]Cl2, is formed by the action of ammonia on iridium trichloride, iridium tetrachloride, or the ehloro-double salts. It may also be prepared from chloro-pentammino-iridium sulphate by treating it with barium chloride. Prepared by the first method it separates in wine-coloured crystals, whilst by the second method it is yellow. The red colour of the first product is due to a small quantity of iridium trichloride, which separates with the chloro-ehloride and may be removed by heating the hot aqueous solution with hydrogen sulphide. It crystallises in 1 Jorgensen, J. prakt. Ghem., 1888, 34, 394 Palmaer, Ber., 1891, 24, 2090. 

Chlor o - pentammino - iridium Bromide, [Ir(NH3)5Cl]Br2, is obtained by decomposing chloro-pentammino-iridium sulphate with the theoretical quantity of barium bromide. It separates in pale yellow rhombic crystals and is soluble in water. 

Bro mo-pentammino-iridium Sulphate, [Ir(NH3)5Br]SO4.H20, is produced by triturating the nitrite with excess of sulphuric acid. On the addition of water slender needle-shaped crystals separate on cooling. They appear to be the acid sulphate, but are converted into the normal salt by redissolving in water and precipitating with alcohol. The substance crystallises in shining yellowish plates which lose their water of hydration at 100° C-.1... 

Dichloro - tetrammino - iridium Sulphate, [Ir(NH3)4Cl2]2 S04.H20, is obtained by rubbing the chloride with concentrated sulphuric acid till no more hydrochloric acid is evolved. The residue... 

Iridium sulphate unites with the sulphates of the alkali metals to yield a series of well-defined crystalline salts known as alums. These are isomorphous with the better known alums of aluminium, chromium, and iron. Application of Mitscherlich s Law, therefore, points to their having a composition represented by the general formula ... 

It is also formed by adding potassium nitrite to a solution of iridium sulphate in the warm.1 It is a white powder, which readily dissolves in boiling water, but which is insoluble in potassium chloride solution. 

Ammonium Iridi-nitrite, (NHJ3Ir(NOJ)6, may be prepared by addition of sodium nitrite and ammonium sulphate to a warm solution of iridium sulphate. It separates out as a white powder, similar to the potassium salt.4 When boiled with water it evolves nitrogen, and heated in the dry state it detonates. 

Platinum alloys containing from 0 5 to 20 per cent, of tantalum are hard, withstand heat, as well as the action of adds and fused potassium hydrogen sulphate, and are more resistant to the action of aqua-regia than platinum.8 They possess the mechanical properties off platinum-iridium alloys and are less expensive the relative quantities, of tantalum and iridium required to produce the same hardness and mechanical resistance are stated to be 1 5. Platinum-tantalum alloys, hence have been recommended for various purposes in place of platinum or platinum-iridium. Tantalum can also be coated with platinum, andl can then be utilised in high-temperature work. ... 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

Pyrogallol (pyrogallic acid).—According to A. G. Perkin and F. M. Perkin,3 purpurogallin CuEgOs, can readily be obtained by electrochemical oxidation of pyrogallol in dilute sulphuric acid with addition of sodium sulphate at a platinum-iridium anode. 

Cobalt, rhodium, and iridium yield sulphates of the type R2(S04)a, and these combine with sulphates of the alkali metals to produce alums, of the general formula MaS01.R2(S01)3.24H80. These are well-defined... 

Fusion with potassium hydrogen sulphate is without effect upon ruthenium, although in like circumstances rhodium, palladium, and iridium are attacked. 

Rhodium is insoluble in acids, even in aqua regia, although when its alloys are attacked by this latter mixture a portion of the rhodium passes into solution. When fused -with potassium hydrogen sulphate, rhodium dissolves, yielding the sulphate. This reaction is interesting as affording a convenient method of separating the metal from iridium and platinum (see p. 34-1). 

Rhodium sulphate, like its analogues the sulphates of cobalt and iridium, yields stable salts with sulphates of the alkali metals known as alums. These are well-defined crystalline salts, isomorphous with the better known iron and aluminium alums. They thus form an interesting link between these metals and the central vertical column in Group VIII, of which rhodium is the middle member. These alums... 

This alum is of interest inasmuch as its formation renders it easy to separate rhodium from iridium. The sulphates of the metals, dissolved in acidulated water, are treated with caesium sulphate and evaporated. The rhodium alum crystallises out in a pure state, entirely free from iridium.1... 

Metallic iridium is thus obtained together with oxide of iron. The whole is heated to redness with potassium hydrogen sulphate, which removes the iron and any remaining traces of rhodium. The residue is well washed with water, then with chlorine water to remove any traces of gold, and finally with hydrochloric acid to take out any silica which may have accidentally been introduced with the alkalies or have come from the vessels employed. The resulting iridium is calcined with charcoal and melted into an ingot. 

Iridium tetrachloride is readily reduced to the trichloride. Its aqueous solution, on dilution, yields hypoehlorous acid and the trichloride. On boiling, a precipitate of oxychloride is obtained. Addition of excess of alkali precipitates part of the iridium as dioxide, the remainder staying in solution as sesquioxide, being precipitated only upon neutralisation of the alkali. Addition of alcohol to the alkaline solution precipitates metallic iridium, aldehydes and alkali formates being simultaneously produced. Reducing agents, such as stannous chloride, sulphur dioxide, nitric oxide, hydrogen sulphide, ferrous sulphate, etc, convert the tetrachloride into trichloride. 

A second substance containing sulphur dioxide is described by Birnbaum as formed as a black, amorphous, insoluble residue during the preparation of the foregoing sesquisulphite. He regards it as a basic sulphite of tetravalent iridium, Ir02.S02.4H20, but it may equally well be a basic sulphate. 

Digestion with dilute aqua regia effects the solution of the platinum and lead, leaving a residue of impure iridium. The solution is filtered, evaporated, and the lead converted into sulphate by addition of sulphuric acid in requisite quantity. The platinic chloride is extracted with water, and ammonium chlor-platinate precipitated in the usual way with excess of ammonium chloride containing sodium chloride. The whole is heated to 80° C. and allowed to stand for several days, most of the rhodium remaining in solution and imparting to the liquid a rose-coloured hue. 

Fused potassium hydrogen sulphate, which readily attacks rhodium, palladium and iridium, is without action upon ruthenium. 

Detection.—Metallic Iridium, like rhodium, is insoluble in all acids, save that in a very finely divided condition it is slowly attacked by aqua regia. Fusion with potassium hydrogen sulphate oxidises the metal but does not effect its solution (contrast ruthenium and rhodium). When fused with a mixture of potassium nitrate and hydroxide an insoluble residue containing the sesquioxide, lr203, with alkali is obtained. 

Addition of reducing agents such as ferrous sulphate or stannous chloride to solutions of the alkali chlor-iridates reduces them to double salts of iridium trichloride, namely, 3MCl.IrCls or M3IrCl6, known generally as chlor-iridites. The solution is simultaneously decolorised, and the salts crystallise out on cooling. 

The sulphates of rhodium and iridium form yellow alums with the sulphates of K, NH4, Rb, Cs and TF. The salts themselves are formulated Rh2(S04)g.l5H20 and 2(804)3121120. Like palladium(II) sulphate, PdS04.2H20, they are made by the action of H2SO4 on the metals. 

Various supported platinum group metal systems have been tested for the SCR process.101 Among them, supported platinum systems appear to be the most active when jointly considering the NOx reduction level achieved and the temperature range at which the catalyst is active, while palladium, rhodium and iridium also show catalytic activity for the process and Rh and Ir apparently present higher selectivity to N2.101>i03-i07 Support effects are observed which generally depend on the type of hydrocarbon employed, the presence or absence of SO2 in the reactant mixture or the type of impurities present in the support.101 In this respect, a variety of materials like SiC>2, AI2O3, ZrC>2, sulphated alumina, zeolitic materials and activated carbons have been employed as supports of the metals and tested for the process.101-112... 

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