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RHODIUM [symbol Rh; atomic weight 102...

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Originally appearing in Volume V23, Page 269 of the 1911 Encyclopedia Britannica.
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RHODIUM [See also:symbol Rh; atomic See also:weight 102.9 (0=16)] , in See also:chemistry, a metallic chemical See also:element found, associated with the other elements of the See also:platinum See also:group, in crude platinum ore, wherein it was discovered in 1803 by W. H. See also:Wollaston (Phil. Trans., 1804, p. 419). It may be obtained from the residues of platinum ore after treatment with aqua regia and removal of the platinum as chlorplatinate. The See also:mother liquors are decomposed by treatment with metallic See also:iron, the precipitate obtained being warmed with concentrated nitric See also:acid and heated in an iron crucible with concentrated See also:caustic potash. The See also:residue thus obtained is mixed with See also:salt and 1 Unless otherwise stated, the See also:place of publication is See also:London. heated in a current of See also:chlorine, any See also:iridium See also:present being converted into its chloride by treatment with nitric acid and precipitated by ammonium chloride, whilst rhodium ammonium chloride goes into See also:solution with its characteristic See also:rose-red See also:colour (C. E. Claus,. Jour. prakt.

Chem., 1843-1845). For other methods of extraction see See also:

Gibbs, ib., 186x, 84, p. 65; 1865, 94, p. 10; T. Wilm, See also:Bull. See also:soc. chim., 188o (2), 34, p. 679; E. See also:Fremy, Comptes rendus, 1854, 38, p. 1oo8, &c.). The See also:metal itself is best obtained by the reduction of chlorpurpureo rhodium chloride, (C12Rh2.10NH3) • CL,, in a current of See also:hydrogen, the metal after reduction being cooled in a stream of See also:carbon dioxide (S. M. Jorgensen, Zeii. anorg. Chem., 1903, 34, p.

82). It somewhat resembles See also:

aluminium in colour ; its specific gravity varies from 11 to 12.1; and its specific See also:heat is 0•o5527 (V. H. See also:Regnault, See also:Ann. chins. phys., 1861, 63, p. 15). It is less fusible than platinum. It oxidizes superficially when heated, and may be distilled in the electric See also:furnace. It is insoluble in acids, but forms a soluble sulphate when fused with See also:potassium bisulphate (a reaction which distinguishes it from the other metals of the platinum group).. It oxidizes when fused with potassium hydroxide and potassium nitrate, to the dioxide, RhO2. It absorbs hydrogen readily. Rhodium See also:black is obtained by reducing rhodium salts with formic acid; by See also:alcohol in the presence of See also:alkali; or by precipitation with See also:zinc and iron. A colloidal rhodium may be prepared by reducing the sesquichloride with See also:hydrazine See also:hydrate.

Rhodium salts may be recognized by their characteristic reaction with freshly prepared See also:

sodium hypochlorite solution. A yellow precipitate is obtained, which on shaking for some See also:time with acetic acid gradually dissolves to an See also:orange-coloured solution. This solution after a See also:short time deposits a See also:grey precipitate, and the supernatant liquid becomes See also:azure See also:blue in colour (E. Demarcay, Comptes rendus, 1885, 101, p. 951). Several oxides of rhodium are known. The monoxide, RhO, formed when the hydrated sesquioxide is heated (Claus) or when finely divided rhodium is heated in a current of See also:air (Wilm), is a grey See also:powder which is insoluble in acids. The sesquioxide, Rh203, is a black insoluble powder, formed when the corresponding hydrate is heated. This hydrate, Rh2(OH)6, is obtained as a yellow powder, by decomposing rhodium salts(not the sulphate)with dilute solutions of the caustic alkalis. It is soluble in acids, and in the moist See also:condition is also soluble in concentrated alkalis. A hydrated rhodium dioxide, RhO2.2H2O, is formed when chlorine is passed into a solution of the sesquioxide in concentrated caustic potash, or by adding an alkaline hypochlorite to a concentrated alkaline solution of rhodium and sodium chlorides. It is a greenish-black powder which is soluble in hydrochloric acid.

Rhodium chloride, Rh2Cls, is obtained impure by See also:

heating the metal to dull redness in a current of chlorine, or, purer, by heating an alloy of rhodium and See also:tin in chlorine or by heating the See also:double ammonium rhodium chloride in chlorine at 440° C. (E. Leidie, Ann. chim. phys., 1889, (6), 17, p. 265; Comptes rendus, 1899, 129, p. 1249). It is a red powder, which decomposes at a red heat, leaving a residue of the metal. It is insoluble in See also:water and acids, but dissolves in concentrated solutions of potassium See also:cyanide. 'The hydrated See also:form Rh2Cl6.8H20 is obtained impure by dissolving the hydrated sesquioxide in hydrochloric acid, by the See also:action of hydrofluosilicic acid on potassium rhodium chloride, and by the action of chlorine on rhodium in the presence of sodium chloride. In the last method the product is dissolved in a dilute hydrochloric acid (1:1), and the solution saturated with hydrochloric acid See also:gas at o° C., allowed to stand for some time, decanted, and finally evaporated in vacuo (Leidie, loc. cit.). It forms a very deliquescent, red, amorphous See also:mass, which decomposes on exposure. It is very soluble in water, forming a yellow solution. It forms double salts with the alkaline chlorides.

Rhodium monosulphide, RhS, is formed when rhodium or rhodium ammonium chloride are heated with See also:

sulphur, and also by precipitating rhodium salts with sulphuretted hydrogen, the precipitate being dissolved in ammonium sulphide and thrown down again by dilute sulphuric acid (Lecoq de Boisbaudran, Ber., 1883, 16, p. 579). It is a dark-coloured powder which is insoluble in acids and other solvents. It loses all its sulphur when heated in air. The sesquisulphide, Rh2S3, is prepared by heating anhydrous rhodium chloride, Rh2Cls, in a current of sulphuretted hydrogen at 36o° C., or by passing the gas into a boiling solution of the chloride. It is a black powder which is insoluble in acids and in alkaline sulphides. It decomposes when strongly heated. Rhodium sulphate, Rh2(SO4)3, is prepared by oxidizing the sulphide, by fusing the metal with acid potassium sulphate, or by the action of concentrated sulphuric acid on an alloy of rhodium and See also:lead, or on the hydrated sesquioxide. It is a red powder which decomposes when heated or when boiled with muchwater. It forms alums (Leidie, Comptes rendus, 0888, 107, p. 234). Rhodium potassium See also:alum, Rh2(SO4)3•K2SO4.24H2O, is obtained by dissolving the sesquioxide in sulphuric acid and adding two-thirds of the calculated amount of potassium sulphate to the solution (A.

Piccini and L. See also:

Marino, Zeit. anorg. Chem., 1901, 27, p. 62). It crystallizes in cubes. Rhodium cyanide, Rh2(CN)s, is a See also:carmine-red powder formed when rhodium potassium cyanide is boiled with acetic acid. Rhodium potassium cyanide, KsRh2(CN)12, is formed when the sesquioxide is dissolved in caustic potash and an excess of hydrocyanic acid added gradually, the solution being then evaporated in vacuo. It is a colourless crystalline solid soluble in water, and isomorphous with the corresponding iron, See also:cobalt, See also:chromium and See also:manganese compounds. The rhodium See also:ammonia salts correspond almost with the similar cobalt compounds and may be divided into three See also:series—namely, hexammine salts (luteo-salts), [Rh(NH3)s]X3; aquopentammine salts (roseo-salts), [Rh(NH3)3•H20]X3; and pentammine salts (purpureosalts), [Rh(NH3)6X]X2. (See S. F. Jorgensen, C.

W. Blomstrand, Jour. prak. Chem., 1882, at seq.) The atomic weight of rhodium has been determined by S. F. Jorgensen (Jour. prakt. Chem., 1883, 27, p. 486), by the See also:

analysis of chlorpurpureo rhodium chloride, the mean value obtained being 103 ; whilst K. Seubert and K. Kobbe (Ann., 189o, 26o, p. 314), by analysis of the double chloride and sulphate, obtained as a mean value See also:Io2.86.

End of Article: RHODIUM [symbol Rh; atomic weight 102.9 (0=16)]

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