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PALLADIUM [symbol Pd, atomic weight I...

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Originally appearing in Volume V20, Page 637 of the 1911 Encyclopedia Britannica.
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PALLADIUM [See also:symbol Pd, atomic See also:weight Io6.7 (O=16)] , in See also:chemistry, a metallic See also:element associated with the See also:platinum See also:group. It is found in platinum ores, and also in the native See also:condition and associated with See also:gold and See also:silver in Brazilian gold-bearing See also:sand. Many methods have been devised for the See also:isolation of the See also:metal from platinum ore. R. See also:Bunsen (See also:Ann., 1868, 146, p. 265), after removing most of the platinum as ammonium platinochloride, precipitates the residual metals of the group by See also:iron; the resulting precipitate is then heatedwith ammonium chloride and evaporated with fuming nitric See also:acid, the See also:residue taken up in See also:water, and the palladium precipitated as See also:potassium palladium chloride. This is purified by dissolving it in hot water and evaporating the See also:solution with oxalic acid, taking up the residue in potassium chloride, and filtering off any potassium platinochloride formed. The filtrate deposits potassium palladium chloride, which on See also:heating in a current of See also:hydrogen leaves a residue of the metal. Roessler (Zeit. f. chemie, 1866, p. 175) precipitates both platinum and palladium as See also:double chlorides, the resulting mixed chlorides being reduced to the metals by ignition in hydrogen, taken up in aqua regia, the solution neutralized, and the palladium precipitated by mercuric See also:cyanide. See also T. Wilm (Ber., 188o, 13, p.

1198; 1881, 14, p. 629; 1882, 15, p. 241) on its separation as palladosammine chloride, and See also:

Cox (Phil. Meg., 1843, 23, p. 16) on the separation of palladium from Brazilian gold sand. Pure palladium may be obtained by the reduction of the double chloride (NH4)2 PdC14 in a current of hydrogen, or of palladious chloride with formic acid. It is a ductile metal of silvery lustre, with a specific gravity of 11.97 (o°C.). It is the most easily fusible of the metals of the platinum group, its melting-point being about 1530-1550 C. (L. See also:Holborn and F. Henning, Sitzb. Akad.

See also:

Berlin, 1905, p. 311). It readily distils when heated in the electric See also:furnace. Its mean specific See also:heat between o° and t°C. is 0.0582 + o•00001ot (J. Violle, Comptes rendus, 1879, 89, p. 702). Palladium finds application in the See also:form of See also:alloys for astronomical See also:instruments, in See also:dentistry, and in the construction of springs and movements of clocks. Native palladium is dimorphous. It is soluble in nitric acid, more especially if the acid contains oxides of See also:nitrogen, and when obtained in the finely divided condition by reduction of its salts, it is to some extent soluble in hydrochloric acid. It also dissolves in boiling concentrated sulphuric acid and in hydriodic acid. It oxidizes when fused with See also:caustic alkalis. It combines with See also:fluorine and with See also:chlorine at a dull red heat, but not with See also:iodine, whilst See also:bromine has scarcely any See also:action on the metal.

It combines with See also:

sulphur directly, and according to T. Wilm (Ber., 1882, 15, p. 2225) forms the See also:oxide Pd2O, when heated in a current of See also:air. Two See also:series of salts are known, namely, palladious salts and palladic salts, corresponding to the two oxides PdO and PdO2. Of these the palladious salts only are See also:stable, the palladic salts readily passing into the palladious form on boiling with water. The palladium compounds show a See also:complete See also:analogy with the corresponding platinum salts. All the salts of the metal when heated decompose and leave a residue of the metal; the metal may also be obtained from solutions of the salts by the addition of See also:zinc, iron, formic acid, See also:phosphorus and hot See also:alcohol. Sulphuretted hydrogen gives with palladium salts a precipitate of palladium sulphide which is insoluble in ammonium sulphide; mercuric chloride gives the characteristic yellowish precipitate of palladious chloride, and potassium iodide the See also:black palladious iodide which dissolves on addition of excess of the precipitant. These two latter reactions may be used for the recognition of palladium, as may also the behaviour of the salts with See also:ammonia, this reagent giving a See also:brown precipitate, which turns to a red shade, and is soluble in a large excess of the precipitant to a clear solution, from which by adding hydrochloric acid a yellow precipitate of palladosammine chloride, Pd(NH3)2C12, is obtained. Palladium is permeable to hydrogen at a temperature of 240° C. and upwards. It absorbs hydrogen and other gases, the heat of occlusion being 4640 calories per See also:gram of hydrogen. The occluded hydrogen is strongly See also:bound to the metal, only traces of the See also:gas being given off on See also:standing in vacuo, but it is easily removed when heated to See also:loo° C.

T. See also:

Graham (Phil. Mag., 1866–1869) was of the See also:opinion that the occluded hydrogen underwent See also:great condensation and behaved as a quasi-metal (to which he gave the name " hydrogenium "). forming an alloy with the palladium; but L. Troost and P. Hautefeuille (Ann. See also:chime phys., 1874, (5) 2, p. 279) considered that a definite See also:compound of See also:composition Pd2H was formed. The more See also:recent See also:work of C. Hoitsema (Zeit. phys. chim., 1895, 17, p. I) however, appears to disprove the formation of a definite compound (see also J. See also:Dewar, Phil. Mag., 1874, (4) 47, pp. 324, 342).

A palladium hydride was obtained by Graham by the reduction of palladious sulphate with See also:

sodium hypophosphite. It is an unstable black See also:powder, which readily loses hydrogen at o° C. C. Paal and J. Gerum (See also:Ben., 1908, 41, p. 818) have shown that-when palladium black is suspended in water one See also:volume of the metal combines with 1204 volumes of hydrogen, or in the atomic proportion Pd/H =1/•98. Palladious oxide, PdO, is a black powder formed by heating spongy palladium to a dull red heat in a current of See also:oxygen or by See also:gentle ignition of the nitrate. It is insoluble in acids, is easily reduced, and decomposes when heated. Palladic oxide, PdO2, is obtained in the hydrated condition, Pd02•nH2O, by the action of See also:ozone on palladious chloride; by the electrolytic oxidation of palladious nitrate in slightly acid solution (L. See also:Wohler) ; and by the action of caustic potash on potassium See also:palladio-chloride, the liquid being neutralized with acetic acid (I. Bellucci, Zeit. anorg. Chem., 1905, 47, p.

287). It is a dark red or brown coloured powder, which loses oxygen on heating. When boiled with water it passes into the See also:

lower oxide. It is an energetic oxidizing See also:agent, and when freshly prepared is soluble in dilute See also:mineral acids. A hydrated form of the monoxide, PdO•nH2O, is obtained by hydrolyzing a faintly acid solution of the nitrate (L. Wohler, Zeit. anorg. Chem., 1905, 46, p. 323), or by the action of a slight excess of caustic soda on the double chloride K2PdC16. It is a dark brown powder which loses its water of hydration when dried in air, and in the dry condition is difficultly soluble in acids. By the electrolytic oxidation of palladious nitrate L. Wohler and F. See also:Martin (Ib., 1908, 57, P.

398), obtained a hydrated oxide, Pd203•nH2O, as a dark brown powder which dissolves in hydrochloric acid, forming an unstable chloride. Palladious chloride, PdC12, is obtained as a deliquescent crystalline See also:

mass when spongy palladium is heated to dull redness in a current of dry chlorine. A hydrated form, of composition PdCl2.2H2O, results on dissolving palladium in aqua regia, containing only a small proportion of nitric acid. It crystallizes from water as a reddish-brown solid. It absorbs hydrogen and is easily reduced. It combines with See also:carbon monoxide to form compounds of composition PdC12.2CO; 2PdC12.3CO; PdC12•CO (E. Fink, Comptes Rendus, 1898, 126, p. 646), and can be used for the determination of the amount of carbon monoxide in air (Potain and R. Drouin, Ib., 1898, 126, p. 938). On treatment with dry ammonia gas it yields palladodiammine chloride, Pd(NH3)4C12. Palladious chloride combines with See also:hydroxylamine to form the compounds Pd(NH30)4C12 and Pd(NH3O)2C12.

The first results from the action of hydroxyl-amine on the chloride in the presence of sodium carbonate, and may be isolated as the See also:

free See also:base. The other is thrown down as a yellow granular precipitate when a small quantity of dilute hydrochloric acid is added to the base, Pd(NH3O)4(OH)2 (S. Feisel and A. Nowak, Ann., 1907, 351, p. 439). The chloride PdC14 is only known in acid solution, and is obtained when palladium is dissolved in aqua regia or when palladic oxide is dissolved in concentrated hydrochloric acid. The solution is brown in See also:colour and gradually loses chlorine, being converted into palladious chloride. Both chlorides combine with many other metallic chlorides to form characteristic double salts, the double potassium salts having the formulae K2PrICl4 and K2PdC16. The former may be prepared by adding an excess of potassium chloride to palladious chloride, or by boiling K2PdCI6 with a large excess of water. It crystallizes in prisms which are readily soluble in water but are practically insoluble in See also:absolute alcohol. It is decomposed by See also:direct heating, and also by heating in a current of hydrogen. The latter compound is formed when chlorine is passed into a warm aqueous solution of the former or by dissolving palladium in aqua regia and saturating the solution with potassium chloride.

It crystallizes in See also:

scarlet octahedra which darken on heating, and decompose when strongly heated. It is slightly soluble in See also:cold water, but dissolves in warm dilute hydrochloric acid. When boiled with alcohol it is reduced to the metallic condition. The subsulphide, Pd2S, is obtained as a hard, See also:green coloured mass when palladosammine chloride is fused with sulphur or when the sulphide PdS is fused with sulphur and ammonium chloride. It loses sulphur slowly when heated and is insoluble in acids. Palladious sulphide, PdS, is obtained by precipitation of the corresponding salts with sulphuretted hydrogen, or by the action of dry sulphuretted hydrogen gas on palladosammine chloride. As pre-pared in the dry way it is a hard, See also:blue coloured, insoluble mass, but if obtained by precipitation is of a brownish-black colour and is soluble in nitric acid. When heated in air it oxidizes to a basic sulphate. The disulphide, PdS2, is a brownish-black crystalline powder which is formed when the double ammonium palladium chloride (NH4)2PdC16 is heated to redness with caustic soda and sulphur. It combines with the alkaline sulphides. It gradually loses sulphur on heating, and is easily soluble in aqua regia. A sulphide of composition Pd3S4 has been described (R.

See also:

Schneider, Fogg. Ann., 1873, 148, p. 625). Palladium sulphate, PdSO4.2H2O, is obtained by dissolving the oxide in sulphuric acid, or by the action of nitric and sulphuric acids on the metal. It forms a reddish-brown, deliquescent, crystalline mass, and is easily soluble in water, but in the presence of a large excess of water yields a basic sulphate. Palladium nitrate, Pd(NO3)2, crystallizes in brownish-yellow deliquescent prisms and is obtained by dissolving the metal in nitric acid. It is very soluble in water, and its aqueous solution decomposes on boiling, with precipitation of a basic nitrate. Palladium cyanide, Pd(CN)2, is obtained as a yellowish precipitate when palladium chloride is precipitated by mercuric cyanide. It is insoluble in water, and on heating decomposes into palladium and See also:cyanogen. It is soluble in solutions of the alkaline cyanides, with formationof double cyanides of the type K2Pd(CN)4. On See also:account of its insolubility and its stability it is useful for the separation of palladium from the other metals of the platinum group. The palladium salts combine with ammonia to form characteristic compounds, which may be grouped into two See also:main divisions: (I) the palladammines (palladosammines) of type [Pd(NH3)2X21, and (2) the palladodiammines [Pd(NH3)41X2.

The palladosammines are obtained by adding a large excess of ammonia to the palladious salts, the resulting clear solution being then precipitated by the mineral acid corresponding to the See also:

salt used. This method of preparation serves well for the chloride, from which other salts may be obtained by double decomposition. These salts are fairly stable, and are red, yellow or See also:orange in colour. The palladodiammine salts are mostly colourless, and are not very stable; acids convert them into the palladosammines, and they lose two molecules of ammonia very easily. They are formed by the action of a large excess of ammonia on the palladious salts or on the corresponding palladosammine salts in the presence of water. Numerous determinations of the atomic weight of palladium have been made, the values obtained varying from I05.7 to 107.249 (see Amer. Chem. Jour., 1899, 21, p. 943 ; Ann., 1905, 341, p. 235; Jour. Chem. See also:Soc., 1894, 65, p.

20). The See also:

International See also:Commission on Atomic Weights, 1909, recount several new determinations: Haas (Dissertation, See also:Erlangen, 1908) from reduction of palladosammine bromide obtained the value 106.7; Kemmerer (Thesis, See also:Pennsylvania, 1908), from reduction of the corresponding chloride and cyanide obtains a mean value of 106.434; whilst A. Gutbier and his collaborators, from analyses of palladosammine chloride and bromide, obtained the values Io6.640.03 and Io6.65to•o2 from the chloride, and Io6.655 from the bromide (Jour. pr. chem., 1909, ii. 79, PP. 235, 457).

End of Article: PALLADIUM [symbol Pd, atomic weight Io6.7 (O=16)]

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