# 1911 Encyclopædia Britannica/Palladium (chemistry)

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​PALLADIUM [symbol Pd, atomic weight 106·7 (0＝16)], in chemistry, a metallic element associated with the platinum group. It is found in platinum ores, and also in the native condition and associated with gold and silver in Brazilian gold-bearing sand. Many methods have been devised for the isolation of the metal from platinum ore. R. Bunsen (Ann., 1868, 146, p. 265), after removing most of the platinum as ammonium platinochloride, precipitates the residual metals of the group by iron; the resulting precipitate is then heated with ammonium chloride and evaporated with fuming nitric acid, the residue taken up in water, and the palladium precipitated as potassium palladium chloride. This is purified by dissolving it in hot water and evaporating the 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 heating in a current of hydrogen leaves a residue of the metal. Roessler (Zeit. f. chemie, 1866, p. 175) precipitates both platinum and palladium as 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 cyanide. See also T. Wilm (Ber., 1880, 13, p. 1198; 1881, 14, p. 629; 1882, 15, p. 241) on its separation as palladosammine chloride, and Cox (Phil. Mag., 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 (NH$_4$)$_2$ PdCl$_4$ 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 (0°C.). It is the most easily fusible of the metals of the platinum group, its melting-point being about 1530–1550° C. (L. Holborn and F. Henning, Sitzb. Akad. Berlin, 1905, p. 311). It readily distils when heated in the electric furnace. Its mean specific heat between 0° and t°C. is 0·0582 + 0·000010t (J. Violle, Comptes rendus, 1879, 89, p. 702). Palladium finds application in the form of alloys for astronomical instruments, in 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 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 caustic alkalis. It combines with fluorine and with chlorine at a dull red heat, but not with iodine, whilst bromine has scarcely any action on the metal. It combines with sulphur directly, and according to T. Wilm (Ber., 1882, 15, p. 2225) forms the oxide Pd$_2$O, when heated in a current of air.
Two series of salts are known, namely, palladious salts and palladic salts, corresponding to the two oxides PdO and PdO$_2$. Of these the palladious salts only are stable, the palladic salts readily passing into the palladious form on boiling with water. The palladium compounds show a complete 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 zinc, iron, formic acid, phosphorus and hot 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 black palladious iodide which dissolves en 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 ammonia, this reagent giving a 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(NH$_3$)$_2$Cl$_2$, 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 gram of hydrogen. The occluded hydrogen is strongly bound to the metal, only traces of the gas being given off on standing in vacuo, but it is easily removed when heated to 100° C. T. Graham (Phil. Mag., 1866–1869) was of the opinion that the occluded hydrogen underwent 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. chim. phys., 1874. (5) 2, p. 279) considered that a definite compound of composition Pd$_2$H was formed. The more recent work of C. Hoitsema (Zeit. phys. chim., 1895, 17, p. 1) however, appears to disprove the formation of a definite compound (see also J. Dewar, Phil. Mag., 1874, (4) 47. pp. 324, 342). A palladium hydride was obtained by Graham by the reduction of palladious sulphate with sodium hypophosphite. It is an unstable black powder, which readily loses hydrogen at 0° C. C. Paal and J. Gerum (Ber., 1908, 41, p. 818) have shown that when palladium black is suspended in water one 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 oxygen or by gentle ignition of the nitrate. It is insoluble in acids, is easily reduced, and decomposes when heated. Palladic oxide, PdO$_2$, is obtained in the hydrated condition, PdO$_2$·nH$_2$O, by the action of ozone on palladious chloride; by the electrolytic oxidation of palladious nitrate in slightly acid solution (L. Wöhler); and by the action of caustic potash on potassium 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 lower oxide. It is an energetic oxidizing agent, and when freshly prepared is soluble in dilute mineral acids. A hydrated form of the monoxide, PdO·nH$_2$O, is obtained by hydrolyzing a faintly acid solution of the nitrate (L. Wöhler, Zeit. anorg. Chem., 1905, 46, p. 323), or by the action of a slight excess of caustic soda on the double chloride K$_2$PdCl$_6$. 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. Wöhler and F. Martin (Ib., 1908, 57, p. 398), obtained a hydrated oxide, Pd$_2$O$_3$·nH$_2$O, as a dark brown powder which dissolves in hydrochloric acid, forming an unstable chloride.
Palladious chloride, PdCl$_2$, is obtained as a deliquescent crystalline mass when spongy palladium is heated to dull redness in a current of dry chlorine. A hydrated form, of composition PdCl$_2$·nH$_2$O, 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 carbon monoxide to form compounds of composition PdCl$_2$·2CO; 2PdCl$_2$·3CO; PdCl$_2$·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, lb., 1898, 126, p. 938). On treatment with dry ammonia gas it yields palladodiammine chloride, Pd(NH$_3$)$_4$Cl$_2$. Palladious chloride combines with hydroxylamine to form the compounds Pd(NH$_3$O)$_4$Cl$_2$ and Pd(NH$_3$O)$_2$Cl$_2$. The first results from the action of hydroxylamine on the chloride in the presence of sodium carbonate, and may be isolated as the free 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(NH$_3$O)$_4$(OH)$_2$ (S. Feisel and A. Nowak, Ann., 1907, 351, p. 439). The chloride PdCl$_4$ 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 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 K$_2$PdCl$_4$ and K$_2$PdCl$_6$. The former may be prepared by adding an excess of potassium chloride to palladious chloride, or by boiling K$_2$PdCl$_6$ with a large excess of water. It crystallizes in prisms which are readily soluble in water but are practically insoluble in absolute alcohol. It is decomposed by 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 scarlet octahedra which darken on heating, and decompose when strongly heated. It is slightly soluble in cold water, but dissolves in warm dilute hydrochloric acid. When boiled with alcohol it is reduced to the metallic condition.
The subsulphide, Pd$_2$S, is obtained as a hard, 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 prepared in the dry way it is a hard, 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, PdS$_2$, is a brownish-black crystalline powder which is formed when the double ammonium palladium chloride (NH$_4$)$_2$PdCl$_6$ 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 Pd$_3$S$_4$ has been described (R. Schneider, Pogg. Ann., 1873, 148, p. 625).
Palladium sulphate, PdSO$_4$·2H$_2$O, 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(NO$_3$)$_2$, 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 cyanogen. It is soluble in solutions of the alkaline cyanides, with formation of double cyanides of the type K$_2$Pd(CN)$_4$. On 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 main divisions: (1) the palladammines (palladosammines) of type [Pd(NH$_3$)$_2$X$_2$], and (2) the palladodiammes [Pd(NH$_3$)$_4$]X$_2$. 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 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 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 105·7 to 107·249 (see Amer. Chem. Jour., 1899, 21, p. 943; Ann., 1905, 341, p. 235; Jour. Chem. Soc., 1894, 65, p. 20). The International Commission on Atomic Weights, 1909, recount several new determinations: Haas (Dissertation, Erlangen, 1908) from reduction of palladosammine bromide obtained the value 106·7; Kemmerer (Thesis, 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 106·64±0·03 and 106·65±0·02 from the chloride, and 106·655 from the bromide (Jour. pr. chem., 1909, ii. 79, pp. 235, 457).