PALLADIUM is a metal always present to some extent in platinum ores, which sometimes consist almost wholly of this metal ; it is occasionally, but rarely, found in small amounts in osmiridium and sometimes occurs in small quantities combined with gold in the Harz Mountains. (Symbol Pd, atomic number 46, atomic weight, 106.7.) It is also present in appreciable amounts in most nickel ores, generally associated with a smaller amount of platinum, and in the Mond process of nickel extraction the residues (after removal of nickel) are worked up for platinum metals, so that considerable quantities of both palladium and platinum are obtained from this source. Palladium was first isolated by Wollaston in 1802, who instead of publishing his discovery in the usual way placed a small amount on sale in Lon don as a new metal, naming it palladium after the recently dis covered planet Pallas. An English chemist, Chenevix, examined the substance and concluded that it was merely a platinum amalgam. The matter was cleared up when Wollaston disclosed the source of the metal in a paper in which he also announced the discovery of rhodium (Phil. Trans., 1804, 428; 1805, 316).
Palladium is readily obtained in the pure state by several methods. It alone of the platinum metals forms an insoluble cyanide ; in ordinary treatment, after most of the osmium, iridium and ruthenium have been removed, the solution contain ing platinum, palladium and rhodium is neutralized and precipi tated with mercuric cyanide (alkaline cyanides are not so suitable, as soluble double cyanides are formed), palladium cyanide, being precipitated, which on simple ignition yields pure palladium. Another method of separating palladium is to add to a nearly neutral solution a large excess of ammonia. To the clear solution hydrochloric acid is added in excess when spar ingly soluble palladosammine chloride, is slowly precipitated, which on ignition gives pure spongy palladium. Pal ladium is a whitish metal very similar in appearance to platinum, it is malleable and ductile and has a specific gravity of about I1.5. It melts more readily than any other of the platinum metals, fusing at 1,549° C and being easily volatilized in an electric fur nace. Palladium sponge is produced by ignition either of the cyanide or palladosammine chloride or ammonium palladium chlo ride (N11,),PdC1,; palladium black is obtained by reduction of any of its salts in solution by formic acid. A stable black colloidal
solution of palladium is produced by reducing with hydrazine hydrate a solution of palladous chloride in the presence of a pro tective colloid, or in a less stable colloidal solution by passing an arc between palladium terminals under water containing a trace of caustic soda. Palladium differs from other platinum metals in being readily soluble in strong nitric acid, especially when this contains lower oxides of nitrogen. In the finely divided state of palladium black it is soluble even in hydrochloric acid. It is readily attacked by fusion with caustic alkalis, and when heated in a current of air or oxygen is said to form the lowest oxide, PdO. It is attacked by fluorine at the ordinary temperature and at a dull red heat by chlorine ; on heating it combines directly with sulphur.
A very remarkable property of palladium is its power of ab sorbing hydrogen. Graham showed that a palladium wire at a dull red heat was capable of absorbing 935 times its volume of hydrogen, thereby increasing in length by 1.6% or in bulk by just over 4%. Another experiment gave an increase in bulk of nearly The maximum amount of hydrogen absorbable by palladium is roughly represented by the formula and it was at one time supposed that a definite compound was formed. This, how ever, has now been completely disproved chiefly by the work of Silverts and his co-workers 0910, who have shown that the amount of hydrogen absorbed is a function of both temperature and pressure and that above 138° C the amount of hydrogen ab sorbed is a constant agreeing closely with the formula (T=temperature, P=pressure). Hence the composition of the substance is variable and it cannot be a true compound. This property of palladium is shared but to a much smaller extent by other allied metals, e.g., the platinum group, nickel, iron, copper and tantalum, and it is on this property that the catalytic reduc tion by hydrogen of many substances depends, for the hydrogen molecule, appears to be dissociated into two atoms when in this condition and thus becomes enormously more reactive (see CATALYSIS). Owing to this property also, hydrogen is capable of diffusing through sheets of these metals at a high temperature.