Chemical Properties of Palladium
|Both chlorine and fluorine attack palladium at high temperatures, yielding the respective halogenides. Chlorine water attacks it slowly, and an alcoholic solution of iodine effects the formation of a superficial layer of palladous iodide. Iodine vapour tarnishes the metal, yielding the iodide, whilst gently heating iodine and finely divided palladium causes them to unite more or less imperfectly. |
Concentrated hydrochloric acid attacks the compact metal but slowly, whilst aqua regia rapidly effects its solution. Dilute nitric acid has but little action, although, when present in certain alloys, such as those with silver or copper, palladium will dissolve in it. Concentrated nitric acid readily attacks the metal. Boiling concentrated sulphuric acid converts it into palladous sulphate. Palladium is unique in that it displaces mercury from its cyanide.
When heated to dull redness in oxygen, the monoxide, PdO, is formed, although at ordinary temperatures palladium is quite permanent in dry or moist air, and is not even attacked by ozone. On solidifying from the molten condition in an oxidising atmosphere, palladium "spits" just like silver. When heated in the oxyhydrogen flame it volatilises in greenish vapours.
Palladium, like rhodium, but unlike platinum, dissolves in fused potassium hydrogen sulphate, yielding palladous sulphate. Fusion with sodium peroxide converts it into palladium monoxide. Heated with sulphur combination takes place with incandescence.
Although closely resembling platinum in appearance, palladium is readily distinguished from the latter metal since an iodine solution upon evaporation in the warm produces a black stain on palladium but not on platinum.
Compounds of Palladium
|Three classes of palladium salts are known, namely, palladous salts, in which the metal functions as a divalent atom, and palladic salts, in which the metal is trivalent and tetravalent. The palladous salts are stable, but upon ignition yield metallic palladium. |
Palladic salts are less well known in the free state, being highly unstable, although, combined with other salts, they yield stable compounds. Examples of these are the chlor-palladates, of general formulae M2PdCl5 and M2PdCl6. The latter correspond to the chlor-platinates, with which indeed they are isomorphous but possess less stability.
Palladous salts are readily oxidised to palladic compounds, particularly when warmed. For example, a solution of palladous nitrate warmed with concentrated persulphate solution is oxidised to a brown, palladic compound, which decomposes hydrogen peroxide and ammonia vigorously, gaseous products being evolved. With hydrochloric acid it yields free chlorine.
When a solution of potassium chlor-palladite or brom-palladite is warmed with a persulphate in the presence of the corresponding halogen acid the halogen palladate is precipitated on cooling. This reaction is interesting as affording a method of separating palladium from certain other metals.
Palladous salts possess catalytic properties. For example, they accelerate the ox dation of oxalic acid by persulphates and, to a less extent, by nitric acid. This is explained on the assumption that the palladous salt is alternately oxidised to a palladic derivative by the oxidising agent and reduced by the oxalic acid.
Ammonia is likewise oxidised by persulphates in the presence of a palladous salt.
Palladium resembles both nickel and cobalt in yielding double nitrites and cyanides.
Palladium salts are reduced to metallic palladium by hydrazine, whether in acid or alkaline solution, and the product is entirely soluble in aqua regia.