Three oxides of palladium are known, Pd0 and Pd02; of these the first is unimportant but the others are basic and give rise to the formation of two series of salts closely analogous to the corresponding platinum compounds, but the palladic salts are less stable than the corresponding platinic ones. Palladous chloride, formed by the action of chlorine at a dull red heat upon the metal or upon the sulphide PdS, or by the action of hot dilute aqua regia or a mixture of hydrochloric acid and chlorine upon the finely divided metal, crystallizes in reddish brown crystals with two molecules of water. It combines readily with chlorides of the alkali metals giving double chlorides of the composition A solution of readily absorbs car bon monoxide, yielding definite compounds. This reaction is used in analysis as a method of determining carbon monoxide in gaseous mixtures. Palladic chloride, is prepared by dis solving the metal in cold strong aqua regia, when palladichloric acid, H,PdC1„, is formed. The simple chloride can be prepared in solution by the action of chlorine upon palladous chloride or by dissolving palladic oxide, in strong hydrochloric acid. The chloride combines easily with alkaline chlorides giving palladi chlorides of the composition Palladous iodide, is formed by precipitating a solution of any palladous salt with po tassium iodide, as a black flocculent precipitate soluble in excess of the reagent to a cherry-red solution.
Palladous sulphide, PdS, precipitated from solutions of pal ladous salts by sulphuretted hydrogen, is also formed by heating the metal in sulphur vapour ; it is insoluble in yellow ammonium sulphide. Palladic sulphide, can be prepared by dry methods only, such as by fusing the lower sulphide with a mixture of sul phur and sodium carbonate. Palladium salts combine with am
monia in two different proportions forming compounds of the types (palladosammines) and (palladodiam mines). Compounds of the former group are stable but those of the latter group easily lose ammonia and pass into the lower com pounds. The ordinary salts of both series are well known and the water-soluble hydroxides of both behave as strong bases.
The uses to which palladium has been put are many and various. It is frequently used in astronomical and other instru ments of precision as the basis of graduated surfaces on account of its unalterability and comparative lightness. It is also used for coating silver goods, for it is not affected by sulphuretted hydrogen as is silver, and is sometimes deposited on glass for the production of permanent mirrors. Palladium is used to a certain extent in dentistry, and during the World War considerable quan tities were used for jewellery as platinum was then unobtainable.
It is being used more and more in various aeroplane parts. The yellow colour of gold is destroyed by alloying it with small quantities of platinum or palladium; the product, white gold, is extensively used for jewellery. The whitening effect of palladium in gold is considerably greater than that of platinum. Palladium is sometimes used for the springs of clocks and watches, for it has the advantage over steel of not being magnetised under any conditions. During the World War, when platinum was controlled and practically unpurchasable, attempts were made to use pal ladium instead of platinum in the manufacture of sulphuric acid. This was only partly successful; when the exact conditions were found, palladium was just as efficient as platinum, but slight alteration from the perfect conditions caused a great falling-off in efficiency. (F. E. M.)