Palladium, in Greek legends, a wooden image of Pallas or Minerva, thrown down to earth by Jupiter. It fell in the neighborhood of Troy, where Ilus the founder of that city, who had just prayed for favorable omens, regarding it in that light, took possession of it and built for it a sanctuary. It was a tradition that Troy could never be taken while this image remained in the city, and therefore Ulysses and Diomedes were commissioned to steal it, and succeeded. There are numerous other accounts of its fate.
Palladium, a metal of the platinum group, discovered by Wollaston in 1803. It is sometimes found pure in small quantities in the form of octahedrons, mixed with grains of platinum in Brazilian ore, but usually as an alloy. It exists in platinum ore from the Ural and Santo Domingo, and it is also found, mixed with gold and selenide of lead, in the Hartz, and in auriferous ore from Zacotinga and Ooudonga in Brazil, mixed with specular iron. It is also alloyed with gold and silver in the oro pudre of Por-pez, Brazil, often amounting to 10 per cent. It is extracted from platinum ore by digesting this in nitro-muriatic acid, precipitating the platinum from the decanted liquor by chloride of ammonium, and the palladium from the filtrate by cyanide of mercury, and then calcining the cyanide thus obtained. From the pallado-auriferous ore of Brazil it is extracted by fusing this with an equal weight of silver and some nitre, which reduces the baser metals and earthy parts to slag. The alloy is cast into bars and again fused in black-lead crucibles with an equal weight of silver, so that the gold shall amount to one fourth of the mixture.
This alloy is then granulated by pouring it into water through a sieve, when it is heated with twice its weight of equal quantities of nitric acid and water, the liquor decanted, and the residue boiled with pure nitric acid in quantity equal to two thirds the weight of granules used. From these nitric acid solutions the silver is precipitated by common salt, and the palladium and copper from the filtrate by zinc, in wooden vessels. The resulting black powder is dissolved in nitric acid, the solution supersaturated with ammonia, and the filtrate from this saturated with hydrochloric acid, which precipitates the greater part of the palladium as a yellow ammonio-protochloride, which is then washed in cold water and reduced to a metallic state by ignition. The remainder of the palladium and the whole of the copper may be precipitated from the hydrochloric acid solution by iron. - The symbol of palladium is Pd: its atomic weight, 106.5; sp. gr., 11.4 to 11.8. It is the most fusible of all the metals of the platinum group, beginning to fuse in the forge, and easily melting before the oxyhydrogen blowpipe at 2,480°. Its color is intermediate between silver and platinum.
When obtained from the cyanide, or from the ammonio-protochloride by ignition, it has the form of a spongy gray mass, which when finely divided floats on water, and has a blood-red color by transmitted light. It is dimorphous, having the form of cubes and octahedrons, and also of six-sided tables, with cleavage parallel to the terminal faces. It is about as hard as platinum, but somewhat less ductile. When heated on lime to the melting point of iridium, it volatilizes in green vapors, which condense to a bistre-colored dust of metal and oxide. It oxidizes at a lower temperature than silver, and is easily oxidized by hydrated alkalies. Its alloys with iron, tin, lead, arsenic, and bismuth are very fusible and brittle. With twice its weight of silver it forms a ductile alloy not liable to tarnish, and well adapted for the construction of small weights. Palladium is also used for the construction of graduated scales for astronomical instruments. Its alloy with gold is hard, and remarkable for its whiteness. With mercury it forms a fluid amalgam. Palladium has the remarkable property of absorbing many times its volume of hydrogen, yielding it again at a high temperature, and was employed by Graham in experiments on the occlusion of hydrogen.
Palladium foil heated for three hours between 195° and 106° F. absorbed 643 volumes of hydrogen; and if the metal after having been heated to redness was allowed to cool in vacuo, it absorbed at common temperatures 376 volumes of the gas. No alteration was produced in the metallic appearance of the foil. Spongy palladium absorbed 686 volumes of hydrogen, but no oxygen or nitrogen. When a wire of the metal is made the negative pole of a voltaic cell decomposing water acidulated with sulphuric acid, a still greater quantity of hydrogen can be absorbed, as much as 936 volumes to one of palladium, the metal increasing in bulk from 100 to nearly 105 volumes, or 16 times as much as if heated from 32° to 212°. When the galvanic current is reversed, and the piece of palladium becomes the positive pole, the hydrogen is rapidly converted into water by union with the nascent oxygen; and by applying a clamp with a movable index, the expansion and contraction of the metal on changing the current can be easily observed. - Palladium, like platinum, forms two classes of compounds: the palla-dious compounds, in which it is bivalent, and the palladic, in which it is quadrivalent. The dichloride, or palladious chloride, PdCl2, is obtained by the action of nitro-muriatic acid.
The tetrachloride or palladic chloride exists only in solution and in combination With alkaline chlorides. It is formed by digesting the dichloride in nitro-muriatic acid, has an intense brown color, and is decomposed by evaporation. Palladious iodide is precipitated from the chloride or nitrate, as a black mass, by soluble iodides. Palladium salts are employed for the quantitative analysis of iodine, as chlorine and bromine are not precipitated by them. The oxides of palladium are the monoxide, or palladious oxide, PdO, and the dioxide or palladic oxide, Pd02. The latter is not obtainable in a separate condition, but exists as a hydrated palladic oxide, which obstinately retains a portion of alkali when precipitated from solutions of palladic chloride by the action of alkalies. There are three sulphides, PdS, PdS2, and Pd2S. Palladious nitrate has the form of rhombic prisms, soluble in a small quantity of water, but decomposing and forming a basic nitrate in a large quantity. The other salts are of little interest.