Palladium

“ Palladium closely resembles platinum in appearance, fusibility, malleability,1231 and ductility; but it is less dense, and has a rather more silvery colour than that metal; it is freely soluble in aqua regia, and is slowly attacked by nitric acid, but the other acids exert little or no action on it; heated to redness in the air, a very superficial blue or purple film of oxide forms on the surface, which is again reduced at a white heat. It melts at 156°—Wedgwood. Sp. gr. 11·3 to 12·1 (11·8—Wollaston; 12·14—Vauquelin) . It readily unites with copper, silver, and some other metals, by fusion. ”

“ It is then ready for galvanizing or drying in bran or beech sawdust. When articles united with soft solder are pickled in nitric acid, the solder receives a gray-black color.
Palladiumizing Watch Movements.—Palladium is successfully employed for coating parts of timepieces and other pieces of metals to preserve them against oxidation. To prepare a palladium bath use the following ingredients: Chloride of palladium, 10 parts, by weight; phosphate of ammonia, 100 parts; phosphate of soda, 300 parts; benzoic acid, 8 parts; water, 2,000 parts. ”

“ 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 (NH4) 2 PdCl4 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 (o°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. ”

“ Palladious chloride, PdCl2, 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 PdCl2•nH20, 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. ”

“ The metal obtained by this process is purer than that by the former; and if it be fused in a crucible along with borax, by the heat of a powerful air-furnace or forge, a button of malleable and ductile palladium will be produced. When a slip of it is heated to redness, it takes a bronze-blue shade of greater or less intensity, as the slip is cooled more or less slowly; but if it be suddenly chilled, as by plunging it into water, it resumes instantly its white lustre. ”

“ The hydrogen diffuses from the point of higher concentration, outside of the vessel, through the palladium, into the interior where the concentration of the hydrogen is lower. The experiment may be carried out at 280°, a temperature at which palladium readily dissolves hydrogen and is permeable to it. The metal does not dissolve nitrogen and is not permeable to it. The volume of the enclosed gas is kept constant by raising the mercury level in the outside arm of the manometer, and the total pressure of the enclosed gas is measured when equilibrium is reached. ”

“ Now, you ought to know that these metals, such as platinum, iridium, and palladium, have a strong affinity for such metals as lead and tin, and upon this a great deal depends. Very much depends upon the platinum throwing out its impurities of iron and so forth, by being taken up with the lead present in it. That you may have a notion of the great power that platinum has of combining with other metals, I will refer you to a little of the chemist's experience—his bad experience. ”

“ Gold and Palladium.—These metals are stated to alloy in all proportions. According to Chenevix, the alloy composed of equal parts of the two metals is grey, is less ductile than its constituent metals and has the specific gravity 11.08. The alloy of 4 parts of gold and 1 part of palladium is white, hard and ductile. Graham showed that a wire of palladium alloyed with from 24 to 25 parts of gold does not exhibit the remarkable retraction which, in pure palladium, attends its loss of occluded hydrogen. ”

“ Nickel, palladium, and platinum are very difficult to fuse (far more so than iron, ruthenium, and osmium, which stand before them) . Copper, silver, and gold melt far more easily in a strong heat than the three preceding metals, and zinc, cadmium, and mercury melt still more easily. Nickel, palladium, and platinum are very slightly volatile; copper, silver, and gold are more volatile; and zinc, cadmium, and mercury are among the most volatile metals. ”

“ In past discussion of the uses of platinum some confusion has resulted from the lack of appreciation that all commercial platinum is not the pure metal. The pure metal is required for chemical work of all sorts, but for other uses the iridium alloys are used. Electrical platinum contains 15 to 50 per cent. iridium, but averages 25 per cent., and jeweler’s platinum carries about 10 per cent. iridium. Palladium, another of the platinum group metals, is also of importance, chiefly in the form of palladium-gold alloys, which can be used to replace platinum in the dental and jewelry industries. ”

“ The chemical qualities of this product are also remarkable. It precipitates mercury from a solution of its chloride, and in general acts as a strong reducing agent. Exposed to the action of chlorine, bromine, or iodine, the hydrogen leaves the palladium and enters into direct union with these elements. Moreover, although the compound is readily decomposed by heat, the gas can not be expelled from the metal by simple mechanical means. ”

“ That curious metal, platinum, and also palladium, possesses a property of maintaining a slow combustion, which the discoverer of the safety-lamp proposed to render available to a very important purpose. If we take a coil of platinum wire, and, having made it red-hot, plunge it into an explosive atmosphere of carburetted hydrogen and common air, it continues to glow with considerable brightness, producing, by this very peculiar influence, a combination of the gases, which is discovered by the escape of pungent acid vapours. ”

“ 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. ”

“ 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. ”

“ The palladium having been precipitated from the muriatic solution of the platinum ore previously saturated with soda, by the cyanide of mercury, muriatic acid is to be poured into the residuary liquid, and the mixture is to be evaporated to dryness, to expel the hydrocyanic acid, and convert the metallic salts into chlorides. The dry mass is to be reduced to a very fine powder, and washed with alcohol of specific gravity 0·837. ”

“ Hoitsema has expressed the opinion that the occlusion of hydrogen by palladium is a process of continuous absorption, the peculiar form of the curve—the flat middle portion—being possibly due to a condensation of the gas, even at temperatures far above the critical temperature of liquid hydrogen. ”

“ Behavior of Metals with Hydrogen.—From researches carried on conjointly by Messrs. Troost and Hautefeuille, and reported to the French Academy of Sciences, it appears that potassium, sodium, and palladium, combine with hydrogen, while a considerable number of other metals merely dissolve this gas. Iron, nickel, and manganese, offer striking analogies in their behavior with hydrogen at different temperatures. The facility with which they absorb or give off hydrogen gas depends greatly on their physical condition. ”

“ A high current-density being employed, the turn-over of gold is rapid—an essential factor of success when the costliness of the metal is taken into account. Platinum and palladium dissolved from the anode accumulate in the solution, and are removed at intervals of, say, a few months by chemical precipitation. It is essential that the bath should not contain more than 5% of palladium, or some of this metal will deposit with the gold. The slimes are treated chemically for the separation of the metals contained in them. ”

“ The six metals: ruthenium, Ru, rhodium, Rh, palladium, Pd, osmium, Os, iridium, Ir, and platinum, Pt, are met with associated together in nature. Platinum always predominates over the others, and hence they are known as the platinum metals. By their chemical character their position in the periodic system is in the eighth group, corresponding with iron, cobalt, and nickel. ”

“ All the platinum metals, like those of the iron group, are grey, with a comparatively feeble metallic lustre, and are very infusible. In this respect they stand in the same order as the metals of the iron series; nickel is more fusible and whiter than cobalt and iron, so also palladium is whiter and more fusible than rhodium and ruthenium, and platinum is comparatively more fusible and whiter than iridium or osmium. ”

“ He especially examined the alloys formed by the union of platinum and palladium with other metals, in order to determine the true nature of palladium, and to establish his claim as the discoverer of a new metal. ”

“ The metal has been obtained in the crystalline condition by distillation in the electric furnace, or by decomposing its fluoride at a red heat (H. Moissan) , Platinum, like palladium, absorbs large quantities of hydrogen and other gases, the occluded gas then becoming more “active”, for this reason platinum is used largely as a catalytic agent. Several forms of platinum, other than the massive form, may be obtained. Spongy platinum is produced when ammonium platinochloride is ignited; platinum black on the reduction of acid solutions of platinum salts ”

“ Oxide of Palladium.
The discoverer of this metal, Dr. Wollaston, has given us its distinguishing chemical properties; but we are indebted to Berzelius and Vauquelin for the proportions of oxygen and sulphur which combine with the metal (Vid. Annal. de Chimie, 77 and 78.) Few chemists have had an opportunity of making experiments on this metal, owing to its great scarcity and the consequent high price of it (1 shilling per grain) . It does not seem desireable that any but those skilled in the more delicate chemical manipulations should operate upon articles such as the present. ”

“ Passing over the discovery of carbon disulphide by W.A. Lampadius in 1796, of chromium by L.N. Vauquelin in 1797, and Klaproth’s investigation of tellurium in 1798, the next important series of observations was concerned with platinum and the allied metals. Platinum had been described by Antonio de Ulloa in 1748, and subsequently discussed by H.T. Scheffer in 1752. In 1803 W.H. Wollaston discovered palladium, especially interesting for its striking property of absorbing (“occluding”) as much as 376 volumes of hydrogen at ordinary temperatures, and 643 volumes at 90°. ”

“ He suggested that it is an element having an atomic weight almost incomparably small compared with hydrogen, incapable of forming compounds, but possessing the property, owing to its small atomic weight and extremely high velocity of its atoms, of penetrating and pervading all other substances, just as argon and helium enter into and dissolve in water and other liquids, or hydrogen passes through platinum and palladium. ”

“ See Lemon juice.
RHO′DIUM. A whitish metal discovered by Wollaston, in 1803, associated with palladium in the ore of platinum.
It is chiefly employed for tipping the nibs of metallic pens (‘rhodium’ or ‘everlasting pens’) . A very small quantity added to steel is said to improve its closeness, hardness, and toughness, and to render it less easily corrodible by damp. ”

“ Platinum itself he discovered how to work on a practical scale, and he is said to have made a fortune from the secret, which, however, he disclosed in a posthumous paper (1829) ; and he was the first to detect the metals palladium (1804) and rhodium (1805) in crude platinum. ”

“ If such a palladium mass be half immersed in water containing benzol, hæmoglobin, potassium iodide, or indigo, in the depths of the fluid strong reductions ensue, with at the surface equally violent oxidations. ”

“ The full list of metals discovered since 1800 may be found under “Chemistry.” The more important only are here given. Palladium and rhodium were reduced by Wollaston in 1804. Potassium and sodium were first separated in metallic form by Sir Humphrey Davy in 1807, through the agency of the voltaic arc; barium, strontium, calcium and boron by the same scientist in 1808; iodine by Courtois in 1811; selenium by Berzelius in 1817; cadmium by Stromeyer in 1817; silicon by Berzelius in 1823, and bromium by Balard in 1826. ”