1902 Encyclopedia > Cadmium


CADMIUM, a metal closely allied to zinc. It was discovered in 1817 by Stromeyer and Hermann, independ-ently, but in a similar manner. The former chemist, in the execution of his duties as inspector of pharmaceutical products in Hanover, found a substance, sold as oxide, to be really carbonate of zinc, and, applying to the manufacturer for explanation of the reason of the substitution of the latter product for the former was informed that, although the best zinc, in which no iron could be detected, was employed, the oxide could not be produced without a slight discoloration from oxide of iron. On investigation by Stromeyer, it was found that the discoloration was due not to iron but to the oxide of a new metal, which he succeeded in isolating, and named cadmium, from the old chemical name for zinc oxide (Cadmia fossilis). About the same time, the sale of an oxide of zinc supplied by Hermann, a chemical manufacturer, who produced it from the waste of the SUesian zinc furnaces, was stopped in Prussia as being contaminated with arsenic,—the reason obviously being that the acid solution of the substance in question gave a bright yeUow precipitate when heated with sulphu-retted hydrogen. The erroneous character of this inference was, however, soon demonstrated by Hermann, who made a careful investigation of the subject, and discovered the nature of the new metal, but not before Stromeyer had published the results of his observations.

Cadmium does not occur in the metallic state in nature, and there is only one definite mineral known which contains it in quantity, namely, the sulphide, or green-ockite, which occurs at Bishopstown, in Benfrewshire, in small isolated crystals of a bright orange-yellow colour, belonging to the hexagonal system, in a doleritic rock associated with prehnite. This contains 77'7 per cent, of cadmium and 22'3 per cent, of sulphur, corresponding to the formula CdS, and is isomorphous with voltzite, the rhombohedral form of sulphide of zinc. Although an extremely rare mineral in the pure state, being confined to the single locality mentioned above, sulphide of cadmium is often present in zinc blende, the richest varieties contain-ing 3 per cent, of cadmium. Among these are the yellow radiated blende of Przibram in Bohemia, Eaton in New Hampshire, and Engis and Corfali in Belgium. It is also found in the carbonates and silicates of zinc from most of the localities producing these ores, but in what state of com-bination is doubtful, as it is not generally found in quantity sufficient to be appreciated by the analysis of samples,— being only discoverable when the ore is treated for zinc on the large scale, in the first products of the reducing processes.

Cadmium is a white metal with a slight bluish tinge by reflected light; it is whiter than lead or zinc, but less so than silver, has a high lustre when polished, and breaks, under a gradually increasing strain, with the fibrous or scaly fracture characteristic of a soft tough metal. It may be readily crystallized in octohedra, differing in this respect from the allied metal zinc which is rhombohedral. It is somewhat harder than tin, but less so than zinc, and like the former metal it emits a peculiar crackling sound when bent. It is malleable, and may be rolled into thin sheets. The specific gravity after fusion is 8'604, which is increased by hammering to 8'694. The specific heat is 0-05669 (Begnault), or 0-0576 (Dulong and Petit). The electric conductivity is 22-10, or somewhat lower than that of zinc; the thermal conductivity does not appear to have been determined. It melts at a temperature below redness (315° to 320° C), and boils at the temperature of 860° C, giving off a vapour of an orange-yellow tint. The principal coloured lines with their relative intensity observed in the spectrum of cadmium vapour are, according to Huggins's notation, 502*, 6391, 656s, 8895-5, 9181, 9531, 986*, 147310, 151710, 15361, 17471, 184310, 2315s, 2562s, 3239*. The most brilliant of these are chiefly in the green and blue field.

Chemically cadmium belongs to the diatomic group of elements; its symbol is Cd, and its equivalent 58. It unites readily with most of the heavy metals, forming alloys, which with gold, copper, and platinum, are brittle, while those with lead and tin are malleable and ductile. The alloy of |- silver and -| cadmium is very tenacious; but that, in the reverse proportion, of -| silver and f-cadmium is brittle. An alloy of two parts of cadmium, two of lead, and four of tin, known as Wood's fusible metal, melts at a somewhat lower point than the similar alloy where bismuth takes the place of cadmium, or Darcct's fusible metal (see BISMUTH). It forms several amalgams, among which those containing equal parts of mercury and cadmium and two of mercury to one of cadmium are remarkable for their cohesive power and malleabdity; whereas that containing 22 per cent, of cadmium is hard and brittle. The amalgams of the former class have been proposed at different times for use in stop-ping teeth, but are not now so employed. When exposed to damp air cadmium becomes rapidly covered with a dull film of suboxide, but as with zinc the oxidation is only superficial, the crust formed protecting the metal below from further change.. When heated to a redness in air it burns, forming a yellowish brown oxide. It also, when in a state of vapour, decomposes water at a red heat, with the forma-tion of oxide of cadmium, hydrogen being evolved. It is soluble with evolution of hydrogen, in sulphuric, hydro-chloric, nitric, and even acetic acid, forming colourless salts. When treated with an aqueous solution of sulphurous acid, it dissolves without evolution of hydrogen, sulphite and sulphide of cadmium being found in the liquid.

Oxide of cadmium, CdO, is a yellowish brown powder of the specific gravity 6-95, varying in depth of tint accord-ing to the temperature at which it is prepared. It may be produced by burning the metal in air or by calcining the nitrate or carbonate. It is readily reducible by hydrogen or carbon, at a high temperature, but below that necessary for the reduction of zinc oxide. If a mixture of the oxides of the two metals be heated in a current of hydrogen in a glass tube, the oxide of cadmium is reduced, volatilizes, and condenses in the cooler part of the tube, while the oxide of zinc remains unchanged. Oxide of cadmium is a strong base, forming salts similar in constitution to those formed by oxide of zinc, and those of the earthy and alkaline metals. The most important of these, is the sulphate, CdS04, which is produced when the metal or its oxide is dissolved in sulphuric acid, forming crystals containing either one or four atoms of water, the former being deposited from a boiling solution, and the latter at the ordinary temperature of the air. The uses of cadmium salts are very limited; the sulphate is employed to a small extent as a lotion in inflammation of the eyes, similarly to the sulphate of zinc, and the iodide in photography and in medicine for the same purposes as iodide of potassium. The only compound of any real importance is the sulphide, CdS, which produces several brilliant yellow and orange colours. These are quite permanent, unlike the yellow produced by lead, chromium, or other metals, which are all more or less subject to discoloration when exposed to the action of sulphuretted hydrogen in the atmosphere. It is produced when sulphuretted hydrogen, or an alkaline sulphide, is added to the solution of any cadmium salt, as an orange-red powder, which becomes carmine-red when heated. At a white heat it melts, and solidities on cooling in lemon-yellow scales of a micaceous structure. When the precipitated sulphide is heated in hydrogen it is de-composed, forming cadmium vapour and sulphuretted hydrogen, which reunite in the cooler part of the tube, producing crystals exactly simdar to the native mineral greenockite.

The best test for cadmium is afforded by the colour of the deposit formed on charcoal when it is volatdized and oxidized before the blowpipe flame. This is of a reddish brown colour, and usually shows the colours of thin plates from the tenuity of the film; whereas zinc under the same conditions gives a deposit which is bright yellow wlrile hot, but becomes white on cooling. The precipitation as a yellow sulphide from an acid solution is another distin. guishing character, as sulphide of zinc does not separate except from neutral or alkaline solutions. In quantitative analysis it is always estimated as oxide, being separated from solution as carbonate by precipitation with carbonate of sodium, which is converted into oxide by calcination. Cadmium, like lead, may also be separated from its solution in acids by means of zinc, which precipitates it in a dendritic form, like the well-known lead tree.

The production of cadmium is restricted to a very few loca-lities. At Engis in Belgium it occurs in zinc blende to the extent of about 02 per cent. The oxide formed, together with oxide of zinc in the calcination of the blende, is in the subsequent reducing process in the ordinary Belgian zinc furnace (see ZINC), reduced and volatilized in the first period of the operation, before the heat is raised sufficiently to pro-duce much zinc vapour, and the vapour, on coming in contact with the air, burns with a characteristic brown flame as dis-tinguished from that of zinc, which is bluish green. The deposit formed in the condensing tubes, and in the nozzles {allonges) in front of the retorts, during this part of the process is comparatively rich in cadmium oxide, averaging about 1-j- per cent. It is put aside until a sufficiency is collected, when it is enriched by a second distillation up to about 6 per cent., this second product being finally reduced by a third distillation with carbon at a dull red heat. The furnace contains fifteen retorts, four of which are reserved for the reduction of the enriched oxide. Cast-iron tubes are used, as the vapour of the metal readily penetrates clay retorts. The loss on the process is very considerable, only 3012 per cent, of the whole amount of cadmium contained in the material treated being re-covered ; 21 "17 per cent, is left in the residues, and 48'71 per cent, escapes condensation. The total produce of cadmium is very small; about one-half of the amount is produced at Engis, and the remainder in Silesia. In 1874 the production of cadmium in Lower Silesia amounted to 25 cwt., valued at £990 or about £800 per ton ; but owing to the small demand many works had given up the manufacture, (H. B.)

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