Cadmium (symbol Cd, atomic weight 112.4 (O=16)), a metallic element, showing a close relationship to zinc, with which it is very frequently associated. It was discovered in 1817 by F. Stromeyer in a sample of zinc carbonate from which a specimen of zinc oxide was obtained, having a yellow colour, although quite free from iron; Stromeyer showing that this coloration was due to the presence of the oxide of a new metal. Simultaneously Hermann, a German chemical manufacturer, discovered the new metal in a specimen of zinc oxide which had been thought to contain arsenic, since it gave a yellow precipitate, in acid solution, on the addition of sulphuretted hydrogen. This supposition was shown to be incorrect, and the nature of the new element was ascertained.
Cadmium does not occur naturally in the uncombined condition, and only one mineral is known which contains it in any appreciable quantity, namely, greenockite, or cadmium sulphide, found at Greenock and at Bishopton in Scotland, and in Bohemia and Pennsylvania. It is, however, nearly always found associated with zinc blende, and with calamine, although only in small quantities.
The metal is usually obtained from the flue-dust (produced during the first three or four hours working of a zinc distillation) which is collected in the sheet iron cones or adapters of the zinc retorts. This is mixed with small coal, and when redistilled gives an enriched dust, and by repeating the process and distilling from cast iron retorts the metal is obtained. It can be purified by solution in hydrochloric acid and subsequent precipitation by metallic zinc.
Cadmium is a white metal, possessing a bluish tinge, and is capable of taking a high polish; on breaking, it shows a distinct fibrous fracture. By sublimation in a current of hydrogen it can be crystallized in the form of regular octahedra; it is slightly harder than tin, but is softer than zinc, and like tin, emits a crackling sound when bent. It is malleable and can be rolled out into sheets. The specific gravity of the metal is 8.564, this value being slightly increased after hammering; its specific heat is 0.0548 (R. Bunsen), it melts at 310-320° C. and boils between 763-772° C. (T. Carnelley), forming a deep yellow vapour. The cadmium molecule, as shown by determinations of the density of its vapour, is monatomic. The metal unites with the majority of the heavy metals to form alloys; some of these, the so-called fusible alloys, find a useful application from the fact that they possess a low melting-point. It also forms amalgams with mercury, and on this account has been employed in dentistry for the purpose of stopping (or filling) teeth.
The metal is quite permanent in dry air, but in moist air it becomes coated with a superficial layer of the oxide; it burns on heating to redness, forming a brown coloured oxide; and is readily soluble in mineral acids with formation of the corresponding salts. Cadmium vapour decomposes water at a red heat, with liberation of hydrogen, and formation of the oxide of the metal.
Cadmium oxide, CdO, is a brown powder of specific gravity 6.5, which can be prepared by heating the metal in air or in oxygen; or by ignition of the nitrate or carbonate; by heating the metal to a white heat in a current of oxygen it is obtained as a dark red crystalline sublimate. It does not melt at a white heat, and is easily reduced to the metal by heating in a current of hydrogen or with carbon. It is a basic oxide, dissolving readily in acids, with the formation of salts, somewhat analogous to those of zinc.
Cadmium hydroxide, Cd(OH), is obtained as a white precipitate by adding potassium hydroxide to a solution of any soluble cadmium salt. It is decomposed by heat into the oxide and water, and is soluble in ammonia but not in excess of dilute potassium hydroxide; this latter property serves to distinguish it from zinc hydroxide.
The chloride, CdCl, bromide, CdBr, and iodide, CdI, are also known, cadmium iodide being sometimes used in photography, as it is one of the few iodides which are soluble in alcohol. Cadmium chloride and iodide have been shown to behave in an anomalous way in aqueous solution (W. Hittorf, Pogg. Ann., 1859, 106, 513), probably owing to the formation of complex ions; the abnormal behaviour apparently diminishing as the solution becomes more and more dilute, until, at very high dilutions the salts are ionized in the normal manner.
Cadmium sulphate, CdSO, is known in several hydrated forms; being deposited, on spontaneous evaporation of a concentrated aqueous solution, in the form of large monosymmetric crystals of composition 3CdSO·8HO, whilst a boiling saturated solution, to which concentrated sulphuric acid has been added, deposits crystals of composition CdSO·HO. It is largely used for the purpose of making standard electric cells, such for example as the Weston cell.
Cadmium sulphide, CdS, occurs naturally as greenockite (q.v.), and can be artificially prepared by passing sulphuretted hydrogen through acid solutions of soluble cadmium salts, when it is precipitated as a pale yellow amorphous solid. It is used as a pigment (cadmium yellow), for it retains its colour in an atmosphere containing sulphuretted hydrogen; it melts at a white heat, and on cooling solidifies to a lemon-yellow micaceous mass.
Normal cadmium carbonates are unknown, a white precipitate of variable composition being obtained on the addition of solutions of the alkaline carbonates to soluble cadmium salts.
Cadmium nitrate, Cd(NO)·4HO, is a deliquescent salt, which may be obtained by dissolving either the metal, or its oxide or carbonate in dilute nitric acid. It crystallizes in needles and is soluble in alcohol.
Cadmium salts can be recognized by the brown incrustation which is formed when they are heated on charcoal in the oxidizing flame of the blowpipe; and also by the yellow precipitate formed when sulphuretted hydrogen is passed though their acidified solutions. This precipitate is insoluble in cold dilute acids, in ammonium sulphide, and in solutions of the caustic alkalis, a behaviour which distinguishes it from the yellow sulphides of arsenic and tin. Cadmium is estimated quantitatively by conversion into the oxide, being precipitated from boiling solutions by the addition of sodium carbonate, the carbonate thus formed passing into the oxide on ignition. It can also be determined as sulphide, by precipitation with sulphuretted hydrogen, the precipitated sulphide being dried at 100° C. and weighed.
The atomic weight of cadmium was found by O.W. Huntington (Berichte, 1882, 15, p. 80), from an analysis of the pure bromide, to be 111.9. H.N. Morse and H.C. Jones (Amer. Chem. Journ., 1892, 14, p. 261) by conversion of cadmium into the oxalate and then into oxide, obtained values ranging from 111.981 to 112.05, whilst W.S. Lorimer and E.F. Smith (Zeit. für anorg. Chem., 1891, 1, p. 364), by the electrolytic reduction of cadmium oxide in potassium cyanide solution, obtained as a mean value 112.055. The atomic weight of cadmium has been revised by G.P. Baxter and M.A. Hines (Journ. Amer. Chem. Soc., 1905, 27, p. 222), by determinations of the ratio of cadmium chloride to silver chloride, and of the amount of silver required to precipitate cadmium chloride. The mean value obtained was 112.469 (Ag=107.93). The mean value 112.467 was obtained by Baxter, Hines and Frevert (ibid., 1906, 28, p. 770) by analysing cadmium bromide.