CADMIUM, a metallic element, showing a close relationship to zinc, with which it is very frequently associated; (symbol Cd, atomic number 48, atomic weight 112.41, isotopes 114, 112, I I0, 113, III, I16). It was discovered in 1817 by F. Stromeyer in a sample of zinc carbonate yielding a yellow zinc oxide, although quite free from iron. Simultaneously Hermann discovered the new metal in a specimen of zinc oxide which had been thought to con tain arsenic, since it gave a yellow sulphide in acid solution on the addition of sulphuretted hydrogen.
Cadmium does not occur naturally in the uncombined condition, and only one mineral is known which contains it in any appre ciable 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 rarely exceeding 3%.
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 solu tion in hydrochloric acid and subsequent precipitation by metallic zinc. This process is repeated and finally the cadmium is depos ited by electrolysis.
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 crack ling 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 0.0S48, it melts at 322° C. and boils at 778° C., forming a deep yellow va pour. The cadmium molecule, as shown by determinations of the density of its vapour, is monatomic. The metal unites with the ma jority 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. An alloy of lead with I o% of cadmium and io% of tin forms a solder. 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, form ing a brown coloured oxide ; and is readily soluble in mineral acids with formation of the corresponding salts. Cadmium decomposes water vapour at a red heat, with liberations 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, or by heating the metal to a white heat in a current of oxygen, when 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, is obtained as a white precipi tate 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; the latter property serves to distinguish it from zinc hydroxide.
The chloride, bromide, and iodide, are soluble salts ; and cadmium iodide which is one of the few iodides which are soluble in alcohol, is sometimes used in photography. Cadmium chloride and iodide behave in an anomalous way in concentrated aqueous solution probably owing to the formation of complex ions the abnormal behaviour diminishing as the solution becomes more dilute, until, at very high dilutions the salts are ionized in the normal manner.
Cadmium sulphate, 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 3CdSO48H2O, whilst a boiling saturated solution to which concentrated sulphuric acid has been added, deposits crys tals of composition It is largely used for making standard electric cells, such for example as the Weston, which gives a practically constant voltage under normal conditions.
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 precipi tated as a pale yellow amorphous solid. It is used as a pigment (cadmium yellow), for it retains its colour in an atmosphere con taining sulphuretted hydrogen; it melts at a white heat, and on cooling solidifies to a lemon-yellow micaceous mass.
An enamel pigment known as fine red or cadmium red is pre pared from a mixture of 8o-9o% of cadmium sulphide and Io 20% of selenium. The well mixed components are heated at 700° C. The product corresponds in composition to In a wet process a mixture of soluble sulphides and selenides is added to a solution of cadmium salt and the precipitate is ignored.
Normal cadmium carbonate is unknown, a white precipitate of variable composition being obtained on the addition of solutions of the alkaline carbonates to soluble cadmium salts.