Element Cadmium, Cd, Transition Metal

Cadmium is very similar to zinc, occurs in comparatively small amount in nature, associated with that metal. As it is more readily volatile than zinc, it collects in the first portions of the distillate in the preparation of the latter. It is a bluish- white metal, almost as soft as lead; it melts at 320° and boils at 770°. Its vapour density points to a molar weight which is equal to the combining weight, Cd = 112.4; the formula of the element in the vaporous state is therefore Cd, similarly to zinc.

Cadmium forms only one elementary ion, divalent cadmion, Cd^••, The metal dissolves, although very slowly, in aqueous acids with formation of this ion. Cadmion is colourless, and acts as a rather virulent poison on the lower and higher organisms. Its heat of formation from the metal is 77 kj.

The cadmium salts in aqueous solution are distinguished by the fact that many of them are considerably less dissociated into ions than the corresponding salts of the other divalent cations. This is especially noticeable in the case of the halogen compounds.

From the aqueous solutions of the cadmium salts alkali hydroxides precipitate white cadmium hydroxide, which is insoluble in an excess of the precipitant. This is in agreement with the general increase of the basic properties with increasing combining weight in the case of similar elements. Cadmium hydroxide is soluble in excess of ammonia. The solution contains complex cadmium-ammonia ions, Cd(NH₃)_n^••.

By heating the hydroxide, and by the combustion of the metal in the air, cadmium oxide is obtained as a brown powder, which readily dissolves in acids to form cadmium salts.

Of the salts the sulphate should be mentioned. This still exhibits some similarity to the sulphates of the magnesium series, but also considerable divergence. Thus, it crystallises at the ordinary temperature in accordance with the formula 3(CdSO₄) • 8H₂O, for which there is no analogy known in the case of the true "vitriols." The formation also of the typical double salt with potassium or ammonium sulphate does not take place quite readily.

The sulphate is readily soluble in water; in the case of the salt with f- molecules of water of crystallisation the temperature has very little influence on the solubility. It is used in medicine, and is also employed for the construction of electrical " standard cells."

The halogen compounds of cadmium exhibit especially clearly the above-mentioned slight dissociation in aqueous solution. Of the three compounds, cadmiun chloride is most, cadmium iodide least, dissociated. The latter salt forms crystalline laminae of a pearly lustre, which are soluble in alcohol. On account of this property it is employed in photography as an iodising salt.

Apart from the small conductivity, the following experiment demonstrates very clearly the slight degree of dissociation of cadmium iodide. If cadmium hydroxide is brought together with water and litmus or phenolphthalein, no alkaline reaction can be detected, because the hydroxide is too slightly soluble. The same thing is observed on using a solution of potassium nitrate or sulphate instead of water. If we take a neutral solution of potassium iodide, however, a strong alkaline reaction is obtained on shaking up. The reason of this is that the cadmion which passes into solution from the hydroxide is converted into undissociated cadmium iodide. A fresh quantity of hydroxide must therefore pass into solution, and this must go on till equilibrium is attained. In this process the hydroxidion of the hydroxide remains over (along with kalion from the potassium iodide) and the solution must exhibit the reaction of hydroxidion, i.e. must react alkaline. In formulae we have Cd(OH)₂ + 2I' = CdI₂ + 2OH'.

Cadmium, the name of which derives from Latin cadmia, Greek kadmeia, was discovered in Germany in 1817 by Friedrich Stromeyer. At that time C.H. Roloff, a district doctor, was auditing district drugstores in Magdeburg vicinity when he found a specimen of zinc oxide which looked like it contained arsenic. It had been proved when the reaction between hydrogen sulphide and acid solution of zinc oxide yielded a yellow precipitation. Strohmeyer roasted the zinc oxide specimen; it turned from gray to yellow. In Schonebeck factory it was explained that the color change was caused by iron impurities. Nevertheless Strohmeyer was not satisfied by this interpretation. After thorough analysis she found a new metal, which was easily separated from zinc by hydrogen sulphide.

The zinc ores used for preparing this metal usually contain cadmium as an impurity, and when zinc is smelted the more volatile cadmium vapour comes off with the first portions of the zinc. The mixed vapours then burn in the air to a mixture of the two oxides. This contamination of zinc oxide led to the discovery of cadmium.

Certain samples of zinc oxide were observed to have a yellow colour and give a yellow precipitate with hydrogen sulphide when dissolved in acid. Stromeyer, in 1817, isolated a metal from this yellow sulphide and called it cadmium. Many pharmacists had supposed the yellow precipitate to be arsenic sulphide, and Hermann, who prepared large quantities of zinc oxide for pharmaceutical use, had also examined the suspicious precipitate and obtained from it a metal that was identical with Stromeyer's product.

The zinc ore (calamine) used by the ancients to obtain brass by melting it with copper was called " cadmia." Names derived from this were applied to the deposits of zinc oxide obtained during the working of zinc, and the name " cadmium " was derived from these.

Cadmium crustal abundance is 1.6x10^-5%, which is close enough to abundance of antimony 2x10^-5% and is twice of that of mercury 8x10^-6%. Cadmium migrates in underground hot waters with zinc and other elements which are apt to form natural sulphides. It is concentrated in hydrothermal deposits. Volcanic rocks contain 0.2 mg Cadmium per kg. Clays are most cadmium abundant in sedimentary rocks containing up to 0.3 mg/kg of cadmium; limestones and sandstones are less rich - 0.03 mg/kg. Average cadmium soil abundance is 0.06 mg/kg.

Cadmium forms its own minerals - greenockite (Cadmium blend) CdS, otavite CdCO₃, monteponite CdO; however, these minerals do not have their own deposits. Zinc ores are the only commercially significance cadmium source which contain 0.01-5% of cadmium. This element is accumulated also in galena (up to 0.02%), chalcopyrite (up to 0.12%), pyrite (up to 0.02%), and stannite (tin pyrite 0.2%). The worldwide Cadmium reserves are estimated to be 20 million tons, among which 600 thousand tons of commercially important.

Plants contain 10^-4% (per dry matter) cadmium; some animals such as sponges, coelenterates, worms, echinoderms and tunicates, - 4x10^-5-3x10^-3. All vertebrate animals collect Cadmium, especially in liver.

Physiological significance of cadmium is not completely clear; however it has been known that cadmium is important for carbohydrate metabolism, hippuric acid synthesis in liver and enzymatic activity.