SULPHATE AND CARBONATE OF ZINC.

547

1931) is obtained in large quantities as a residue in the ordinary process of procuring hydrogen by the action of diluted sulphuric acid. It may also be prepared by roasting sulphide of zinc at a low temperature, lixiviating the mass and crystallizing. It crystallizes in colourless four-sided prisms, which constitute the white vitriol of commerce. In a dry air it is efflorescent; it is soluble in 24 parts of water at 60°, and melts in its water of crystallization when heated; it may be obtained crystallized with 6, 5, 4, and I atom of water, by varying the temperature at which the crystals are allowed to be formed. Sulphate of zinc is used medicinally in small doses; it is likewise prepared largely for the calico printer. It forms double sulphates with potassium and with ammonium, which crystallize with 6 H2O. Several basic sulphates of zinc may also be obtained.

29

(712) CARBONATE OF ZINC (Zn¤¤ ̧=125, or ZnO,CO2=62*5: Sp. Gr. 44: Composition in 100 parts, ZnO, 648; ЄÐ1⁄2, 35'2; or Zn, 52; EÐ ̧, 48) is found native, both massive and crystallized, in forms derived from the rhombohedron. It is usually of a greyish or yellowish colour, forming one variety of calamine, which is so named from its property of adhering, after fusion, in the form of reeds, to the base of the furnace. It readily loses carbonic anhydride when ignited. No neutral carbonate of zinc can be obtained from the salts of the metal by double decomposition. When a hot solution of a salt of zinc is precipitated by a boiling solution of an alkaline carbonate, a hydrated oxycarbonate is formed, consisting of (8 ZnO,30,. 6 H2O; Schindler). Several other basic carbonates of zinc may be formed.

2

The other variety of calamine becomes electric by heat; it is a hydrated orthosilicate (Zn,SiO,H,, or 2 ZnO, SiO, . HO).

2

(713) CHARACTERS OF THE SALTS OF ZINC.-The salts of zinc are colourless; their solutions have an astringent, metallic taste, and act rapidly and powerfully as emetics.

They are distinguished by giving no precipitate in acid solutions with sulphuretted hydrogen, though the acetate, even when acidulated with acetic acid, gives a white hydrated sulphide when the gas is transmitted: they yield a white hydrated sulphide of zinc with sulphide of ammonium; a white hydrated oxide with potash, soda, or ammonia, soluble in excess of the alkali; a white basic carbonate of zinc with the carbonates of the alkali-metals, soluble in excess of the solution of carbonate of ammonium, but not in that of the carbonates of potassium or sodium; they also yield a white precipitate with ferrocyanide of potassium.

548

TESTS FOR ZINC-CADMIUM.

Before the blowpipe, in the reducing flame on charcoal, the metal is reduced and volatilized, burning into white fumes of oxide of zinc. If placed on charcoal and moistened with a solution of nitrate of cobalt, the compounds of zinc when heated in the oxidating flame leave a green residue, which is not fusible.

(714) Estimation of Zinc.-Zinc is best precipitated for analysis by carbonate of potassium, the whole solution being evaporated down to dryness; the residue, which contains the carbonate of zinc, is washed with boiling water, dried, and converted by ignition into oxide of zinc, which is weighed. The oxide contains, in 100 parts, 80'39 of zinc. If ammoniacal salts be present, an excess of the carbonate of potassium should be used sufficient to decompose the salts of ammonium completely, the ammonium being wholly expelled as carbonate during the process of evaporation. The foregoing process is not applicable to the separation of zinc from any but the alkaline metals.

(715) Separation of Zinc from the Alkalies and Alkaline Earths. -This may be effected by the addition of sulphide of ammonium to the solution after it has been neutralized by ammonia ; the zinc is thus precipitated as hydrated sulphide: it must be washed with a solution of sulphuretted hydrogen, to prevent its oxidation, then redissolved in hydrochloric acid, and evaporated to dryness with excess of carbonate of sodium: the soluble salts must be washed from the carbonate of zinc, which is to be converted into oxide by ignition, and then weighed.

The separation of zinc from aluminum and glucinum may be effected by dissolving all the bases by means of an excess of caustic potash, and adding sulphide of ammonium: in this case sulphide of zinc is alone precipitated: it may be collected and its amount determined in the manner just described.

§ III. CADMIUM: Ed"=112, (or Cd=56); Sp. Gr. 8·6 to 8·69; Boiling-pt. 1580°; calculated Sp. Gr. of vapour, 3.869; observed Sp. Gr. 394; Atomic and Mol. Vol. of Vapour,

.*

(716) CADMIUM was discovered by Stromeyer, in 1818. It is occasionally found as sulphide of cadmium, accompanying the ores of zinc, and is obtained as an accidental product during the extraction of the latter metal. Being more volatile than zinc, the

*This vapour (as well probably as that of zinc) and the other metallic dyads is anomalous in volume, I atom of each metal yielding 2 volumes instead of 1 volume.

CADMIUM-ITS OXIDE AND SULPHIDE.

549

greater part of the cadmium sublimes among the first portions of the distilled metal, from which it may be extracted by dissolving them in sulphuric acid, and precipitating the cadmium as sulphide by means of sulphuretted hydrogen: the sulphide may be dissolved in strong hydrochloric acid, precipitated by carbonate of ammonium, and reduced in an earthen retort by ignition with charcoal; the metal distils over at a heat below redness.

:

Cadmium is of a white colour, resembling tin, and, like it, creaks when a rod of it is bent; it is so soft that it leaves its traces upon paper, and possesses considerable malleability and ductility when heated to about 180°, it becomes very brittle, and may be powdered in a mortar with facility. Cadmium fuses at 242°, and may be obtained in octohedral crystals as it cools. In the atmosphere it undergoes little change, but when thrown into a red-hot crucible it takes fire, depositing brownish-yellow fumes of oxide. It is dissolved with evolution of hydrogen when heated in sulphuric or hydrochloric acid slightly diluted; nitric acid dissolves it still more freely.

The addition of cadmium to the more fusible metals generally yields an alloy of low fusing-point, without destroying the toughness or malleability of the compound. An alloy consisting of 15 parts of bismuth, 8 of lead, 4 of tin, and 3 of cadmium furnishes a silver-white alloy of sp. gr. 9'4: it softens between 131° and 140°, and at about 140° F. is completely liquid; it expands a little as it solidifies. This alloy is somewhat ductile, and may be filed readily without clogging the tool: it preserves its brilliancy in the air. An alloy consisting of 1 part of cadmium, 6 parts of lead, and 7 of bismuth, melts at 180°.

(717) OXIDE OF CADMIUM (Єd=128, or CdO=64; Sp. Gr. 693) Comp. in 100 parts, Ed, 87.5; 12.5.-This oxide is obtained as a brown anhydrous powder, by burning the metal in air, or by igniting the nitrate of cadmium; it is not fusible or volatile in the furnace. A white hydrated oxide of cadmium, Єd,H,, may be obtained by decomposing its salts by a fixed alkali; ammonia in excess redissolves it, but the hydrates of potash and soda have no such effect; even the anhydrous oxide is soluble in ammonia, Carbonate of ammonium does not dissolve oxide of cadmium either in the anhydrous or the hydrated form.

2

SULPHIDE OF CADMIUM (ЄdS=144, or CdS=72) constitutes the mineral known as Greenockite, which occurs crystallized in six-sided prisms. It may be formed artificially by transmitting a current of sulphuretted hydrogen through a solution of a salt

550

CHARACTERS OF THE SALTS OF CADMIUM.

of cadmium; it greatly resembles orpiment in appearance, but is distinguished from it by its want of volatility when heated, and by its insolubility in ammonia and in the sulphides of the alkaline metals. It forms a bright yellow pigment highly valued both for the purity and permanence of its tint.

CHLORIDE OF CADMIUM (EdCl2, 2 H2O, or CdCl,2 HO) crystallizes easily. The iodide (sp. gr. 4'576) may be also obtained without difficulty in crystals which have a pearly lustre. It is anhydrous, and fuses readily on the application of heat. This salt is easily obtained by digesting metallic cadmium in water with free iodine, and evaporating the solution; it is employed for iodizing collodion for photographic purposes (1018).

(718) CHARACTERS OF THE SALTS OF CADMIUM.-The salts of cadmium are colourless, and resemble those of zinc. They may be readily distinguished by the yellow precipitate of sulphide of cadmium which they yield with sulphuretted hydrogen in acid solutions; this precipitate is insoluble either in ammonia or in the alkaline sulphides, or in cyanide of potassium, but soluble in boiling diluted sulphuric acid. Hydrates of potash and soda give a precipitate of white hydrated oxide, insoluble in excess ; ammonia, a similar precipitate very soluble in excess; carbonates of potassium, sodium, and ammonium, a white carbonate, insoluble in excess; oxalic acid, a white precipitate, soluble in ammonia ; ferrocyanide of potassium, a yellowish-white precipitate, soluble in hydrochloric acid.

Before the blowpipe they are decomposed, and on the cool part of the charcoal a ring of brown oxide of cadmium is deposited, due to the reduction and subsequent combustion of the metal.

Estimation of Cadmium.-Cadmium is readily separated from all the foregoing metals by the action of sulphuretted hydrogen, which causes a precipitate of the yellow sulphide of cadmium from an acidulated solution of its salts. This precipitate is redissolved in nitric acid, decomposed by an excess of carbonate of sodium, evaporated to dryness, washed from the soluble salts, and the resulting carbonate of cadmium is heated to redness, by which it is converted into oxide; it is then weighed: 100 grains of oxide of cadmium contain 87.5 of the metal.

CHAPTER XVI.

GROUP V. METALS MORE OR LESS ALLIED TO IRON.

THE metals of this class include those which are distinctly magnetic; uranium, however, appears to be diamagnetic. They decompose water at a red heat, and are soluble with evolution of hydrogen in hydrochloric and diluted sulphuric acid. They form several oxides, two at least of which, except in the case of cobalt and nickel, are soluble in acids. Sulphuretted hydrogen in solutions acidulated with the mineral acids does not precipitate

the metals of this group. Corresponding salts of these metals are isomorphous (See page 339).

§ 1. COBALT: Co′′=59, or Co=29'5. Sp. Gr. 8'95.

(719) COBALT appears to have been first recognized as distinct metal by Brandt, in 1733. It generally occurs in combination with arsenic, as speiss-cobalt or tin-white cobalt (CoAs), but occasionally it is found as cobalt glance, which is a compound of the arsenide and the sulphide of the metal (Єo§„,ЄoAs,). Cobalt is never met with in the native state, except as an ingredient in meteoric iron in small proportion. The black oxide has been found to some extent in the Western States of America, mixed with the sulphide of cobalt and with variable proportions of the oxides of nickel, manganese, iron, and copper. The ores of this metal occur chiefly in the primitive rocks, and are usually very complicated; they contain nickel, iron, and often bismuth and copper, mineralized either by sulphur or by arsenic, or by both together.

Extraction. It is not easy to obtain cobalt in a state of purity. On a small scale the ore may be treated as follows: