Prep. The zinc of commerce is obtained from the native sulphide (zinc blende), or carbonate (calamine), by roasting those ores, and distilling the calx with carbonaceous matter in a covered earthen crucible, having its bottom connected with an iron tube, which terminates over a vessel of water situated beneath the furnace. The first portion that passes over contains cadmium and arsenic, and is indicated by what is technically called ‘brown blaze,’ but when the metallic vapour begins to burn with a bluish-white flame, or the ‘blue blaze’ commences, the volatilised metal is collected.
The following method, by which several pounds of chemically pure zinc may be obtained in about 1⁄4 of an hour, will be found very useful:—Melt the zinc of commerce in a common crucible, and granulate it by throwing it into a tolerably deep vessel of water, taking care that the metal be very hot at the time; dry the metallic grains, and dispose them by layers in a Hessian crucible with 1⁄4 of their weight of nitrate of potassium, using the precaution to place a slight excess at the top and at the bottom; cover the crucible, and secure the lid, then apply heat; after the vivid deflagration which occurs is over, remove the crucible from the fire, separate the dross with a tube, and, lastly, run the zinc into an ingot mould. This zinc, tested in Marsh’s apparatus during entire days, has never given any stain, and in solution the most sensitive reagents, such as hydro-sulphocyanic acid, have never indicated the least atom of iron. (‘Journ. de Pharm.’)
Prop. Zinc is a bluish-white metal, having the sp. gr. 6·8 to 7·2; tough (under some circumstances, brittle) when cold, ductile and malleable at from 250° to 300° Fahr.; brittle and easily pulverised at 400°; fuses at 773° (Daniell); at a white heat it boils, and sublimes unchanged in close vessels; heated to whiteness (941° Daniell) in contact with the
air, it burns with a brilliant green light, and is converted into oxide. It is very soluble in dilute sulphuric and hydrochloric acid, with the evolution of hydrogen gas. It is little acted on by the air, even when moist. The salts of zinc are colourless.
Pur. Commercial zinc is never pure. Its specific gravity is 6·86. It is soluble in nitric, hydrochloric, and dilute sulphuric acids.
Tests. 1. The solutions of zinc give a gelatinous white precipitate with the alkalies and carbonate of ammonium, which is completely redissolved by an excess of the precipitant.—2. The carbonates of potassium and sodium give a white precipitate of carbonate of zinc. All the above precipitates acquire a lemon-yellow colour when dried and heated, but again become white on cooling.—3. Sulphide of ammonium gives, in neutral solutions, a white precipitate, insoluble in excess of the precipitant, or in solutions of hydrate of potassium or ammonium, but freely soluble in the dilute mineral acids.—4. Sulphuretted hydrogen, in neutral and alkaline solutions, also gives a like white precipitate.—5. Ferrocyanide of potassium gives a gelatinous white precipitate.
Estim., &c.—a. 100 gr. are digested in dilute hydrochloric acid in excess, and the insoluble portion, which is chiefly carbon, dried and weighed.
b. The acidulous solution (see a) is next treated with a current of sulphuretted hydrogen until it smells very strongly of that gas; the whole is then left for some time in a warm situation. The precipitate which subsides consists of the sulphides of arsenic, cadmium, copper, lead, &c., if any of these metals were present in the sample.
c. The filtrate from b, after being boiled, is treated with a little nitric acid, after which it is again boiled, and, when cold, is precipitated with carbonate of barium added in excess; the precipitate (ferric hydrate) is then collected, dried, ignited, and weighed. The weight, in grains, multiplied by ·7, gives the percentage of iron in the sample examined.
d. The filtrate from c is next precipitated with dilute sulphuric acid, and solution of carbonate of sodium is added in excess to the filtered liquid; the whole is then boiled, after which the new precipitate is washed, dried, gently ignited for some time, and then cooled and weighed. The weight, in grains, multiplied by ·80247, gives the percentage of pure zinc in the sample.