Compounds
Zinc forms only one oxide, ZnO, from which is derived a well-characterized series of salts. It is chemically related to cadmium and mercury, the resemblance to cadmium being especially well marked; one distinction is that zinc is less basigenic. Zinc is capable of isomorphously replacing many of the bivalent metals—magnesium, manganese, iron, nickel, cobalt and cadmium.
Zinc oxide, ZnO, is manufactured for paint by two processes—directly from the ore mixed with coal by volatilization on a grate, as in the Wetherill oxide process, and by oxidizing the vapour given off by a boiling bath of zinc metal. The oxide made by the latter method has generally a better colour, a finer texture, and a greater covering power. It is also manufactured by the latter process from the metallic zinc liquated out of galvanizer's dross. It is an infusible solid, which is intensely yellow at a red heat, but on cooling becomes white. This at least is true of the oxide produced from the metal by combustion; that produced from the carbonate, if once made yellow at a red heat, retains a yellow shade permanently. By heating the nitrate it is obtained as hemimorphous pyramids belonging to the hexagonal system; and by heating the chloride in a current of steam as hexagonal prisms. It is insoluble in water; it dissolves readily in all aqueous acids, with formation of salts. It also dissolves in aqueous caustic alkalis, including ammonia, forming “zincates” [e.g. Zn(OK)2]. Zinc oxide is used in the arts as a white pigment (zinc white); it has not by any means the covering power of white lead, but offers the advantages of being non-poisonous and of not becoming discoloured in sulphuretted hydrogen. It is used also in medicine.
Zinc hydroxide, Zn(OH)2, is prepared as a gelatinous precipitate by adding a solution of any zinc salt to caustic potash. The alkali must be free from carbonate and an excess of it must be avoided, otherwise the hydrate redissolves. It is a white powder, and is insoluble in water. To acids and to alkalis it behaves like the oxide, but dissolves more readily.
Zinc chloride, ZnCl2, is produced by heating the metal in dry chlorine gas, when it distils over as a white translucent mass, fusing at 250° and boiling at about 400°. Its vapour-density at 900° C. corresponds to ZnCl2. It is extremely hygroscopic and is used in synthetical organic chemistry as a condensing agent. It dissolves in a fraction of its weight of even cold water, forming a syrupy solution. A solution of zinc chloride is easily produced from the metal and hydrochloric acid; it cannot be evaporated to dryness without considerable decomposition of the hydrated salt into oxychloride and hydrochloric acid, but it may be crystallized as ZnCl2·H2O. A concentrated solution of zinc chloride converts starch, cellulose and a great many other organic bodies into soluble compounds; hence the application of the fused salt as a caustic in surgery and the impossibility of filtering a strong ZnCl2 solution through paper (see Cellulose). At a boiling heat, zinc chloride dissolves in any proportion of water, and highly concentrated solutions, of course, boil at high temperatures; hence they afford a convenient medium for the maintenance of high temperatures.
Zinc chloride solution readily dissolves the oxide with the formation of oxychlorides, some of which are used as pigments, cements and for filling teeth in dentistry. A solution of the oxide in the chloride has the property of dissolving silk, and hence is employed for removing this fibre from wool.
Zinc bromide, ZnBr2, and Zinc iodide, ZnI2, are deliquescent solids formed by the direct union of their elements. With ammonia and alkaline bromides and iodides double salts are formed.
Zinc sulphide, ZnS, occurs in nature as blende (q.v.), and is artificially obtained as a white precipitate by passing sulphuretted hydrogen into a neutral solution of a zinc salt. It dissolves in mineral acids, but is insoluble in acetic acid.
Zinc sulphate, ZnSO4+7H2O, or white vitriol, is prepared by dissolving the metal in dilute sulphuric acid. If care be taken to keep the zinc in excess, the solution will be free from all foreign metals except iron and perhaps manganese. Both are easily removed by passing chlorine through the cold solution, to produce ferric and manganic salt, and then digesting the liquid with a washed precipitate of basic carbonate, produced from a small portion of the solution by means of sodium carbonate. The iron and manganese are precipitated as hydroxides, and are filtered off. The filtrate is acidified with a little sulphuric acid and evaporated to crystallization. The salt crystallizes out on cooling with 7 molecules of water, forming colourless orthorhombic prisms, usually small and needle-shaped. They are permanent in the air. According to Poggiale, 100 parts of water dissolve respectively of (7H2O) salt, 115.2 parts at 0°, and 653.6 parts at 100°. At 100° C. the crystals lose 6 of their molecules of water; the remaining molecule goes off at 250°, a temperature which lies close to that at which the salt begins to decompose. The anhydrous salt, when exposed to a red heat, breaks up into oxide, sulphur dioxide and oxygen. An impure form of the salt is prepared by roasting blende at a low temperature. In the arts it is employed in the preparation of varnishes, and as a mordant for the production of colours on calico. A green pigment known as Rinmann's green is prepared by mixing 100 parts of zinc vitriol with 2.5 parts of cobalt nitrate and heating the mixture to redness, to produce a compound of the two oxides. Zinc sulphate, like magnesium sulphate, unites with the sulphates of the potassium metals and of ammonium into crystalline double salts, ZnSO4⋅R2SO4+6H2O, isomorphous with one another and the magnesium salts.
Zinc carbonate, ZnCO3, occurs in nature as the mineral calamine (q.v.), but has never been prepared artificially, basic carbonates, ZnCO3.xZn(OH)2, where x is variable, being obtained by precipitating a solution of the sulphate or chloride with sodium carbonate. To obtain a product free of Cl or SO4, there must be an excess of alkali and the zinc salt must be poured into the hot solution of the carbonate. The precipitate, even after exhaustive washing with hot water, still contains a trace of alkali; but from the oxide, prepared from it by ignition, the alkali can be washed away. The basic carbonate is used as a pigment.
Of zinc phosphates we notice the minerals hopeite, Zn3(PO4)2⋅4H2O, and tarbuttite, Zn3(PO4)2⋅Zn(OH)2, both found in Rhodesia.
Analysis.—From neutral solutions of its salts zinc is precipitated by sulphuretted hydrogen as sulphide, ZnS—a white precipitate, soluble, but by no means readily, in dilute mineral acids, but insoluble in acetic acid. In the case of acetate the precipitation is quite complete; from a sulphate or chloride solution the greater part of the metal goes into the precipitate; in the presence of a sufficiency of free HCl the metal remains dissolved; sulphide of ammonium precipitates the metal completely, even in the presence of ammonium salts and free ammonia. The precipitate, when heated, passes into oxide, which is yellow in the heat and white after cooling; and, if it be moistened with cobalt nitrate solution and re-heated, it exhibits a green colour after cooling.
Zinc may be quantitatively estimated by precipitating as basic carbonate, which is dried and ignited to zinc oxide. It may also be precipitated as zinc ammonium phosphate, NH4ZnPO4, which is weighed on a filter tared at 100°. Volumetric methods have also been devised.
Pharmacology and Therapeutics of Zinc Compounds
Zinc chloride is a powerful caustic, and is prepared with plaster of Paris in the form of sticks for destroying warts, &c. Its use for this purpose at the present day is, however, very rare, the knife or galvanocautery being preferred in most cases. The salt is a corrosive irritant poison when taken internally. The treatment is to wash out the stomach or give such an emetic as apomorphine, and, when the stomach has been emptied, to administer demulcents such as white of egg or mucilage. Numerous other salts of zinc, used in medicine, are of value as containing this metal. Certain others are referred to in relation with the important radicle contained in the salt. Those treated here are the sulphate, oxide, carbonate, oleate and acetate. All these salts are mild astringents when applied externally, as they coagulate the albumen of the tissues and of any discharge which may be present. In virtue of this property they are also mild haemostatics, tending to coagulate the albumens of the blood and thereby to arrest hemorrhage. Lotio Rubra, the familiar “Red Lotion,” a solution of zinc sulphate, is widely used in many catarrhal inflammations, as of the ear, urethra, conjunctiva, &c. There are also innumerable ointments.
These salts have been extensively employed internally, and indeed they are still largely employed in the treatment of the more severe and difficult cases of nervous disease. The sulphate is an excellent emetic in cases of poisoning, acting rapidly and without much nausea or depression. For these reasons it may also be given with advantage to children suffering from acute bronchitis or acute laryngitis.
Bibliography.—For the history of zinc see Bernard Neumann, Die Metalle (1904); A. Rossing, Geschichte der Metalle (1901). For the chemistry see H. Roscoe and C. Schorlemmer, Treatise on Inorganic Chemistry, vol. 2 (1897); H. Moissan, Traité de chimie minerale; O. Dammer, Handbuch der anorganischen Chemie. For the metallurgy see Walter Renton Ingalls, The Metallurgy of Zinc and Cadmium, Production and Properties of Zinc; A. Lodin, Metallurgie du zinc (1905); C. Schnabel, Handbook of Metallurgy, English translation by H. Louis (1907). See also The Mineral Industry (annual).
ZINCITE, a mineral consisting of zinc oxide (ZnO), crystallizing in the hemimorphic-hemihedral class of the rhombohedral system. Distinct crystals are of rare occurrence; they have the form of a hexagonal pyramid terminated at one end only by a basal plane. There is a perfect cleavage parallel to the basal plane, and usually the mineral is found as platy foliated masses. The blood-red colour and the orange-yellow streak are characteristic features. The hardness is 412, and the specific gravity is 5.6. Some manganese is usually present replacing zinc. It is found in the zinc mines at Sterling Hill and Franklin Furnace in Sussex county, New Jersey, where it is associated with franklinite and willemite in crystalline limestone, and is mined as an ore of zinc. Artificial crystals of a white or yellowish colour are not infrequently formed by sublimation in zinc furnaces.
