Zinc

Zinc Carbonate,

occurs in nature as the mineral cala mine (q.v.), but has never been prepared artificially, basic carbo nates, where x is variable, being obtained by precipitating a solution of the sulphate or chloride with sodium carbonate ; the basic salt is used as a pigment.

Of zinc phosphates we notice the minerals hopeite, Zn3(PO4)2.4H20, and tarbuttite, Zn3(PO4)2.Zn(OH)2, both found in Rhodesia.

Analysis.

From neutral solutions of its salts zinc is precipi tated by sulphuretted hydrogen as sulphide, ZnS—a white pre cipitate, soluble, but by no means readily, in dilute mineral acids, but insoluble in acetic acid. In the case of the acetate precipita tion 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 HC1 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 when hot 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, which is filtered on a Gooch crucible and dried at io5°. Volumetric methods have also been devised.

Zinc chloride is a powerful caustic, and is prepared with plaster of Paris in the form of sticks for destroying warts, etc. The salt is a corrosive irritant poison when taken internally. The treat ment is to wash out the stomach or give such an emetic as apo morphine, 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 im portant radical 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 haemorrhage. Lotio Rubra, the familiar "Red Lotion," a solu tion of zinc sulphate, is widely used in many catarrhal inflamma tions, as of the ear, urethra, conjunctiva, etc. There are also innumerable ointments.

These salts are extensively employed internally especially 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. See also Zinc peroxide (above).

chemistry of zinc, see J. W. Mellor, Compre hensive Treatise on Inorganic Chemistry, vol. 4 (1923) ; J. N. Friend, Text-book of Inorganic Chemistry, vol. 3, Pt. 2 (by J. C. Gregory and Mrs. M. S. Burr) (1926) ; Smith, The Zinc Industry (1918) ; • for statis tics and technological advances, see G. A. Roush and A. Butts, The Mineral Industry (annually). (X.; A. D. M.)