Zinc

Zinc, Zincum, chemical element, symbol Zn, atomic number 30, atomic weight 65.38. In nature, it is found in a number of minerals: common smithsonite, Zn₂SiO₄·H₂O; zinc blende, ZnS; smithsonite, ZnCO₃, often mixed with silicate and carbonate salts; zincite, ZnO, and others. In small amounts, zinc is found in plant and animal organisms. It is found in certain species of Viola, Aspergillus, and many, if not all, higher plants. In 1 kg of rye, it contains 17 mg, in 1 kg of wheat - 80 mg, in 1 kg of oats - 38 mg. It has been found in the bodies of both invertebrate and vertebrate animals. In the human body, it is present in small amounts, mainly in the liver, prostate gland, and sperm. In cancerous tumors, an increased content of zinc has been found. Cow's milk contains 2.3-2.9 mg, human milk - 5-13 mg per 1 liter. Metallic zinc is a bluish-white metal with a strongly brilliant, leafy-crystalline fracture, specific gravity 6.9-7.2. It melts at 419.4°, boils at 906°. At ordinary temperatures, zinc is quite hard, at 150° it is malleable, at 200° it is brittle. In moist air, zinc quickly becomes coated with a layer of basic zinc carbonate, which protects the metal surface from further oxidation. When heated in air, zinc burns to zinc oxide. Dilute acids convert metallic zinc into the corresponding salts with the release of hydrogen. Zinc in all its compounds is divalent. Aqueous solutions of zinc salts have an acidic reaction due to hydrolysis and an unpleasant metallic taste. Caustic alkalis dissolve zinc with the release of hydrogen and the formation of zincates, NaOH + Zn + H₂O = NaHZnO₂ + H₂. Detection of zinc can be carried out using the following reactions: 1) Ammonium sulfide or sodium in an alkaline environment or in the presence of acetic acid precipitates white zinc sulfide, insoluble in alkalis and ammonia and acetic acid, easily soluble in strong acids. 2) When zinc salts are strongly calcined, the resulting residue is moistened with a weak solution of cobalt nitrate and calcined again, the residue turns green (Rinmann's green). 3) Yellow blood salt, K₄Fe(CN)₆, gives a white precipitate, insoluble in dilute acids, soluble in caustic alkalis. 4) Carbonates of alkali metals give white precipitates, easily soluble in acids. 5) Ammonia gives a gelatinous white precipitate, soluble in excess of the reagent with the formation of the complex Zn(NH₃)₄²⁺. 6) Microchemically, zinc is detected in a weak acetic acid solution using a complex thiocyanate-mercury-ammonium salt. Quantitative determination of zinc is carried out by precipitation with alkali carbonates, calcination and weighing as ZnO. The method of G. Spacu and T. Dick consists in precipitating zinc with ammonium thiocyanate in the presence of pyridine and weighing as Zn₂(C₅H₅N)₂(SCN)₂. A volumetric-analytical method for determining zinc with ortho-oxine is described. The biological significance of zinc has not been clarified. For therapeutic purposes, zinc is used in the form of salts, mainly as a locally acting agent, externally. All zinc salts are poisonous; soluble zinc salts in strong dilutions have an astringent effect; strong solutions are caustic. The antiseptic action of zinc salts is insignificant. Zinc is rarely taken internally: for various nervous diseases and epilepsy in the form of valerian-zinc salt up to 0.2 per dose, less often in the form of zinc oxide at 0.05-0.5 per dose. Metal vapors are poisonous. Symptoms and treatment of acute poisoning with salts - see Poisoning. In forensic and sanitary-chemical detection of zinc in the presence of organic substances, the latter are destroyed either by burning or oxidation with a mixture of potassium chlorate and hydrochloric acid. The resulting hydrochloric acid filtrate, after removal of chlorine, is saturated with hydrogen sulfide, filtered from the precipitate and supersaturated with ammonia in a small excess. The resulting zinc sulfide precipitate, after removal of iron, can be used for characteristic reactions. Zinc preparations: Chemically pure zinc, Zincum metallicum pro analysi (granulatum, sine arseno), is used in arsenic testing by Marsh's method. - Zinc oxide, ZnO, Zincum oxydatum (Ph. VII), a fine white amorphous powder, insoluble in water, soluble in dilute acids and alkalis. It is widely used externally as a drying, secretion-limiting and slightly astringent agent in ointments, powders and liniments. - Zinc ointment - see Ointments (Ph. VII). - Zinc chloride, ZnCl₂, Zincum chloratum (Ph. VII), a white crystalline mass or similar sticks, deliquescent in air, very easily soluble in water and alcohol. The aqueous solution has a strongly acidic reaction; very dilute solutions are opalescent or turbid due to the formation of insoluble zinc hydroxide: ZnCl₂ + 2H₂O ⇌ Zn(OH)₂ + 2HCl. The turbidity disappears when hydrochloric acid or ammonium chloride is added due to enhancement of the reverse reaction; upon renewed dilution with water, turbidity may reappear. With zinc oxide, it forms a hard mass (cement). It is used externally as an astringent, in strong solutions as a caustic agent or (extremely painfully) in the form of caustic pastes. In a 0.2% solution - for urethral irrigations; in a 0.5-1% solution - for vaginal irrigations in colpitis, leucorrhea and other catarrhal conditions. Internally, due to its causticity, it is not used. - Zinc sulfate, ZnSO₄·7H₂O, Zincum sulfuricum (Ph. VII), white rhombic prisms or small needles, weathering on the surface, soluble in 0.6 parts of water, losing 6 molecules of crystallization water when dried at 100°. It is widely used in the form of irrigations (1%) in urological and gynecological practice; in the form of "eye drops" (0.25-0.5%), often with the addition of boric acid. Rarely as an emetic internally in doses up to 1.0 per dose. - Phenolsulfonic acid zinc, Zincum phenolsulfonicum (inaccurate term - Zincum sulfocarbolicum), (OH-C₆H₄-SO₃)₂Zn·8H₂O (Ph. VII), colorless transparent or slightly pinkish crystals with a very faint phenolic odor or odorless, soluble in water 1:2 with a weakly acidic reaction, used similarly to the previous preparation. - Zinc salicylate, (HO-C₆H₄COO)₂Zn·3H₂O, colorless shiny needles, with a sweetish metallic taste, soluble in 25 parts of water and 4 parts of alcohol. Used externally, similar to the previous one. Internally 0.1 per dose. - Zinc oleate, (C₁₇H₃₃COO)₂Zn, Zincum oleinicum. Usually contains in varying amounts impurities of palmitic and stearic zinc salts. A fine white powder, insoluble in water. Used in ointments and powders for eczema. In dental practice, cements from ZnO with ZnCl₂ or with ZnSO₄ are used; ZnO is added to the powder part of the cement, zinc salt - to the liquid. In industrial conditions, chronic zinc poisoning is observed in foundry workers - see Foundry fever.