Element Zinc, Zn, Transition Metal
|From the metals of the iron group, to which it is in many respects similar, zinc differs in the fact that it cannot form any other stage of combination than that containing divalent ion. In this respect it forms an intermediate member between nickel, which has lost this property for the most part, and magnesium, in the case of which no trace of that property is present. |
Zinc occurs fairly abundantly in nature, both as oxygen compounds (carbonate and silicate), and as sulphide or blende. Zinc is obtained from both these and is a white, fairly soft metal which melts as low as 420° and boils at 950°. In the air and in water it oxidises very quickly. Since, however, the zinc hydroxide or carbonate which is formed covers the underlying metal with a coherent coating, the oxidation proceeds for the most part slowly, and objects made of zinc resist the influence of air and water fairly well.
Cast zinc is coarsely crystalline and brittle. If, however, the metal is heated to somewhat over 100°, it becomes soft and tenacious, and can be hammered and rolled. Having once undergone this treatment, it remains tenacious even at the ordinary temperature. If it is heated to about 300°, it again becomes extremely brittle, and at that temperature can be ground to a powder; on being cooled, it retains a somewhat brittle character.
Zinc is employed not only in the pure state, but also to a large extent in alloys. Its most important alloy is that with copper; it is called brass, and will be treated under copper. With copper and nickel it forms German silver.
Zinc is also used for coating iron in order to protect it from rust; this is then known as " galvanised " iron. Iron objects which remain constantly exposed to the air, such as railings, agricultural implements, etc., are in this way rendered durable. It is true that zinc in contact with iron oxidises more quickly than when alone, but the oxidation is limited to the surface.
At 420° zinc fuses, and this temperature is low enough to allow of the metal being largely used for castings. At 950° zinc is converted into a vapour, which burns in the air with a brilliant blue flame, forming zinc oxide. The density of this vapour yields the molar weight 65.4; as this number also represents the combining weight, the formula of zinc in the vaporous state is Zn. It contains, therefore, only one combining weight, whereas most of the elements in the gaseous or vaporous form have the double formula. The other metals, however, so far as they are known in the vaporous state, exhibit the same peculiarity as zinc.
It is on the volatility of zinc that its manufacture depends. The oxygen ores are heated directly with charcoal; the sulphide, after being converted into zinc oxide by roasting in the air. The metal formed by the reduction of the oxide with charcoal volatilises and is collected in suitable receivers with exclusion of air, while the impurities remain behind in the retort.
In this process a portion of the metal is obtained in a form in which it is often used in the laboratory, viz. as zinc dust. So long as the temperature of the receiver remains below the melting-point of zinc, the metal is deposited in the form of a fine grey powder. (The relations are exactly the same as in the formation of flowers of sulphur.) This powdery form of zinc is more suitable for many chemical purposes than the fused; in using it, however, it must be remembered that it generally contains a considerable amount of zinc oxide in consequence of an incipient oxidation.
Recently many attempts have been made to obtain zinc from its ores by first converting it into a salt and then decomposing this by means of the electric current. The difficulty of obtaining a coherent metal free from oxide in this way does not appear as yet to have been overcome.
|Humanity was familiar with the manufacture of brass from ancient ages. However the metal zinc was not was known. Instead of it Phrygian ash, used for healing sore eyes, was mentioned in the Ebers papyrus from about 1552 BC, the oldest preserved medical document. Brass is known probably since 2 century BC. In Europe it is produced since year 1400 AD. Smelting and extraction of metal zinc was accomplished perhaps since XII century AD in India. Several hypotheses exist about the origin of the name "zinc". Some of scientists suppose that it may also be derived from Persian sing for stone. A Lexicon of Alchemy by Martin Ruland (Martinus Rulandus the Elder) defines zinc as metallic marcasite, a mixture of four elements. By another, less credible version the name derives from ancient or medieval German Zinco or Zinke = sharp point. Some suppose a relation with Zinn, the German word for Tin.|
Zinc belongs to the Transition metals family.
|Average crustal abundance of zinc is 8.3x10-3 mass %. In basic igneous rocks its abundance slightly bigger than in the acidic ones: 1.3x10-2% vs. 6x10-3%. This chalcophilic element is, nevertheless, found in 66 rocks, the most important of them are zincite, sphalerite, willemite, calamine, smithsonite and franklinite ZnFe2O4. Zinc is a very active aquatic migrant, especially in thermal waters with accompanied by Lead from which commercially important zinc sulphides are settled out. Zinc actively migrates also in surface and underground waters, settled out by H2S or, in a less degree, by clay sorption.|
Zinc is an essential element, necessary for sustaining all life. A grown-up human body contains 2 g of zinc. Despite zinc-containing enzymes are omnipresent, its actual concentration is so small, that its importance was fully understood only 100 years ago. Its significant role is based on the fact that zinc may be found in more than 40 essential enzymes. It catalyzes hydrolysis peptides, proteins, some esters and aldehydes. The most important of them are carboxypeptidase A and carbonic anhydrase.
Zinc contributes to carbohydrate metabolism via insulin and activates vitamin A in the organism. It plays noticeable role for bone formation as well as for its antivirus and antitoxic activity.