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Zinc (from ) is a metallic chemical element with the symbol Zn and atomic number 30. It is a first-row transition metal of the group 12 of the periodic table. Although zinc has been used in the copper-zinc alloy brass since Roman times, and the metal was produced in large scale in India around 1200 AD, the pure metal was unknown to Europe until the end of the 16th century. Industrial-scale production in Europe had not started until the late 18th century.
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Encyclopedia
Zinc (from ) is a metallic chemical element with the symbol Zn and atomic number 30. It is a first-row transition metal of the group 12 of the periodic table. Although zinc has been used in the copper-zinc alloy brass since Roman times, and the metal was produced in large scale in India around 1200 AD, the pure metal was unknown to Europe until the end of the 16th century. Industrial-scale production in Europe had not started until the late 18th century. Corrosion-resistant zinc plating of steel is the major application for zinc. Other applications are in batteries and alloys, such as brass. Sphalerite, a zinc sulfide, is the most important zinc ore. Zinc production includes roasting, leaching and, at the end, pyrometallurgic winning or electrowinning.
Zinc is an essential mineral, necessary for sustaining all life. Enzymes with a zinc atom in the reactive center are widespread in biochemistry, such as the alcohol dehydrogenase in humans. Consumption of higher concentrations of zinc can cause ataxia, lethargy and copper deficiency.
A variety of zinc compounds find use industrially, such as zinc chloride (in deodorants), zinc pyrithione (anti-dandruff shampoos), zinc sulfide (in luminescent paints), and zinc methyl or zinc diethyl in the organic laboratory. Roughly one quarter of all zinc output is consumed in the form of zinc compounds.
Characteristics
Physical
Zinc, also referred to in nonscientific contexts as spelter, is a bluish-white, lustrous, diamagnetic metal, though most common commercial grades of the metal have a dull finish. It is somewhat less dense than iron and has a hexagonal crystal structure.
The metal is hard and brittle at most temperatures but becomes malleable between 100 and 150 °C. Above 210 °C, the metal becomes brittle again and can be pulverized by beating. Zinc is a fair conductor of electricity. For a metal, zinc has relatively low melting (420 °C) and boiling points (900 °C). Its melting point is the lowest of all the transition metals aside from mercury and cadmium.
Many alloys contain zinc, including brass, an alloy of zinc and copper. Other metals long known to form binary alloys with zinc are aluminium, antimony, bismuth, gold, iron, lead, mercury, silver, tin, magnesium, cobalt, nickel, tellurium and sodium. While neither zinc nor zirconium are ferromagnetic, the alloy ZrZn2 exhibits ferromagnetism below 35 K.
Occurrence
Zinc makes up about 75 ppm (0.007%) of the Earth's crust, making it the 24th most abundant element there. Soil contains an average of 64 ppm, but actual soils have 5–770 ppm zinc. Seawater has only 30 ppb zinc and the atmosphere contains 0.1 to 4 µg/m3.
The element is normally found in association with other base metals such as copper and lead in ores. Zinc is a chalcophile ("sulfur loving"), meaning the element has a low affinity for oxygen and prefers to bond with sulfur in highly insoluble sulfides. Chalcophiles formed as the crust solidified under the reducing conditions of the early Earth's atmosphere. Sphalerite, which is a form of zinc sulfide, is the most heavily-mined zinc-containing ore because its concentrate contains 60–62% zinc.
Other minerals from which zinc is extracted include smithsonite (zinc carbonate), hemimorphite (zinc silicate), wurtzite (another zinc sulfide), and sometimes hydrozincite (basic zinc carbonate). With the exception of wurtzite, all these other minerals were formed as a result of weathering processes on the primordial zinc sulfides.
Exploitable reserves of zinc exceed 100 million tonnes, with large deposits in Australia, Canada and the United States. At the current rate of consumption, these reserves are estimated to be depleted sometime between 2027 and 2055.
Isotopes
Naturally occurring zinc is composed of 5 stable isotopes, Zn-64, Zn-66, Zn-67, Zn-68 and Zn-70 with Zn-64 being the most abundant (48.63% natural abundance). Although they slowly decay, Zn-64 and Zn-70 are considered stable because their half-lifes are over 4.3×1018 and 1.3×1016 years, respectively. For comparison, the age of the universe is believed to be less than 1.4×1010 years, or less than a millionth the time required for a significant amount of 64Zn to decompose.
Twenty-five radioisotopes have been characterized, with the longest-lived being Zn-65 with a half-life of 243.66 days and Zn-72 with a half-life of 46.5 hours. All of the remaining radioactive isotopes have half-lives that are less than an hour, and the majority of these have half lives that are less than a minute. Zinc has 10 nuclear isomers with the longest-lived being Zn-69m (t½ 13.76 hours). The nuclei of nuclear isomers exist in a delicate equilibrium or metastability due to at least one nucleon having an excited energy state. Zn-61 has three excitation states and Zn-73 has two. The remaining isotopes that have metastable states, Zn-65, Zn-71, Zn-77, and Zn-78 only have one excited state each.
The isotopes of zinc range in mass number from 54 to 83. The most common decay mode of isotopes with mass numbers lower than the most abundant stable isotope, Zn-64, is electron capture, primarily forming copper isotopes (29 protons) as decay products. The most common decay mode for isotopes with mass numbers higher than Zn-64 is beta negative decay, primarily forming gallium isotopes (31 protons) as decay products.
Compounds and chemistry
Reactivity
Zinc has an electronic configuration of [Ar]3d104s2 and is a member of the group 12 of the periodic table. The chemistry of zinc is dominated by the +2 oxidation state and is similar to the alkaline earth metals. Zinc's chemistry is particularly similar to magnesium except that it forms bonds with a greater degree of covalency. It differs in chemistry in most respects to other transition metals due to a stable filled d-shell. Similar to other transition elements, however, zinc forms complexes with O-donors, but unlike magnesium, readily forms complexes with N- and S- donors. In its complexes zinc is found mainly in 4 or 6 coordination although 5 coordinate complexes are known.
Zinc is a moderately reactive metal, sitting above iron in the electrochemical series. The surface of the pure metal tarnishes quickly, eventually forming a passivating layer of the basic zinc carbonate, Zn5(OH)6CO3, by reaction with atmospheric CO2. Zinc burns in air with a bright bluish-green flame, giving off fumes of zinc oxide. Zinc reacts readily with acids, alkalis and other non-metals. Extremely pure zinc reacts only slowly at room temperature with acids. The reaction of zinc with water is prevented by the passivating layer however, when this layer is removed by a strong alkali, the reaction proceeds evolving hydrogen. Similarly oxide free zinc dust reacts readily with water. Zinc metal reacts with hydrochloric and sulfuric acid forming, respectively, the soluble salts zinc chloride and zinc sulfate.
- Zn + 2HCl ? ZnCl2 + H2
- Zn + H2SO4 ? ZnSO4 + H2
The passive layer is destroyed by reaction with mercury(II), forming an amalgam. The Jones reductor, which contains this amalgam, is a useful device for reactions which need a strong reducing agent.
Compounds
Binary compounds of zinc are known for most of the metalloids and all the nonmetals except the noble gases. Of the four halides, has the most ionic character, whereas the others (, , and ) have relatively low melting points and are considered to have more covalent character. The oxide ZnO is a white powder that is nearly insoluble in neutral aqueous solutions, but is amphoteric, dissolving in both strong basic and acidic solutions. The other chalcogenides (ZnS, ZnSe, and ZnTe) have varied applications in electronics and optics. Pnictogenides (, , and ), the peroxide (), the hydride (), and the carbide are also known.
]
In weak basic solutions containing Zn2+ ions, the hydroxide forms as a white precipitate. In stronger alkaline solutions, this hydroxide is dissolved to form zincates . The nitrate , chlorate , sulfate , phosphate , molybdate , cyanide , arsenite Zn(AsO2)2, arsenate and the chromate (one of the few colored zinc compounds) are a few examples of other common inorganic compounds of zinc. One of the simplest examples of an organic compound of zinc is the acetate ().
Organozinc compounds are those that contain zinc-carbon covalent bonds. Diethylzinc is an important reagent in synthetic chemistry. It was first reported in 1848 from the reaction of zinc and ethyl iodide, and is the first compound known to contain a metal-carbon sigma bond.
The volatilization of zinc in combination with zinc chloride at temperatures above 285 °C indicates the formation of Zn2Cl2, a zinc compound with a +1 oxidation state. The first such compound, decamethyldizincocene, contains a strong zinc-zinc bond at room temperature.
History
Ancient use
Ornaments made of alloys that contain 80–90% zinc with lead, iron, antimony, and other metals making up the remainder, have been found that are 2500 years old. A possibly prehistoric statuette containing 87.5% zinc was found in a Dacian archaeological site in Transylvania (modern Romania). The Berne zinc tablet is a votive plaque dating to Roman Gaul made of an alloy that is mostly zinc.
Zinc ores were used to make brass many centuries prior to the discovery of zinc as a separate element. Palestinian brass from the 14th to 10th centuries BC contains 23% zinc. The Book of Genesis, written between the 10th and 5th centuries BC, mentions Tubalcain as an "instructor in every artificer in brass and iron" (Genesis 4:22).
The manufacture of brass was known to the Romans by about 30 BC. They made brass by heating powdered calamine (zinc silicate or carbonate), charcoal and copper together in a crucible. The resulting calamine brass was then either cast or hammered into shape and was used in weaponry. Some coins struck by Romans in the Christian era are made of what is likely calamine brass. In the West, impure zinc was known from antiquity to exist in the remnants in melting ovens, but it was usually discarded, as it was thought to be worthless.
References to medicinal uses of zinc are in the Charaka Samhita, which is believed to have been written as early as 300 BC in India. The zinc mines and smelter of Zawar, near Udaipur in India, were active about 100 years before that and produced an estimated million tonnes of metallic zinc and zinc oxide from the 12th to 16th centuries. The Rasaratna Samuccaya, written in approximately the year 800, mentions two types of zinc-containing ores; one used for metal extraction and another used for medicinal purposes.
Early studies and naming
Zinc was distinctly recognized as a metal under the designation of Fasada in the medical Lexicon ascribed to the Hindu king Madanapala and written about the year 1374. Smelting and extraction of impure zinc by reducing calamine with wool and other organic substances was accomplished in the 13th century in India. The Chinese did not learn of the technique until the 17th century.
Alchemists burned zinc metal in air and collected the resulting zinc oxide on a condenser.
Some alchemists called zinc oxide formed in this way lana philosophica, Latin for "philosopher's wool", because it collected in wooly tufts while others thought it looked like white snow and named it ninx album.
The name of the metal was probably first used by Paracelsus, a Swiss-born German alchemist, who referred to the metal as "zincum" or "zinken" in his book Liber Mineralium II, in the 16th century. The word is probably derived from the German , and supposedly meant "tooth-like, pointed or jagged part" and, as metallic zinc crystals are needle-like. The metal was also called Indian tin, tutanego, calamine, and spinter.
German metallurgist Andreas Libavius received a quantity of what he called "calay" of Malabar from a cargo ship captured from the Portuguese in 1596. Libavius described the properties of the sample, which may have been zinc. Zinc was regularly imported to Europe from the Orient in the 17th and early 18th centuries, but was at times very expensive.[An East India Company ship carrying a cargo of nearly pure zinc metal from the Orient sank off the coast Sweden in 1745.]
Isolation of the pure element
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The isolation of metallic zinc in the West may have been achieved independently by several people. Postlewayt's Universal Dictionary, a contemporary source giving technological information in Europe, did not mention zinc before 1751 but the element was studied before then.
Flemish metallurgist P.M. de Respour reported that he extracted metallic zinc from zinc oxide in 1668. By the turn of the century, Étienne François Geoffroy described how zinc oxide condenses as yellow crystals on bars of iron placed above zinc ore being smelted.
In Britain, Dr. John Lane is said to have carried out experiments, probably at Landore, prior to his bankruptcy in 1726. In 1738, William Champion patented in Great Britain a process to extract zinc from calamine in a vertical retort style smelter. His technology was somewhat similar to that used at Zawar zinc mines in Rajasthan but there is no evidence that he visited the Orient. Champion's process was used through 1851.
German chemist Andreas Marggraf normally gets credit for discovering pure metallic zinc even though Swedish chemist Anton von Swab distilled zinc from calamine four years before. In his 1746 experiment, Marggraf heated a mixture of calamine and charcoal in a closed vessel without copper to obtain a metal. This procedure became commercially practical by 1752.
Later work
was named for Luigi Galvani]]
William Champion's brother, John, patented a process in 1758 for calcining zinc sulfide into an oxide usable in the retort process. Prior to this only calamine could be used to produce zinc. In 1798, Johann Christian Ruberg improved on the smelting process by building the first horizontal retort smelter. Jean-Jacques Daniel Dony built a different kind of horizontal zinc smelter in Belgium, which processes even more zinc.
Italian doctor Luigi Galvani discovered in 1780 that connecting the spinal cord of a freshly dissected frog to an iron rail attached by a brass hook caused the frog's leg to twitch. He incorrectly thought he had discovered an ability of nerves and muscles to create electricity and called the effect "animal electricity." The galvanic cell and the process of galvanization were both named for Luigi Galvani and these discoveries paved the way for electrical batteries, galvanization and cathodic protection.
Galvani's friend, Alessandro Volta, continued researching this effect and invented the Voltaic pile in 1800. The basic unit of Volta's pile was a simplified galvanic cell, which is made of a plate of copper and a plate of zinc connected to each other externally and separated by an electrolyte. These were stacked in series to make the Voltaic cell, which in turn produced electricity by directing electrons from the zinc to the copper and allowing the zinc to corrode.
Production
Mining and processing
Zinc is the fourth most common metal in use, trailing only iron, aluminium, and copper with an annual production of about 10 million tonnes. The world's largest zinc producer is Nyrstar, a merger of the Australian Zinifex and the Belgian Umicore. About 70% of the world's zinc originates from mining, while the remaining 30% comes from recycling secondary zinc. Commercially pure zinc is known as Special High Grade, often abbreviated SHG, and is 99.995% pure.
Worldwide, 95% of the zinc is mined from sulfidic ore deposits, in which sphalerite ZnS is nearly always mixed with the sulfides of copper, lead and iron. There are zinc mines throughout the world, with the main mining areas being China, Australia and Peru. China produced over one-fourth of the global zinc output in 2006.
Zinc metal is produced using extractive metallurgy. After grinding the ore, froth flotation, which selectively separates minerals from gangue by taking advantage of differences in their hydrophobicity, is used to get a ore concentrate. A final concentration of zinc of about 50% is reached by this process with the remainder of the concentrate being sulpher (32%), iron (13%), and SiO2 (5%).
Roasting converts the zinc sulfide concentrate produced during processing to zinc oxide.
- 2 ZnS + 3 O2 ? 2 ZnO + 2 SO2
Top 10 zinc producing countries in 2006 (full list) | Rank | Country | tonnes |
|---|
| 1 | China (PRC) | 2,600,000 | | 2 | Australia | 1,338,000 | | 3 | Peru | 1,201,794 | | 4 | United States | 727,000 | | 5 | Canada | 710,000 | | 6 | Mexico | 480,000 | | 7 | Ireland | 425,700 | | 8 | India | 420,800 | | 9 | Kazakhstan | 400,000 | | 10 | Sweden | 192,400 |
The sulfur dioxide is used for the production of sulfuric acid, which is necessary for the leaching process.
If deposits of zinc carbonate, zinc silicate or zinc spinel, like the Skorpion Deposit in Namibia are used for zinc production the roasting can be omitted.
For further processing two basic methods are used: pyrometallurgy or electrowinning. The first is the older process in which the zinc oxide is reduced by carbon or carbon monoxide at high temperatures of into the metal which is distilled of as zinc vapor. The zinc vapor is collected in a condenser.
- 2 ZnO + C ? 2 Zn + CO2
- 2 ZnO + 2 CO ? 2 Zn + 2 CO2
For the second process the zinc is leached from the ore concentrate by sulfuric acid.
- ZnO + H2SO4 ? ZnSO4 + H2O
After this step electrolysis, electrowinning, is used to produce the zinc metal.
Environmental impact
The production for sulfidic zinc ores produces large amounts of sulfur dioxide and cadmium vapor. Smelter slag and other residues of process also contain significant amounts of heavy metals. About 1,100,000 tonnes of metallic zinc and 130,000 tonnes of lead were mined and smelted in the Belgian towns of La Calamine and Plombières between 1806 and 1882. The dumps of the past mining operations leach significant amounts of zinc and cadmium, and, as a result, the sediments of the Gaul river contain significant amounts of heavy metals.
Levels of zinc in rivers flowing through industrial or mining areas can be as high as 20 ppm. Effective sewage treatment greatly reduces this; treatment along the Rhine, for example, has decreased zinc levels to 50 ppb. Soils contaminated with zinc through the mining of zinc-containing ores, refining, or where zinc-containing sludge is used as fertilizer, can contain several grams of zinc per kilogram (several ‰) of dry soil. Levels of zinc in excess of 500 ppm in soil interferes with the ability of plants to absorb other essential metals, such as iron and manganese. Zinc levels of 2000 ppm to 180,000 ppm (18%) have been recorded in some soil samples.
Applications
Anti-corrosion and batteries
The most important application of zinc and the largest use of the metal is as an anti-corrosion agent. Galvanization, which is the coating of iron or steel as protection against corrosion, is the most familiar form of using zinc in this way. In 2006 in the United States, 56% or 773,000 tonnes of the zinc metal was used for galvanization, while worldwide 47% was used for this purpose.
Zinc is more reactive than iron or steel and thus will attract almost all local oxidation until it completely corrodes away. A protective surface layer of oxide and carbonate (Zn5(OH)6(CO3)2) forms as the zinc corrodes. This protection lasts even after the zinc layer is scratched but degrades through time as the zinc corrodes away. The zinc is applied electrochemically or as molten zinc by hot-dip galvanizing or spraying. Galvanization is used on chain-link fencing, guard rails, suspension bridges, lightposts, metal roofs, heat exchangers, and car bodies.
The relative reactivity of zinc and its ability to attract oxidation to itself also makes it a good sacrificial anode in cathodic protection. Cathodically protecting (CP) buried pipelines requires a solid piece of zinc to be connected by a conductor to a steel pipe. Zinc acts as the anode (negative terminus) by slowly corroding away as it passes electric current to the steel pipeline.[Electric current will naturally flow between zinc and steel but larger pipeline systems require a rectifier that adds an induced DC electric current to the CP system.] Zinc is also used to cathodically protect metals that are exposed to sea water from corrosion. A zinc disc attached to a ship's iron rudder will slowly corrode while the rudder stays unattacked. Other similar uses include a plug of zinc attached to a propeller or the metal protective guard for the keel of the ship.
With a electrochemical potential of −0.7628 volts, zinc makes a good material for the negative terminus or anode in batteries. Powdered zinc is used in this way in alkaline batteries and sheets of zinc metal form the cases for and act as anodes in zinc-carbon batteries.
Alloys
An important alloy of zinc is brass, in which copper is alloyed with anywhere from 3% to 45% zinc, depending upon the type of brass. Brass is generally more ductile and strong than copper and has superior corrosion resistance. These properties make it useful in communication equipment, hardware, instruments, and water valves.
Other widely used alloys that contain zinc include nickel silver, typewriter metal, soft and aluminum solder, and commercial bronze. Zinc is also used in contemporary pipe organs as a substitute for the traditional lead/tin alloy in pipes. Alloys of 85–88% zinc, 4–10% copper, and 2–8% aluminium find limited use in certain types of machine bearings. Zinc is the primary metal used in making American one cent coins since 1982. The zinc core is coated with a thin layer of copper to give the impression of a copper coin. In 1994, 33,200 tons of zinc were used to produce 13.6 billion pennies in the United States.
Alloys of primarily zinc with small amounts of copper, aluminium, and magnesium are useful in die casting as well as spin casting, especially in the automotive, electrical, and hardware industries. These alloys are marketed under the name Zamak. An example of this is zinc aluminium. The low melting point together with the low viscosity of the alloy makes the production of small and intricate shapes possible. The low working temperature leads to rapid cooling of the casted products and therefore fast assembly is possible. Another alloy, marketed under the name Prestal, contains 78% zinc and 22% aluminium and is reported to be nearly as strong as steel but as malleable as plastic. This superplasticity of the alloy allows it to be molded using die casts made of ceramics and cement.
Similar alloys with the addition of a small amount of lead can be cold-rolled into sheets. An alloy of 96% zinc and 4% aluminium is used to make stamping dies for low production run applications for which ferrous metal dies would be too expensive. In building facades, roofs or other applications in which zinc is used as sheet metal and for methods such as deep drawing, roll forming or bending, zinc alloys with titanium and copper are used. Unalloyed zinc is too brittle for these kinds of manufacturing processes.
Cadmium zinc telluride (CZT) is a semiconductive alloy that can be divided into an array of small sensing devices. These devices are similar to an integrated circuit and can detect the energy levels of incoming gamma rays. When placed behind an absorbing mask, the CZT sensor array can also be used to determine the direction of the rays. Zinc is used as the anode or fuel of the zinc-air battery/fuel cell providing the basis of the theorized zinc economy.
Other industrial uses
Roughly one quarter of all zinc output is consumed in the form of zinc compounds; a variety of which are used industrially. Zinc oxide is widely used as a white pigment in paints, and as a catalyst in the manufacture of rubber. It is also used as a heat disperser for the rubber and acts to protect its polymers from ultraviolet radiation (the same UV protection is conferred to plastics containing zinc oxide). The semiconductor properties of zinc oxide make it useful in varistors and photocopying products. The zinc zinc-oxide cycle is a two step thermochemical process based on zinc and zinc oxide for hydrogen production with a typical efficiency around 40%.
Zinc chloride is often added to lumber as a fire retardant and can be used as a wood preservative. It is also used to make other chemicals. Zinc methyl (Zn(CH3)2) is used in a number of organic syntheses.
Sulfur-containing compounds are also important. Zinc sulfide (ZnS) is used in luminescent pigments such as on the hands of clocks, x-ray and television screens, and luminous paints. ZnS is also used in lasers that opperate in the mid-infrared part of the spectrum. Zinc sulphate is an important chemical in dyes and pigments. Zinc pyrithione is used as in antifouling paints.
Zinc powder is sometimes used as a propellant in model rockets. When a compressed mixture of 70% zinc and 30% sulfur powder is ignited there is a violent chemical reaction. This produces zinc sulfide, together with large amounts of hot gas, heat, and light. Zinc sheet metal is used to make zinc bars.
Zinc has been proposed as a salting material for nuclear weapons (cobalt is another, better-known salting material). A jacket of isotopically enriched Zn-64, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope Zn-65 with a half-life of 244 days and produce massive gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several days. Such a weapon is not known to have ever been built, tested, or used. Zn-65 is also used as a tracer to study physical phenomena such as the wearing out of alloys containing zinc, or the path and the role of zinc in organisms.
Zinc dithiocarbamate complexes are used as agricultural fungicides, these include Zineb, Metiram, Propineb and Ziram. Zinc naphthenate is used as wood preservative.
Medicinal
is one compound used for the delivery of zinc as a dietary supplement]]
Zinc is included in most single tablet over-the-counter daily vitamin and mineral supplements. It is believed to possess antioxidant properties, which protect against premature aging of the skin and muscles of the body, although studies differ on effectiveness. In larger amounts, taken as zinc alone in other proprietaries, it is believed by some to speed up the healing process after an injury. Zinc gluconate glycine and zinc acetate are used in throat lozenges or tablets to reduce the duration and the severity of cold symptoms. Preparations include zinc oxide, zinc acetate and zinc gluconate.
Zinc preparations can protect against sunburn in the summer and windburn in the winter. Applied thinly to a baby's diaper area (perineum) with each diaper change, it can protect against rash.
The Age-Related Eye Disease Study determined that zinc can be part of an effective treatment for age-related macular degeneration. Zinc supplementation is also an effective treatment for acrodermatitis enteropathica, a genetic disorder affecting zinc absorption that was previously fatal to babies born with it.
Zinc lactate is used in toothpaste to prevent halitosis. Zinc pyrithione is widely applied in shampoos because of its anti-dandruff function.
Zinc salts are effective against pathogens in direct application. Gastroenteritis is strongly attenuated by ingestion of zinc, and this effect could be due to direct antimicrobial action of the zinc ions in the GI tract, or to the absorption of the zinc and re-release from immune cells (all granulocytes secrete zinc), or both.[In clinical trials, both zinc gluconate and zinc gluconate glycine (the formulation used in lozenges) have been shown to shorten the duration of symptoms of the common cold.
]-
The amount of glycine can vary from two to twenty moles per mole of zinc gluconate. One review of the research found that out of nine controlled experiments using zinc lozenges, the results were positive in four studies, and no better than placebo in five.
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This review also suggested that the research is characterized by methodological problems, including differences in the dosage amount used, and the use of self-report data. The evidence suggests that zinc supplements may be most effective if they are taken at the first sign of cold symptoms.
Biological role
Uptake and use
Zinc is an essential trace element, necessary for sustaining all animal life and is thought to help plants resist drought and disease. Concentrations of zinc in humans are highest in the eye, kidney, liver, muscle, and prostate. It is particularly rich in semen and is a key factor in prostate gland function and reproductive organ growth.
The intestines absorb 15 to 40% of the zinc that passes through them, with higher absorption rates when zinc levels are low in the body. Once absorbed, it may be held in metallothionein reserves within the intestines or the liver. Excess zinc is also stored in bones and the spleen but it can not be easily released from these tissues to make up for a dietary deficiency. About 1% of total zinc is lost from the body per day; 90% of that is excreted through the intestines and the rest via urine (5%) and sweat (5%).
Zinc is transported through the blood by albumin and transferrin. Since transferrin also transports iron, excessive iron reduces zinc absorption, and vice-versa. A similar reaction occur with zinc and copper. The concentration of zinc in blood plasma stays relatively constant regardless of zinc intake.
The most-important types of protein that contain zinc are enzymes and transcription factors; two examples are metalloenzyme and zinc finger, respectively. There are over 200 enzymes that contain zinc as a cofactor and about the same number of transcription factors. Zinc-containing enzymes are used by the body to regulate growth and development, promote fertility, and aid digestion and nucleic acid synthesis.
Carbonic anhydrase uses zinc to quickly catalyze the waste gas carbon dioxide into bicarbonate and protons in vertebrate blood. The non-related ß-carbonic anhydrase is required in plants for leaf formation, the synthesis of indole acetic acid (auxin) and anaerobic respiration (alcoholic fermentation). Alcohol dehydrogenase uses zinc in the liver to metabolize alcohol.
Zinc is a good lewis acid, making it a useful catalytic agent in hydroxylation and other enzymatic reactions. Zinc also has a flexible coordination geometry, allowing enzymes using zinc to rapidly shift conformations and perform biological reactions.
Cells in the salivary gland, prostate, immune system and intestine use zinc signaling. Zinc is especially needed by tissues that have a high rate of new cell development, such as bone marrow, immune system cells and the lining of the gut.
Food sources
Red meats, especially beef, lamb and liver have some of the highest concentrations of zinc in food. A turkey's neck and beef's chuck or shank also contain significant amounts of zinc. Tubers, such as potatoes or cassava, have very low levels of zinc.
The concentration of zinc in plants varies based on levels of the element in soil. When there is adequate zinc in the soil, the food plants that contain the most zinc are wheat (20–60 ppm), sweet corn (about 20 ppm) and lettuce (12 ppm) while fruits have the lowest zinc content; apples and oranges only contain 1 ppm zinc or less (all fresh weights). Zinc is also found in beans, nuts, almonds, whole grains, pumpkin seeds, sunflower seeds and blackcurrant. Soil conservation is needed to make sure that crop rotation will not deplete the zinc in soil.
Other sources include fortified food and dietary supplements, which come in various forms. A 1998 review concluded that zinc oxide, one of the most common supplements in the United States, and zinc carbonate are nearly insoluble and poorly absorbed in the body. This review also cited studies which found low plasma zinc concentrations after zinc oxide and zinc carbonate were consumed compared with those seen after consumption of zinc acetate and sulfate salts.
For fortification, however, a 2003 review recommended zinc oxide in cereals as cheap, stable, and as easily absorbed as more expensive forms. A 2005 study found that various compounds of zinc, including oxide and sulfate, did not show statistically significant differences in absorption when added as fortificants to maize tortillas.
Deficiency
Zinc deficiency occurs where insufficient zinc is available for metabolic needs. It is usually nutritional, but can be associated with malabsorption, acrodermatitis enteropathica, chronic liver disease, chronic renal disease, sickle cell disease, diabetes, malignancy, and other chronic illnesses. Phytic acid decreases zinc absorption by forming insoluble zinc phytate. Diets rich in foods that contain a lot of phytic acid, such as cereals and legumes, decrease zinc absorption into the body. There is a "paucity" of adequate zinc biomarkers, and the "most widely used and accepted" indicator, plasma zinc, has poor sensitivity.
Assuming 30% of zinc ingested is absorbed for a particular diet, then men require 7.5 mg of zinc in their food per day and women need 5.5 mg. An average diet contains about 10 mg zinc per day for men and around 8 mg per day for women. Absorption of zinc during pregnancy and lactation increases, resulting in sufficient zinc assuming an average diet is maintained.[Human breast milk contains the 2 mg zinc needed by the baby each day but cows milk needs to be fortified in zinc to provide adequate nutrition.]
About half of the world's population is at risk for zinc deficiency. The World Health Organization advocates zinc supplementation for severe malnutrition and diarrhea. Zinc supplements help prevent disease and reduce mortality, especially among children with low-birth weight or stunted growth. However, zinc supplements should not be administered alone, since many in the developing world have several deficiencies, and zinc interacts with other micronutrients.
Zinc deficiency is found in other organisms as well. Animals with a diet deficient in zinc require twice as much food to attain the same weight gain as animals given sufficient zinc. Symptoms of chronic zinc deficiency are stunted growth and retarded sexual development. The condition may be on factor responsible for low sperm counts. Plants that grow in soils that are zinc-deficient are more susceptible to disease. Many fruit and nut-bearing trees grown in the western United States, for example, were diseased in the 19th century. The cause was thought to be copper deficiency but treatment with a copper solution had mixed results; some trees responded well while others did not respond at all. Researchers later found that trees that responded well were treated with copper solution applied by using buckets galvanized with zinc. Zinc was thereafter applied directly.
Precautions
Toxicity
Even though zinc is an essential requirement for a healthy body, excess zinc can be harmful. Excessive absorption of zinc suppresses copper and iron absorption. The free zinc ion in solution is highly toxic to plants, invertebrates, and even vertebrate fish. The Free Ion Activity Model (FIAM) is well-established in the literature, and shows that just micromolar amounts of the free ion kills some organisms. A recent example showed 6 micromolar killing 93% of all Daphnia in water.
The free zinc ion is a powerful Lewis acid up to the point of being corrosive. Stomach acid contains hydrochloric acid, in which metallic zinc dissolves readily to give corrosive zinc chloride. Swallowing a post-1982 American one cent piece (97.5% zinc) can cause damage to the stomach lining due to the high solubility of the zinc ion in the acidic stomach.
There is evidence of induced copper deficiency at low intakes of 100–300 mg Zn/d. The USDA RDA is 15 mg Zn/d. Even lower levels, closer to the RDA, may interfere with the utilization of copper and iron or to adversely affect cholesterol. Levels of zinc in excess of 500 ppm in soil interferes with the ability of plants to absorb other essential metals, such as iron and manganese. There is also a condition called the zinc shakes or "zinc chills" that can be induced by the inhalation of freshly formed zinc oxide formed during the welding of galvanized materials.
Poisoning
In 1983, the United States Mint began minting pennies coated in copper but made primarily of zinc. With the new zinc pennies, there is the potential for zinc toxicosis, which can be fatal. One reported case of chronic ingestion of 425 pennies (over 1 kg of zinc) resulted in death due to gastrointestinal bacterial and fungal sepsis, while another patient, who ingested 12 grams of zinc, only showed lethargy and ataxia (gross lack of coordination of muscle movements). Several other cases are reported of humans suffering zinc intoxication by the ingestion of zinc coins.
Pennies and other small coins are sometimes ingested by dogs, resulting in the need for medical treatment to remove the foreign body. The zinc content of some coins can also cause zinc toxicity, which is commonly fatal in dogs, where it causes a severe hemolytic anemia, also liver or kidney damage; vomiting and diarrhea are possible symptoms. Zinc is highly toxic in parrots and poisoning can often be fatal. Storing fruit juices in galvanized cans has resulted in mass poisonings of zinc.
Bibliography
External links
- Information about the uses and properties of zinc.
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