Silver (BioDeep_00000006290)

描述信息

Among metals, pure silver has the highest thermal conductivity (the non-metal diamond and superfluid helium II are higher) and one of the highest optical reflectivity. (Aluminium slightly outdoes silver in parts of the visible spectrum, and silver is a poor reflector of ultraviolet light). Silver also has the lowest contact resistance of any metal. Silver halides are photosensitive and are remarkable for their ability to record a latent image that can later be developed chemically. Silver is stable in pure air and water, but tarnishes when it is exposed to air or water containing ozone or hydrogen sulfide to form a black layer of silver sulfide which can be cleaned off with dilute hydrochloric acid. The most common oxidation state of silver is +1 (for example, silver nitrate: AgNO3); in addition, +2 compounds (for example, silver(II) fluoride: AgF2) and +3 compounds (for example, potassium tetrafluoroargentate: K[AgF4]) are known.; Hippocrates, the "father of medicine", wrote that silver had beneficial healing and anti-disease properties, and the Phoenicians used to store water, wine, and vinegar in silver bottles to prevent spoiling. In the early 1900s people would put silver dollars in milk bottles to prolong the milks freshness. Its germicidal effects increased its value in utensils and as jewellery. The exact process of silvers germicidal effect is still not well understood, although theories exist. One of these is the oligodynamic effect, which explains the effect on microorganisms but would not explain antiviral effects.; Jewellery and silverware are traditionally made from sterling silver (standard silver), an alloy of 92.5\\% silver with 7.5\\% copper. In the United States, only an alloy consisting of at least 92.5\\% fine silver can be marketed as "silver". Sterling silver is harder than pure silver, and has a lower melting point (893 °C) than either pure silver or pure copper. Britannia silver is an alternative hallmark-quality standard containing 95.8\\% silver, often used to make silver tableware and wrought plate. With the addition of germanium, the patented modified alloy Argentium Sterling Silver is formed, with improved properties including resistance to firescale.; Silver bromide is a yellow, low hardness salt.; Silver is a chemical element with the chemical symbol Ag (Latin: argentum) and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal. The metal occurs naturally in its pure, free form (native silver), as an alloy with gold (electrum) and other metals, and in minerals such as argentite and chlorargyrite. Most silver is produced as a by-product of copper, gold, lead, and zinc refining.; Silver is a constituent of almost all colored carat gold alloys and carat gold solders, giving the alloys paler colour and greater hardness. White 9 carat gold contains 62.5\\% silver and 37.5\\% gold, while 22 carat gold contains up to 8.4\\% silver or 8.4\\% copper.; Silver is a very ductile and malleable (slightly harder than gold) monovalent coinage metal with a brilliant white metallic luster that can take a high degree of polish. It has the highest electrical conductivity of all metals, even higher than copper, but its greater cost and tarnishability have prevented it from being widely used in place of copper for electrical purposes, though 13,540 tons were used in the electromagnets used for enriching uranium during World War II (mainly because of the wartime shortage of copper). Another notable exception is in high-end audio cables.; Silver is commonly used in catheters. Silver alloy catheters are more effective than standard catheters for reducing bacteriuria in adults in hospital having short term catheterisation.This meta-analysis clarifies discrepant results among trials of silver-coated urinary catheters by revealing that silver alloy catheters are significantly more effective in preventing urinary tract infectio...
Silver is widely distributed in the earths crust and is found in soil, fresh and sea water, and the air. It is readily absorbed into the human body with food and drink and through inhalation, but the low levels of silver commonly present in the bloodstream (< 2.3 b.mu g/L) and in key tissues like liver and kidney have not been associated with any disease or disability. Silver is not an acknowledged trace element in the human body and fulfills no physiological or biochemical role in any tissue even though it interacts with several essential elements including zinc and calcium. Physiologically, it exists as an ion in the body. Silver has a long history in the treatment of human diseases, including epilepsy, neonatal eye disease, venereal diseases, and wound infections. It has been employed in water purification and is currently used to safeguard hospital hot water systems against Legionella infections. Principle routes of human exposure to silver nowadays are through its widespread use as an antimicrobial agent in wound care products and medical devices, including in-dwelling catheters, bone cements, cardiac valves and prostheses, orthopedic pins, and dental devices. In each case, the antimicrobial properties of silver are dependent upon release of biologically active silver ion (Ag*) from metallic silver (including nanocrystalline forms), silver nitrate, silver sulfadiazine, and other silver compounds incorporated in the various devices, and its lethal effect on pathogenic organisms. Experience has shown that a large proportion of the silver ion released from medical devices not required for antimicrobial action is disseminated into tissue fluids and exudates, where it combines with albumins and macroglobulins. These silver-protein complexes are absorbed into the systemic circulation to be deposited in key soft tissues, including the skin, liver, kidney, spleen, lungs, and brain. As a xenobiotic material, silver must be presumed to present a health risk to exposed persons under some circumstances. Unlike the well-documented neurotoxic metals including lead and mercury, silver does not appear to be a cumulative poison and is eliminated from the body through the urine and feces. Excretion of silver by these routes may be a measure of mean daily intake, but since this view is based largely on the clinical use of silver nitrate and silver sulfadiazine used in burn wound therapy, its true relevance in the metabolism of silver used in the wider context of medical devices is questionable. Argyria is the most widely publicized clinical condition associated with silver accumulation in blood and soft tissues. It commonly occurs in individuals exposed to high levels of silver occupationally (metallurgy, photography, and mining industries), or consuming or inhaling silver hygiene products (including colloidal silver products) for long periods. Silver is absorbed into the body and deposited in the perivascular regions of the skin and other soft tissues as black granules of silver sulfide or silver selenide. The resulting slate grey discoloration of the skin occasionally associated with melanogenic changes, is semipermanent and cosmetically undesirable but is not known to be life-threatening. (PMID: 17453933).
D - Dermatologicals > D08 - Antiseptics and disinfectants > D08A - Antiseptics and disinfectants > D08AL - Silver compounds
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