Arsenic
ArsenicPlace In Periodic Table
Phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi) form a group of four elements in Group 5A of the periodic table. They exhibit increasing metallic properties going down the group. Nitrogen (N), which heads the group, is a colorless, odorless, and tasteless gas. Phosphorus is a highly reactive nonmetal, arsenic and antimony are poisonous metalloids, and bismuth is a true metal. Because of the arrangement of the outer electrons in their atoms, each of these elements can form up to five chemical bonds with other elements or groups of elements. Arsenic has an atomic number 33, atomic mass is 74.9216, and it sublimes (passes directly into a vapor without melting) at 613� C.
History
The Earth's crust contains relatively little arsenic, only about 5.5 parts per million. Arsenic and some arsenic compounds have been known for a long time. Aristotle thought that arsenic was a kind of sulfur. The Latin word arsenicum means yellow orpiment (a pigment containing arsenic and sulfur). While knowledge of arsenic dates back to ancient Greece, it wasn't until the Middle Ages that its poisonous characteristics were described. It was identified by Albert Magnus about 1250, and he described the way to manufacture it. Since then the method has scarcely changed: the mineral arsenopyrite is heated and decomposes with the liberation of arsenic gas. The gas can be condensed on a cold surface. Metallic Arsenic was first produced in the 17th century by heating arsenic with potash and soap.
General Properties
Arsenic is very similar to antimony and bismuth. It exists in bright, metallic forms that are stable in air. It is found free in nature or in combination with other elements, usually sulfur. It is most often used to improve the strength and hardness of alloys, which are combinations of metals.Arsenic is a gray, shiny metalloid, which is a moderately good conductor of heat and electricity, but gray arsenic is brittle and breaks easily. This is the ordinary, stable form of the element. There are two other allotropes (solid forms)--yellow arsenic and black arsenic, whose modifications have no metallic properties. Occasionally found free in nature, arsenic usually occurs in combination with sulfur, oxygen or certain metals like cobalt, copper, nickel, iron, silver, and tin. In combination, such arsenic is referred to as inorganic arsenic. Arsenic combined with carbon and hydrogen is referred to as organic arsenic. The organic forms are usually less toxic than inorganic forms. The principal arsenic-containing mineral is arsenopyrite. The most widely used arsenic compound is white arsenic, also called arsenic trioxide. It is usually produces as a by-product of the smelting (melting)of copper or lead. At about 400� C it burns with a bluish flame, forming the As2O3 (arsenic trioxide), which is used as a rat poison. In water, arsenic combinations range from being quite soluble (sodium arsenite and arsenic acid) to practically insoluble (arsenic trisulfide). Twenty-one arsenic compounds are considered to be of concern because of their toxicity and/or presence in the environment.
Commercial Uses
Compounds of arsenic have been used since ancient times for many purposes, including medicines and poisons. In Aristotle's time it was used to harden copper. Orpiment and realgar have long been used as depilatories in the leather industry. When orpiment is rubbed on silver, it gives the surface a golden color. Orpiment thus appears to have one of the properties attributed to the philosophers' stone, and it was therefore an important material for alchemists. Nowadays, it is used in the manufacture of fungicides, weed killers, rat poisons, herbicides, pesticides and insecticides. It is also used to manufacture lead gun shot, to harden the lead, and used in certain types of electrical equipment and to increase the strength of certain alloys. Arsenic is also blended with gallium to produce semiconductors.
Effect On Humans
Arsenic is a deadly poison and its toxic quality has also been known since ancient times. In the human body it accumulates in the hair and the nails, where it can be detected-even in the bodies of people long dead-by the Marsh test. The Marsh test was devised as a forensic test, where gas arsine is heated to form a metallic mirror of arsenic. Arsenic poisoning may be either acute or chronic. Acute poisoning occurs when a person ingests a large quantity of arsenic at one time. This condition is characterized by vomiting, diarrhea and cramps, and may lead to shock, coma and even death. Chronic poisoning occurs over a longer period of time. In cases of chronic poisoning, aneamia and paralysis may appear. Other symptoms include skin lesions that are noncancerous and tingling, and numbness of the soles and palms that develops into a painful condition called neuritis. With neuritis, reflexes in the extremities may be impaired and even lost. Upon identification and treatment of the condition, the patient generally recovers within months, although recovery is not always complete. Prolonged low-dose exposure to arsenic can also cause cancer, usually skin and lung cancer. Breathing arsenic can irritate the nose and throat; eye contact can cause red watery eyes and irritation. Long-term exposure can cause an ulcer or hole in the 'bone' dividing the inner nose, hoarseness, and sore eyes. BAL (British Anti-Lewisite) was developed as an antidote against arsenic-containing war gas Lewisite, but it also proved useful in treating common arsenic poisoning. In medicine, 4-aminobenzene arsenic and 4-hydroxybenzene arsenic compounds are used in certain infections. An arsenical is one of a group of drugs that contain arsenic and have been used as a medicine. The best known is Salvarsan, an antisyphilis drug. Carbarsone is an arsenical used in treating amebic dysentery. Arsenical now are being replaced with other drugs.
Supply
Worlds production of arsenic trioxide in 1998 were estimated at 42,000 tonnes, with China contributing 33%, Belgium 14%, followed by Ghana, 12%, France 7% and Mexico with 7%, at an estimated price of $0.40/lb. World resources of copper and lead were estimated to contain about 11 million tonnes of arsenic. Substantial arsenic resources occur in copper ores in Peru and Philippinesand in copper-gold ores in Chile. Canada also has substantial arsenic resources, according to the U.S. Geological Survey. The United States imports all of its arsenic and compounds with more than 95% coming into the country as arsenic trioxide.
Ground Water Problem
In many places, arsenic is causing a serious problem, that is very hard to control, that is contaminating ground water. Throughout the world, arsenic in ground water often comes from natural sources such as bedrock. In some areas, levels of arsenic are increasing in ground water because of seepage from hazardous waste sites, and arsenic pesticide runoff also produces elevated arsenic levels in ground water. So, populations relying on ground water or surface water near geological or man-made sources of arsenic may receive higher than typical exposure. These areas include industrialized areas and areas where large quantities of arsenic are disposed of in the landfills, areas of high historical pesticide use, with soil low in available ferrous and aluminum hydroxides, and areas of high natural levels of arsenic containing mineral deposits. Population in the area of copper and other types of metal smelters may be exposed to above-average levels of arsenic both through the air and as a result of the atmospheric deposition in the soil and water. Individuals with protein-poor diets or chlorine (of the Vitamin B complex) deficiency may be more sensitive to arsenic than the general population.
Milestones
Due to this, and to its being carcinogenic, but also because of the toxicity of arsenic and its compounds in general, environmental regulation is expected to become increasingly stringent. While this might adversely affect arsenic demand in the long term, it should only have a minor near-time effect.