Phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi) form a group offour elements in Group 5A of the periodic table. They exhibit increasingmetallic properties going down the group. Nitrogen (N), which heads the group,is a colorless, odorless, and tasteless gas. Phosphorus is a highly reactivenonmetal, arsenic and antimony are poisonous metalloids, and bismuth is a truemetal.
Because of the arrangement of the outer electrons in their atoms, each ofthese elements can form up to five chemical bonds with other elements or groupsof elements. Arsenic has an atomic number 33, atomic mass is 74. 9216, and itsublimes (passes directly into a vapor without melting) at 613? C. History TheEarth’s crust contains relatively little arsenic, only about 5. 5 parts permillion. Arsenic and some arsenic compounds have been known for a long time.
Aristotle thought that arsenic was a kind of sulfur. The Latin word arsenicummeans yellow orpiment (a pigment containing arsenic and sulfur). While knowledgeof arsenic dates back to ancient Greece, it wasn’t until the Middle Ages thatits poisonous characteristics were described. It was identified by Albert Magnusabout 1250, and he described the way to manufacture it. Since then the methodhas scarcely changed: the mineral arsenopyrite is heated and decomposes with theliberation of arsenic gas.
The gas can be condensed on a cold surface. MetallicArsenic was first produced in the 17th century by heating arsenic with potashand soap. General Properties Arsenic is very similar to antimony and bismuth. Itexists in bright, metallic forms that are stable in air. It is found free innature or in combination with other elements, usually sulfur. It is most oftenused to improve the strength and hardness of alloys, which are combinations ofmetals.
Arsenic is a gray, shiny metalloid, which is a moderately good conductorof heat and electricity, but gray arsenic is brittle and breaks easily. This isthe ordinary, stable form of the element. There are two other allotropes (solidforms)–yellow arsenic and black arsenic, whose modifications have no metallicproperties. Occasionally found free in nature, arsenic usually occurs incombination with sulfur, oxygen or certain metals like cobalt, copper, nickel,iron, silver, and tin.
In combination, such arsenic is referred to as inorganicarsenic. Arsenic combined with carbon and hydrogen is referred to as organicarsenic. The organic forms are usually less toxic than inorganic forms. Theprincipal arsenic-containing mineral is arsenopyrite. The most widely usedarsenic compound is white arsenic, also called arsenic trioxide. It is usuallyproduces as a by-product of the smelting (melting)of copper or lead.
At about400? 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 beingquite soluble (sodium arsenite and arsenic acid) to practically insoluble(arsenic trisulfide). Twenty-one arsenic compounds are considered to be ofconcern because of their toxicity and/or presence in the environment. CommercialUses Compounds of arsenic have been used since ancient times for many purposes,including medicines and poisons.
In Aristotle’s time it was used to hardencopper. Orpiment and realgar have long been used as depilatories in the leatherindustry. When orpiment is rubbed on silver, it gives the surface a goldencolor. Orpiment thus appears to have one of the properties attributed to thephilosophers’ stone, and it was therefore an important material for alchemists. Nowadays, it is used in the manufacture of fungicides, weed killers, ratpoisons, herbicides, pesticides and insecticides.
It is also used to manufacturelead gun shot, to harden the lead, and used in certain types of electricalequipment and to increase the strength of certain alloys. Arsenic is alsoblended with gallium to produce semiconductors. Effect On Humans Arsenic is adeadly poison and its toxic quality has also been known since ancient times. Inthe human body it accumulates in the hair and the nails, where it can bedetected-even in the bodies of people long dead-by the Marsh test.
The Marshtest was devised as a forensic test, where gas arsine is heated to form ametallic mirror of arsenic. Arsenic poisoning may be either acute or chronic. Acute poisoning occurs when a person ingests a large quantity of arsenic at onetime. This condition is characterized by vomiting, diarrhea and cramps, and maylead to shock, coma and even death. Chronic poisoning occurs over a longerperiod of time.
In cases of chronic poisoning, aneamia and paralysis may appear. Other symptoms include skin lesions that are noncancerous and tingling, andnumbness of the soles and palms that develops into a painful condition calledneuritis. With neuritis, reflexes in the extremities may be impaired and evenlost. Upon identification and treatment of the condition, the patient generallyrecovers within months, although recovery is not always complete. Prolongedlow-dose exposure to arsenic can also cause cancer, usually skin and lungcancer. Breathing arsenic can irritate the nose and throat; eye contact cancause red watery eyes and irritation.
Long-term exposure can cause an ulcer orhole in the ‘bone’ dividing the inner nose, hoarseness, and sore eyes. BAL(British Anti-Lewisite) was developed as an antidote against arsenic-containingwar gas Lewisite, but it also proved useful in treating common arsenicpoisoning. In medicine, 4-aminobenzene arsenic and 4-hydroxybenzene arseniccompounds are used in certain infections. An arsenical is one of a group ofdrugs that contain arsenic and have been used as a medicine.
The best known isSalvarsan, an antisyphilis drug. Carbarsone is an arsenical used in treatingamebic dysentery. Arsenical now are being replaced with other drugs. SupplyWorlds production of arsenic trioxide in 1998 were estimated at 42,000 tonnes,with China contributing 33%, Belgium 14%, followed by Ghana, 12%, France 7% andMexico with 7%, at an estimated price of $0. 40/lb.
World resources of copper andlead were estimated to contain about 11 million tonnes of arsenic. Substantialarsenic resources occur in copper ores in Peru and Philippinesand in copper-goldores in Chile. Canada also has substantial arsenic resources, according to theU. S. Geological Survey. The United States imports all of its arsenic andcompounds with more than 95% coming into the country as arsenic trioxide.
GroundWater Problem In many places, arsenic is causing a serious problem, that is veryhard to control, that is contaminating ground water. Throughout the world,arsenic in ground water often comes from natural sources such as bedrock. Insome areas, levels of arsenic are increasing in ground water because of seepagefrom hazardous waste sites, and arsenic pesticide runoff also produces elevatedarsenic levels in ground water. So, populations relying on ground water orsurface water near geological or man-made sources of arsenic may receive higherthan typical exposure. These areas include industrialized areas and areas wherelarge quantities of arsenic are disposed of in the landfills, areas of highhistorical pesticide use, with soil low in available ferrous and aluminumhydroxides, and areas of high natural levels of arsenic containing mineraldeposits.
Population in the area of copper and other types of metal smelters maybe exposed to above-average levels of arsenic both through the air and as aresult of the atmospheric deposition in the soil and water. Individuals withprotein-poor diets or chlorine (of the Vitamin B complex) deficiency may be moresensitive to arsenic than the general population. Milestones Due to this, and toits being carcinogenic, but also because of the toxicity of arsenic and itscompounds in general, environmental regulation is expected to becomeincreasingly stringent. While this might adversely affect arsenic demand in thelong term, it should only have a minor near-time effect. BibliographyChemistry Today: The World Book Encyclopedia Of Science.
Chicago: World BookInc. , 1992. Lexicon Universal Encyclopedia. New York: Lexicon Publications Inc.
,1985. The World Book Encyclopedia
