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.
Bibliography
Chemistry Today: The World Book Encyclopedia Of Science. Chicago: World Book
Inc., 1992. Lexicon Universal Encyclopedia. New York: Lexicon Publications Inc.,
1985. The World Book Encyclopedia
