Chemicals Analysis
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