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Title: Chemistry/Elements/Chromium - Lenntech: Chromium Physical data, chemical properties, health and environmental effects.
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Chromium (Cr) - Chemical properties, Health and Environmental effects
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Chemical properties of chromium
- Health
effects of chromium -
Environmental effects of chromium
Atomic number
24
Atomic mass
51.996 g.mol -1
Electronegativity
1.6
Density
7.19 g.cm-3 at 20°C
Melting point
1907 °C
Boiling point
2672 °C
Vanderwaals
radius
0.127 nm
Ionic radius
0.061 nm (+3) ; 0.044 nm (+6)
Isotopes
6
Electronic
shell
[ Ar ] 3d4 4s2
Energy of
first ionisation
651.1 kJ.mol -1
Energy of
second ionisation
1590.1 kJ.mol -1
Energy of
first ionisation
2987 kJ.mol -1
Standard
potential
- 0.71 V (Cr3+ / Cr )
Discovered by
Vaughlin in 1797
Chromium
Chromium
is a lustrous, brittle, hard metal. Its colour is silver-gray and
it can be highly
polished. It does not tarnish in air, when heated it borns and forms the green chromic oxide. Chromium is unstable in oxygen,
it immediately produces a thin oxide layer that is impermeable to oxygen and protects
the metal below.
ApplicationsChromium main uses are in alloys such
as stainless steel, in chrome plating and in metal ceramics.
Chromium plating was once widely used to give steel a
polished silvery mirror coating. Chromium is used in
metallurgy to impart corrosion resistance and a shiny
finish; as dyes and paints, its salts colour glass an
emerald green and it is used to produce synthetic rubies; as
a catalyst in dyeing and in the tanning of leather; to make
molds for the firing of bricks. Chromium (IV) oxide (CrO2)
is used to manufacture magnetic tape.
Chromium in the environment
Chromium is mined as chromite (FeCr2O4)
ore. Chromium ores are mined today in South Africa,
Zimbabwe, Finland, India, Kazakihstan and the Philippines. A
total of 14 million tonnes of chromite ore is extracted.
Reserves are hestimated to be of the order of 1 billion
tonnes with unexploited deposits in Greenland, Canada e USA.
Health
effects of chromium
People can be exposed to chromium through
breathing, eating or drinking and through skin contact with
chromium or chromium compounds. The level of chromium in air and
water is generally low. In drinking water the level of chromium is
usually low as well, but contaminated well water may contain the
dangerous chromium(IV); hexavalent chromium. For most people
eating food that contains chromium(III) is the main route of
chromium uptake, as chromium(III) occurs naturally in many
vegetables, fruits, meats, yeasts and grains. Various ways of food
preparation and storage may alter the chromium contents of food.
When food in stores in steel tanks or cans chromium concentrations
may rise.
Chromium(III) is an essential nutrient for humans and shortages
may cause heart conditions, disruptions of metabolisms and
diabetes. But the uptake of too much chromium(III) can cause
health effects as well, for instance skin rashes.
Chromium(VI) is a danger to human health, mainly for people who
work in the steel and textile industry. People who smoke tobacco
also have a higher chance of exposure to chromium.
Chromium(VI) is known to cause various health effects. When it is
a compound in leather products, it can cause allergic reactions,
such as skin rash. After breathing it in chromium(VI) can cause
nose irritations and nosebleeds.
Other health problems that are caused by chromium(VI) are:
- Skin rashes
- Upset stomachs and ulcers
- Respiratory problems
- Weakened immune systems
- Kidney and liver damage
- Alteration of genetic material
- Lung cancer
- Death
The health
hazards associated with exposure to chromium are dependent on its oxidation state. The metal form (chromium
as it exists in this product)
is of low toxicity. The hexavalent form is toxic. Adverse effects
of the
hexavalent form on the skin may include ulcerations, dermatitis,
and allergic skin reactions. Inhalation of
hexavalent chromium compounds can result in ulceration and
perforation of the mucous membranes of the nasal septum,
irritation of the pharynx and larynx, asthmatic bronchitis,
bronchospasms and edema. Respiratory symptoms
may include coughing and wheezing,
shortness of breath, and nasal itch.
Carcinogenicity- Chromium and most
trivalent chromium compounds have been
listed by the National Toxicology Program (NTP) as having inadequate evidence for carcinogenicity in
experimental animals. According to NTP,
there is sufficient evidence for carcinogenicity in experimental animals for the following
hexavalent chromium compounds;
calcium
chromate, chromium trioxide, lead chromate,
strontium chromate,and
zinc chromate. International Agency for Research on Cancer (IARC)
has listed chromium metal and its trivalent compounds within Group
3 (The agent is not classifiable as to its
carcinogenicity to humans.) Chromium is not
regulated as a carcinogen by OSHA (29 CFR 1910 Subpart Z). ACGIH
has classified chromium metal and trivalent
chromium compounds as A4,not classifiable as a human carcinogen.
Environmental effects of chromium
There are several different kinds of chromium
that differ in their effects upon organisms. Chromium enters the
air, water and soil in the chromium(III) and chromium(VI) form
through natural processes and human activities.
The main human activities that increase the concentrations of
chromium (III) are steal, leather and textile manufacturing. The
main human activities that increase chromium(VI) concentrations
are chemical, leather and textile manufacturing, electro painting
and other chromium(VI) applications in the industry. These
applications will mainly increase concentrations of chromium in
water. Through coal combustion chromium will also end up in air
and through waste disposal chromium will end up in soils.
Most of the chromium in air will eventually settle and end up in
waters or soils. Chromium in soils strongly attaches to soil
particles and as a result it will not move towards groundwater. In
water chromium will absorb on sediment and become immobile. Only a
small part of the chromium that ends up in water will eventually
dissolve.
Chromium(III) is an essential element for organisms that can
disrupt the sugar metabolism and cause heart conditions, when the
daily dose is too low. Chromium(VI) is mainly toxic to organisms.
It can alter genetic materials and cause cancer.
Crops contain systems that arrange the chromium-uptake to be low
enough not to cause any harm. But when the amount of chromium in
the soil rises, this can still lead to higher concentrations in
crops. Acidification of soil can also influence chromium uptake by
crops. Plants usually absorb only chromium(III). This may be the
essential kind of chromium, but when concentrations exceed a
certain value, negative effects can still occur.
Chromium is not known to accumulate in the bodies of fish, but
high concentrations of chromium, due to the disposal of metal
products in surface waters, can damage the gills of fish that swim
near the point of disposal.
In animals chromium can cause respiratory problems, a lower
ability to fight disease, birth defects, infertility and tumor
formation.
Read more on
chromium in water
Back to the
periodic table of elements.
Recommended daily intake of
chromium
[ Home
] [ Back ] [ More
Information or question on Chromium? ]
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