Chromium is a chemical element which has the
symbol Cr and atomic number 24. It is the
first element in Group 6. It is a steely-gray,
lustrous, hard and brittle metal which takes
a high polish, resists tarnishing, and has
a high melting point. The name of the element
is derived from the Greek word χρῶμα,
chrōma, meaning colour, because many of its
compounds are intensely coloured.
Chromium oxide was used by the Chinese in
the Qin dynasty over 2,000 years ago to coat
metal weapons found with the Terracotta Army.
Chromium was discovered as an element after
it came to the attention of the western world
in the red crystalline mineral crocoite chromate),
discovered in 1761 and initially used as a
pigment. Louis Nicolas Vauquelin first isolated
chromium metal from this mineral in 1797.
Since Vauquelin's first production of metallic
chromium, small amounts of native chromium
metal have been discovered in rare minerals,
but these are not used commercially. Instead,
nearly all chromium is commercially extracted
from the single commercially viable ore chromite,
which is iron chromium oxide. Chromite is
also now the chief source of chromium for
chromium pigments.
Chromium metal and ferrochromium alloy are
commercially produced from chromite by silicothermic
or aluminothermic reactions, or by roasting
and leaching processes. Chromium metal has
proven of high value due to its high corrosion
resistance and hardness. A major development
was the discovery that steel could be made
highly resistant to corrosion and discoloration
by adding metallic chromium to form stainless
steel. This application, along with chrome
plating currently comprise 85% of the commercial
use for the element, with applications for
chromium compounds forming the remainder.
Trivalent chromium) ion is possibly required
in trace amounts for sugar and lipid metabolism,
although the issue remains in debate. In larger
amounts and in different forms, chromium can
be toxic and carcinogenic. The most prominent
example of toxic chromium is hexavalent chromium).
Abandoned chromium production sites often
require environmental cleanup.
Characteristics
Physical
Chromium is remarkable for its magnetic properties:
it is the only elemental solid which shows
antiferromagnetic ordering at room temperature.
Above 38 °C, it transforms into a paramagnetic
state.
Passivation
Chromium metal left standing in air is passivated
by oxygen, forming a thin protective oxide
surface layer. This layer is a spinel structure
only a few atoms thick. It is very dense,
and prevents the diffusion of oxygen into
the underlying material. This barrier is in
contrast to iron or plain carbon steels, where
the oxygen migrates into the underlying material
and causes rusting. The passivation can be
enhanced by short contact with oxidizing acids
like nitric acid. Passivated chromium is stable
against acids. The opposite effect can be
achieved by treatment with a strong reducing
agent that destroys the protective oxide layer
on the metal. Chromium metal treated in this
way readily dissolves in weak acids.
Chromium, unlike metals such as iron and nickel,
does not suffer from hydrogen embrittlement.
However, it does suffer from nitrogen embrittlement,
reacting with nitrogen from air and forming
brittle nitrides at the high temperatures
necessary to work the metal parts.
Occurrence
Chromium is the 22nd most abundant element
in Earth's crust with an average concentration
of 100 ppm. Chromium compounds are found in
the environment, due to erosion of chromium-containing
rocks and can be distributed by volcanic eruptions.
The concentrations range in soil is between
1 and 300 mg/kg, in sea water 5 to 800 µg/liter,
and in rivers and lakes 26 µg/liter to 5.2 mg/liter.
Chromium is mined as chromite ore. About two-fifths
of the chromite ores and concentrates in the
world are produced in South Africa, while
Kazakhstan, India, Russia, and Turkey are
also substantial producers. Untapped chromite
deposits are plentiful, but geographically
concentrated in Kazakhstan and southern Africa.
Although rare, deposits of native chromium
exist. The Udachnaya Pipe in Russia produces
samples of the native metal. This mine is
a kimberlite pipe, rich in diamonds, and the
reducing environment helped produce both elemental
chromium and diamond.
The relation between Cr(III) and Cr(VI) strongly
depends on pH and oxidative properties of
the location, but in most cases, the Cr(III)
is the dominating species, although in some
areas the ground water can contain up to 39 µg/liter
of total chromium of which 30 µg/liter is
present as Cr(VI).
Isotopes
Naturally occurring chromium is composed of
three stable isotopes; 52Cr, 53Cr and 54Cr
with 52Cr being the most abundant. 19 radioisotopes
have been characterized with the most stable
being 50Cr with a half-life of 1.8×1017 years,
and 51Cr with a half-life of 27.7 days. All
of the remaining radioactive isotopes have
half-lives that are less than 24 hours and
the majority of these have half-lives that
are less than 1 minute. This element also
has 2 meta states.
53Cr is the radiogenic decay product of 53Mn.
Chromium isotopic contents are typically combined
with manganese isotopic contents and have
found application in isotope geology. Mn-Cr
isotope ratios reinforce the evidence from
26Al and 107Pd for the early history of the
solar system. Variations in 53Cr/52Cr and
Mn/Cr ratios from several meteorites indicate
an initial 53Mn/55Mn ratio that suggests Mn-Cr
isotopic composition must result from in-situ
decay of 53Mn in differentiated planetary
bodies. Hence 53Cr provides additional evidence
for nucleosynthetic processes immediately
before coalescence of the solar system.
The isotopes of chromium range in atomic mass
from 43 u to 67 u. The primary decay mode
before the most abundant stable isotope, 52Cr,
is electron capture and the primary mode after
is beta decay. 53Cr has been posited as a
proxy for atmospheric oxygen concentration.
Compounds
Chromium is a member of the transition metals,
in group 6. Chromium(0) has an electronic
configuration of 4s13d5, owing to the lower
energy of the high spin configuration. Chromium
exhibits a wide range of possible oxidation
states, where the +3 state is most stable
energetically; the +3 and +6 states are most
commonly observed in chromium compounds, whereas
the +1, +4 and +5 states are rare.
The following is the Pourbaix diagram for
chromium in pure water, perchloric acid or
sodium hydroxide:
Chromium(III)
A large number of chromium(III) compounds
are known. Chromium(III) can be obtained by
dissolving elemental chromium in acids like
hydrochloric acid or sulfuric acid. The Cr3+
ion has a similar radius to the Al3+ ion,
so they can replace each other in some compounds,
such as in chrome alum and alum. When a trace
amount of Cr3+ replaces Al3+ in corundum,
the red-colored ruby is formed.
Chromium(III) ions tend to form octahedral
complexes. The colors of these complexes is
determined by the ligands attached to the
Cr centre. The commercially available chromium(III)
chloride hydrate is the dark green complex
[CrCl2(H2O)4]Cl. Closely related compounds
have different colours: pale green [CrCl(H2O)5]Cl2
and the violet [Cr(H2O)6]Cl3. If water-free
green chromium(III) chloride is dissolved
in water then the green solution turns violet
after some time, due to the substitution of
water by chloride in the inner coordination
sphere. This kind of reaction is also observed
with solutions of chrome alum and other water-soluble
chromium(III) salts.
Chromium(III) hydroxide3) is amphoteric, dissolving
in acidic solutions to form [Cr(H2O)6]3+,
and in basic solutions to form [Cr(OH)
6]3−. It is dehydrated by heating to form
the green chromium(III) oxide, which is the
stable oxide with a crystal structure identical
to that of corundum.
Chromium(VI)
Chromium(VI) compounds are powerful oxidants
at low or neutral pH. Most important are chromate
anion (CrO2−
4) and dichromate anions, which exist in equilibrium:
2 [CrO4]2- + 2 H+ [Cr2O7]2- + H2O
Chromium(VI) halides are known also and include
the hexafluoride CrF6 and chromyl chloride
(CrO
2Cl
2).
Sodium chromate is produced industrially by
the oxidative roasting of chromite ore with
calcium or sodium carbonate. The dominant
species is therefore, by the law of mass action,
determined by the pH of the solution. The
change in equilibrium is visible by a change
from yellow to orange, such as when an acid
is added to a neutral solution of potassium
chromate. At yet lower pH values, further
condensation to more complex oxyanions of
chromium is possible.
Both the chromate and dichromate anions are
strong oxidizing reagents at low pH:
Cr
2O2−
7 + 14 H
3O+ + 6 e− → 2 Cr3+ + 21 H
2O
They are, however, only moderately oxidizing
at high pH:
CrO2−
4 + 4 H
2O + 3 e− → Cr(OH)
3 + 5 OH−
Chromium(VI) compounds in solution can be
detected by adding an acidic hydrogen peroxide
solution. The unstable dark blue chromium(VI)
peroxide is formed, which can be stabilized
as an ether adduct CrO
5·OR
2.
Chromic acid has the hypothetical formula
H
2CrO
4. It is a vaguely described chemical, despite
many well-defined chromates and dichromates
being known. The dark red chromium(VI) oxide
CrO
3, the acid anhydride of chromic acid, is
sold industrially as "chromic acid". It can
be produced by mixing sulfuric acid with dichromate,
and is a strong oxidizing agent.
Chromium(V) and chromium(IV)
The oxidation state +5 is only realized in
few compounds but are intermediates in many
reactions involving oxidations by chromate.
The only binary compound is the volatile chromium(V)
fluoride. This red solid has a melting point
of 30 °C and a boiling point of 117 °C.
It can be synthesized by treating chromium
metal with fluorine at 400 °C and 200 bar
pressure. The peroxochromate(V) is another
example of the +5 oxidation state. Potassium
peroxochromate4]) is made by reacting potassium
chromate with hydrogen peroxide at low temperatures.
This red brown compound is stable at room
temperature but decomposes spontaneously at
150–170 °C.
Compounds of chromium(IV) are slightly more
common than those of chromium(V). The tetrahalides,
CrF4, CrCl4, and CrBr4, can be produced by
treating the trihalides (CrX
3) with the corresponding halogen at elevated
temperatures. Such compounds are susceptible
to disproportionation reactions and are not
stable in water.
Chromium(II)
Many chromium(II) compounds are known, including
the water-stable chromium(II) chloride, CrCl
2, which can be made by reduction of chromium(III)
chloride with zinc. The resulting bright blue
solution is only stable at neutral pH. Many
chromous carboxylates are also known, most
famously, the red chromous acetate4), which
features a quadruple bond.
Chromium(I)
Most Cr(I) compounds are obtained by oxidation
of electron-rich, octahedral Cr(0) complexes.
Other Cr(I) complexes contain cyclopentadienyl
ligands. As verified by X-ray diffraction,
a Cr-Cr quintuple bond  pm) has also been
described. Extremely bulky monodentate ligands
stabilize this compound by shielding the quintuple
bond from further reactions.
Chromium(0)
Many chromium(0) compounds are known. Most
are derivatives of chromium hexacarbonyl or
bis(benzene)chromium.
History
Weapons found in burial pits dating from the
late 3rd century B.C. Qin Dynasty of the Terracotta
Army near Xi'an, China have been analyzed
by archaeologists. Although buried more than
2,000 years ago, the ancient bronze tips of
crossbow bolts and swords found at the site
showed unexpectedly little corrosion, possibly
because the bronze was deliberately coated
with a thin layer of chromium oxide. However,
this oxide layer was not chromium metal or
chrome plating as we know it.
Chromium minerals as pigments came to the
attention of the west in the 18th century.
On 26 July 1761, Johann Gottlob Lehmann found
an orange-red mineral in the Beryozovskoye
mines in the Ural Mountains which he named
Siberian red lead. Though misidentified as
a lead compound with selenium and iron components,
the mineral was in fact crocoite with a formula
of PbCrO4.
In 1770, Peter Simon Pallas visited the same
site as Lehmann and found a red lead mineral
that had useful properties as a pigment in
paints. The use of Siberian red lead as a
paint pigment then developed rapidly. A bright
yellow pigment made from crocoite also became
fashionable.
In 1797, Louis Nicolas Vauquelin received
samples of crocoite ore. He produced chromium
trioxide by mixing crocoite with hydrochloric
acid. In 1798, Vauquelin discovered that he
could isolate metallic chromium by heating
the oxide in a charcoal oven, making him the
discoverer of the element. Vauquelin was also
able to detect traces of chromium in precious
gemstones, such as ruby or emerald.
During the 1800s, chromium was primarily used
as a component of paints and in tanning salts.
At first, crocoite from Russia was the main
source, but in 1827, a larger chromite deposit
was discovered near Baltimore, United States.
This made the United States the largest producer
of chromium products till 1848 when large
deposits of chromite were found near Bursa,
Turkey.
Chromium is also known for its luster when
polished. It is used as a protective and decorative
coating on car parts, plumbing fixtures, furniture
parts and many other items, usually applied
by electroplating. Chromium was used for electroplating
as early as 1848, but this use only became
widespread with the development of an improved
process in 1924.
Metal alloys now account for 85% of the use
of chromium. The remainder is used in the
chemical industry and refractory and foundry
industries.
Production
Approximately 23.3 million metric tons of
marketable chromite ore were produced in 2011,
and converted into 9.5 Mt of ferrochromium.
According to John F. Papp, writing for the
USGS, "Ferrochromium is the leading end use
of chromite ore, [and] stainless steel is
the leading end use of ferrochromium."
The largest producers of chromium ore have
been South Africa India, Kazakhstan Zimbabwe,
Finland Iran and Brazil with several other
countries producing the rest of less than
10% of the world production.
The two main products of chromium ore refining
are ferrochromium and metallic chromium. For
those products the ore smelter process differs
considerably. For the production of ferrochromium,
the chromite ore is reduced in large scale
in electric arc furnace or in smaller smelters
with either aluminium or silicon in an aluminothermic
reaction.
For the production of pure chromium, the iron
has to be separated from the chromium in a
two step roasting and leaching process. The
chromite ore is heated with a mixture of calcium
carbonate and sodium carbonate in the presence
of air. The chromium is oxidized to the hexavalent
form, while the iron forms the stable Fe2O3.
The subsequent leaching at higher elevated
temperatures dissolves the chromates and leaves
the insoluble iron oxide. The chromate is
converted by sulfuric acid into the dichromate.
4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4
+ 2 Fe2O3 + 8 CO2
2 Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 +
H2O
The dichromate is converted to the chromium(III)
oxide by reduction with carbon and then reduced
in an aluminothermic reaction to chromium.
Na2Cr2O7 + 2 C → Cr2O3 + Na2CO3 + CO
Cr2O3 + 2 Al → Al2O3 + 2 Cr
Applications
Metallurgy
The strengthening effect of forming stable
metal carbides at the grain boundaries and
the strong increase in corrosion resistance
made chromium an important alloying material
for steel. The high-speed tool steels contain
between 3 and 5% chromium. Stainless steel,
the main corrosion-proof metal alloy, is formed
when chromium is added to iron in sufficient
concentrations, usually above 11%. For its
formation, ferrochromium is added to the molten
iron. Also nickel-based alloys increase in
strength due to the formation of discrete,
stable metal carbide particles at the grain
boundaries. For example, Inconel 718 contains
18.6% chromium. Because of the excellent high-temperature
properties of these nickel superalloys, they
are used in jet engines and gas turbines in
lieu of common structural materials.
The relative high hardness and corrosion resistance
of unalloyed chromium makes it a good surface
coating, being still the most "popular" metal
coating with unparalleled combined durability.
A thin layer of chromium is deposited on pretreated
metallic surfaces by electroplating techniques.
There are two deposition methods: Thin, below
1 µm thickness, layers are deposited by
chrome plating, and are used for decorative
surfaces. If wear-resistant surfaces are needed
then thicker chromium layers are deposited.
Both methods normally use acidic chromate
or dichromate solutions. To prevent the energy-consuming
change in oxidation state, the use of chromium(III)
sulfate is under development, but for most
applications, the established process is used.
In the chromate conversion coating process,
the strong oxidative properties of chromates
are used to deposit a protective oxide layer
on metals like aluminium, zinc and cadmium.
This passivation and the self-healing properties
by the chromate stored in the chromate conversion
coating, which is able to migrate to local
defects, are the benefits of this coating
method. Because of environmental and health
regulations on chromates, alternative coating
method are under development.
Anodizing of aluminium is another electrochemical
process, which does not lead to the deposition
of chromium, but uses chromic acid as electrolyte
in the solution. During anodization, an oxide
layer is formed on the aluminium. The use
of chromic acid, instead of the normally used
sulfuric acid, leads to a slight difference
of these oxide layers. The high toxicity of
Cr(VI) compounds, used in the established
chromium electroplating process, and the strengthening
of safety and environmental regulations demand
a search for substitutes for chromium or at
least a change to less toxic chromium(III)
compounds.
Dye and pigment
The mineral crocoite was used as a yellow
pigment shortly after its discovery. After
a synthesis method became available starting
from the more abundant chromite, chrome yellow
was, together with cadmium yellow, one of
the most used yellow pigments. The pigment
does not photodegrade, but it tends to darken
due to the formation of chromium(III) oxide.
It has a strong color, and was used for school
buses in the US and for Postal Service in
Europe. The use of chrome yellow declined
due to environmental and safety concerns and
was replaced by organic pigments or alternatives
free from lead and chromium. Other pigments
based on chromium are, for example, the bright
red pigment chrome red, which is a basic lead
chromate2). A very important chromate pigment,
which was used widely in metal primer formulations,
was zinc chromate, now replaced by zinc phosphate.
A wash primer was formulated to replace the
dangerous practice of pretreating aluminium
aircraft bodies with a phosphoric acid solution.
This used zinc tetroxychromate dispersed in
a solution of polyvinyl butyral. An 8% solution
of phosphoric acid in solvent was added just
before application. It was found that an easily
oxidized alcohol was an essential ingredient.
A thin layer of about 10–15 µm was applied,
which turned from yellow to dark green when
it was cured. There is still a question as
to the correct mechanism. Chrome green is
a mixture of Prussian blue and chrome yellow,
while the chrome oxide green is chromium(III)
oxide.
Chromium oxides are also used as a green color
in glassmaking and as a glaze in ceramics.
Green chromium oxide is extremely light-fast
and as such is used in cladding coatings.
It is also the main ingredient in IR reflecting
paints, used by the armed forces, to paint
vehicles, to give them the same IR reflectance
as green leaves.
Synthetic ruby and the first laser
Natural rubies are corundum crystals that
are colored red due to chromium ions. A red-colored
artificial ruby may also be achieved by doping
chromium(III) into artificial corundum crystals,
thus making chromium a requirement for making
synthetic rubies. Such a synthetic ruby crystal
was the basis for the first laser, produced
in 1960, which relied on stimulated emission
of light from the chromium atoms in such a
crystal.
Wood preservative
Because of their toxicity, chromium(VI) salts
are used for the preservation of wood. For
example, chromated copper arsenate is used
in timber treatment to protect wood from decay
fungi, wood attacking insects, including termites,
and marine borers. The formulations contain
chromium based on the oxide CrO3 between 35.3%
and 65.5%. In the United States, 65,300 metric
tons of CCA solution have been used in 1996.
Tanning
Chromium(III) salts, especially chrome alum
and chromium(III) sulfate, are used in the
tanning of leather. The chromium(III) stabilizes
the leather by cross linking the collagen
fibers. Chromium tanned leather can contain
between 4 and 5% of chromium, which is tightly
bound to the proteins. Although the form of
chromium used for tanning is not the toxic
hexavalent variety, there remains interest
in management of chromium in the tanning industry
such as recovery and reuse, direct/indirect
recycling, use of less chromium or "chrome-less"
tanning are practiced to better manage chromium
in tanning.
Refractory material
The high heat resistivity and high melting
point makes chromite and chromium(III) oxide
a material for high temperature refractory
applications, like blast furnaces, cement
kilns, molds for the firing of bricks and
as foundry sands for the casting of metals.
In these applications, the refractory materials
are made from mixtures of chromite and magnesite.
The use is declining because of the environmental
regulations due to the possibility of the
formation of chromium(VI).
Catalysts
Several chromium compounds are used as catalysts
for processing hydrocarbons. For example the
Phillips catalysts for the production of polyethylene
are mixtures of chromium and silicon dioxide
or mixtures of chromium and titanium and aluminium
oxide. Fe-Cr mixed oxides are employed as
high-temperature catalysts for the water gas
shift reaction. Copper chromite is a useful
hydrogenation catalyst.
Other use
Chromium(IV) oxide is a magnetic compound.
Its ideal shape anisotropy, which imparts
high coercivity and remnant magnetization,
made it a compound superior to the γ-Fe2O3.
Chromium(IV) oxide is used to manufacture
magnetic tape used in high-performance audio
tape and standard audio cassettes. Chromates
can prevent corrosion of steel under wet conditions,
and therefore chromates are added to drilling
muds.
Chromium(III) oxide is a metal polish known
as green rouge.
Chromic acid is a powerful oxidizing agent
and is a useful compound for cleaning laboratory
glassware of any trace of organic compounds.
It is prepared in situ by dissolving potassium
dichromate in concentrated sulfuric acid,
which is then used to wash the apparatus.
Sodium dichromate is sometimes used because
of its higher solubility. The use of dichromate
cleaning solutions is now phased out due to
the high toxicity and environmental concerns.
Modern cleaning solutions are highly effective
and chromium free. Potassium dichromate is
a chemical reagent,used as a titrating agent.
It is also used as a mordant for dyes in fabric.
Biological role
Recently, a paradigm shift has occurred in
terms of the status of trivalent chromium
or Cr3+). While first proposed to be an essential
element in the late 1950s and accepted as
a trace element in the 1980s, scientific studies
have continued to fail to produce convincing
evidence of this status. Trivalent chromium
occurs in trace amounts in foods and waters,
and appears to be benign. In contrast, hexavalent
chromium or Cr6+) is very toxic and mutagenic
when inhaled. Cr(VI) has not been established
as a carcinogen when in solution, although
it may cause allergic contact dermatitis.
Chromium deficiency, involving a lack of Cr(III)
in the body, or perhaps some complex of it,
such as glucose tolerance factor is controversial,
or is at least extremely rare. Chromium has
no verified biological role and has been classified
by some as not essential for mammals. However,
other reviews have regarded it as an essential
trace element in humans.
Chromium deficiency has been attributed to
only three people on long-term parenteral
nutrition, which is when a patient is fed
a liquid diet through intravenous drips for
long periods of time.
Although no biological role for chromium has
ever been demonstrated, dietary supplements
for chromium include chromium(III) picolinate,
chromium(III) polynicotinate, and related
materials. The benefit of those supplements
is questioned by some studies. The use of
chromium-containing dietary supplements is
controversial, owing to the absence of any
verified biological role, the expense of these
supplements, and the complex effects of their
use. The popular dietary supplement chromium
picolinate complex generates chromosome damage
in hamster cells. In the United States the
dietary guidelines for daily chromium uptake
were lowered in 2001 from 50–200 µg for
an adult to 35 µg and to 25 µg.
No comprehensive, reliable database of chromium
content of food currently exists. Data reported
prior to 1980 is unreliable due to analytical
error. Chromium content of food varies widely
due to differences in soil mineral content,
growing season, plant cultivar, and contamination
during processing. In addition, large amounts
of chromium leech into food cooked in stainless
steel.
Precautions
Water insoluble chromium(III) compounds and
chromium metal are not considered a health
hazard, while the toxicity and carcinogenic
properties of chromium(VI) have been known
for a long time. Because of the specific transport
mechanisms, only limited amounts of chromium(III)
enter the cells. Several in vitro studies
indicated that high concentrations of chromium(III)
in the cell can lead to DNA damage. Acute
oral toxicity ranges between 1.5 and 3.3 mg/kg.
The proposed beneficial effects of chromium(III)
and the use as dietary supplements yielded
some controversial results, but recent reviews
suggest that moderate uptake of chromium(III)
through dietary supplements poses no risk.
Cr(VI)
The acute oral toxicity for chromium(VI) ranges
between 50 and 150 µg/kg. In the body, chromium(VI)
is reduced by several mechanisms to chromium(III)
already in the blood before it enters the
cells. The chromium(III) is excreted from
the body, whereas the chromate ion is transferred
into the cell by a transport mechanism, by
which also sulfate and phosphate ions enter
the cell. The acute toxicity of chromium(VI)
is due to its strong oxidational properties.
After it reaches the blood stream, it damages
the kidneys, the liver and blood cells through
oxidation reactions. Hemolysis, renal and
liver failure are the results of these damages.
Aggressive dialysis can improve the situation.
The carcinogenity of chromate dust is known
for a long time, and in 1890 the first publication
described the elevated cancer risk of workers
in a chromate dye company. Three mechanisms
have been proposed to describe the genotoxicity
of chromium(VI). The first mechanism includes
highly reactive hydroxyl radicals and other
reactive radicals which are by products of
the reduction of chromium(VI) to chromium(III).
The second process includes the direct binding
of chromium(V), produced by reduction in the
cell, and chromium(IV) compounds to the DNA.
The last mechanism attributed the genotoxicity
to the binding to the DNA of the end product
of the chromium(III) reduction.
Chromium salts are also the cause of allergic
reactions in some people. Chromates are often
used to manufacture, amongst other things,
leather products, paints, cement, mortar and
anti-corrosives. Contact with products containing
chromates can lead to allergic contact dermatitis
and irritant dermatitis, resulting in ulceration
of the skin, sometimes referred to as "chrome
ulcers". This condition is often found in
workers that have been exposed to strong chromate
solutions in electroplating, tanning and chrome-producing
manufacturers.
Environmental issues
As chromium compounds were used in dyes and
paints and the tanning of leather, these compounds
are often found in soil and groundwater at
abandoned industrial sites, now needing environmental
cleanup and remediation per the treatment
of brownfield land. Primer paint containing
hexavalent chromium is still widely used for
aerospace and automobile refinishing applications.
In 2010, the Environmental Working Group studied
the drinking water in 35 American cities.
The study was the first nationwide analysis
measuring the presence of the chemical in
U.S. water systems. The study found measurable
hexavalent chromium in the tap water of 31
of the cities sampled, with Norman, Oklahoma,
at the top of list; 25 cities had levels that
exceeded California's proposed limit. Note:
Concentrations of Cr(VI) in US municipal drinking
water supplies reported by EWG are within
likely, natural background levels for the
areas tested and not necessarily indicative
of industrial pollution, as asserted by EWG.
This factor was not taken into consideration
in their report.
Notes
^ Most common oxidation states of chromium
are in bold. The right column lists a representative
compound for each oxidation state.
References
External links
ATSDR Case Studies in Environmental Medicine:
Chromium Toxicity U.S. Department of Health
and Human Services
IARC Monograph "Chromium and Chromium compounds"
It's Elemental – The Element Chromium
The Merck Manual – Mineral Deficiency and
Toxicity
National Institute for Occupational Safety
and Health – Chromium Page
Chromium at The Periodic Table of Videos
