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= Chromium =
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Introduction
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Chromium is a chemical element; it has symbol Cr and atomic number 24.
It is the first element in group 6. It is a steely-grey, lustrous,
hard, and brittle transition metal.
Chromium is valued for its high corrosion resistance and hardness. A
major development in steel production was the discovery that steel
could be made highly resistant to corrosion and discoloration by
adding metallic chromium to form stainless steel. Stainless steel and
chrome plating (electroplating with chromium) together comprise 85% of
the commercial use. Chromium is also greatly valued as a metal that is
able to be highly polished while resisting tarnishing. Polished
chromium reflects almost 70% of the visible spectrum, and almost 90%
of infrared light. The name of the element is derived from the Greek
word χρῶμα, 'chrōma', meaning color, because many chromium compounds
are intensely colored.
Industrial production of chromium proceeds from chromite ore (mostly
FeCr2O4) to produce ferrochromium, an iron-chromium alloy, by means of
aluminothermic or silicothermic reactions. Ferrochromium is then used
to produce alloys such as stainless steel. Pure chromium metal is
produced by a different process: roasting and leaching of chromite to
separate it from iron, followed by reduction with carbon and then
aluminium.
Trivalent chromium (Cr(III)) occurs naturally in many foods and is
sold as a dietary supplement, although there is insufficient evidence
that dietary chromium provides nutritional benefit to people. In 2014,
the European Food Safety Authority concluded that research on dietary
chromium did not justify it to be recognized as an essential nutrient.
While chromium metal and Cr(III) ions are considered non-toxic,
chromate and its derivatives, often called "hexavalent chromium", is
toxic and carcinogenic. According to the European Chemicals Agency
(ECHA), chromium trioxide that is used in industrial electroplating
processes is a "substance of very high concern" (SVHC).
Atomic
========
Gaseous chromium has a ground-state electron configuration of [Ar] 3d5
4s1. It is the first element in the periodic table whose configuration
violates the Aufbau principle. Exceptions to the principle also occur
later in the periodic table for elements such as copper, niobium and
molybdenum.
Chromium is the first element in the 3d series where the 3d electrons
start to sink into the core; they thus contribute less to metallic
bonding, and hence the melting and boiling points and the enthalpy of
atomisation of chromium are lower than those of the preceding element
vanadium. Chromium(VI) is a strong oxidising agent in contrast to the
molybdenum(VI) and tungsten(VI) oxides.
Bulk
======
Chromium is the third hardest element after carbon (diamond) and
boron. Its Mohs hardness is 8.5, which means that it can scratch
samples of quartz and topaz, but can be scratched by corundum.
Chromium is highly resistant to tarnishing, which makes it useful as a
metal that preserves its outermost layer from corroding, unlike other
metals such as copper, magnesium, and aluminium.
Chromium has a melting point of 1907 °C (3465 °F), which is relatively
low compared to the majority of transition metals. However, it still
has the second highest melting point out of all the period 4 elements,
being topped by vanadium by 3 °C (5 °F) at 1910 °C (3470 °F). The
boiling point of 2671 °C (4840 °F), however, is comparatively lower,
having the fourth lowest boiling point out of the Period 4 transition
metals alone behind copper, manganese and zinc. The electrical
resistivity of chromium at 20 °C is 125 nanoohm-meters.
Chromium has a high specular reflection in comparison to other
transition metals. In infrared, at 425 μm, chromium has a maximum
reflectance of about 72%, reducing to a minimum of 62% at 750 μm
before rising again to 90% at 4000 μm. When chromium is used in
stainless steel alloys and polished, the specular reflection decreases
with the inclusion of additional metals, yet is still high in
comparison with other alloys. Between 40% and 60% of the visible
spectrum is reflected from polished stainless steel. The explanation
on why chromium displays such a high turnout of reflected photon waves
in general, especially the 90% in infrared, can be attributed to
chromium's magnetic properties. Chromium has unique magnetic
properties; it is the only elemental solid that shows
antiferromagnetic ordering at room temperature and below. Above 38 °C,
its magnetic ordering becomes paramagnetic. The antiferromagnetic
properties, which cause the chromium atoms to temporarily ionize and
bond with themselves, are present because the body-centric cubic's
magnetic properties are disproportionate to the lattice periodicity.
This is due to the magnetic moments at the cube's corners and the
unequal, but antiparallel, cube centers. From here, the
frequency-dependent relative permittivity of chromium, deriving from
Maxwell's equations and chromium's antiferromagnetism, leaves chromium
with a high infrared and visible light reflectance.
Passivation
=============
Chromium metal in air is passivated: it forms a thin, protective
surface layer of chromium oxide with the corundum structure.
Passivation can be enhanced by short contact with oxidizing acids like
nitric acid. Passivated chromium is stable against acids. Passivation
can be removed 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.
The surface chromia scale, is adherent to the metal. In contrast,
iron forms a more porous oxide which is weak and flakes easily and
exposes fresh metal to the air, causing continued rusting. At room
temperature, the chromia scale is a few atomic layers thick, growing
in thickness by outward diffusion of metal ions across the scale.
Above 950 °C volatile chromium trioxide forms from the chromia scale,
limiting the scale thickness and oxidation protection.
Chromium, unlike 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.
Isotopes
==========
Naturally occurring chromium is composed of four stable isotopes;
50Cr, 52Cr, 53Cr and 54Cr, with 52Cr being the most abundant (83.789%
natural abundance). 50Cr is observationally stable, as it is
theoretically capable of decaying to 50Ti via double electron capture
with a half-life of no less than 1.3 years. Twenty-five radioisotopes
have been characterized, ranging from 42Cr to 70Cr; the most stable
radioisotope is 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 less than 1 minute. Chromium also has two
metastable nuclear isomers. The primary decay mode before the most
abundant stable isotope, 52Cr, is electron capture and the primary
mode after is beta decay.
53Cr is the radiogenic decay product of 53Mn (half-life 3.74 million
years). Chromium isotopes are typically collocated (and compounded)
with manganese isotopes. This circumstance is useful in isotope
geology. Manganese-chromium isotope ratios reinforce the evidence from
26Al and 107Pd concerning 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. 53Cr has been posited as a proxy for atmospheric oxygen
concentration.
Chemistry and compounds
======================================================================
Chromium is a member of group 6, of the transition metals. The +3 and
+6 states occur most commonly within chromium compounds, followed by
+2; charges of +1, +4 and +5 for chromium are rare, but do
nevertheless occasionally exist.
Common oxidation states
=========================
colspan=2|Oxidation states
−4 (d10) |Na4[Cr(CO)4]
−2 (d8)
−1 (d7)
0 (d6) bis(benzene)chromium
+1 (d5)
| **+2 (d4)**||Chromium(II) chloride
| **+3 (d3)** || Chromium(III) chloride
+4 (d2)
+5 (d1) 4}
|-
| +6 (d0) || Potassium chromate
|}
Chromium(0)
=============
Many Cr(0) complexes are known. Bis(benzene)chromium and chromium
hexacarbonyl are highlights in organochromium chemistry.
Chromium(II)
==============
Chromium(II) compounds are uncommon, in part because they readily
oxidize to chromium(III) derivatives in air. Water-stable
chromium(II) chloride that can be made by reducing chromium(III)
chloride with zinc. The resulting bright blue solution created from
dissolving chromium(II) chloride is stable at neutral pH. Some other
notable chromium(II) compounds include chromium(II) oxide , and
chromium(II) sulfate . Many chromium(II) carboxylates are known. The
red chromium(II) acetate (Cr2(O2CCH3)4) is somewhat famous. It
features a Cr-Cr quadruple bond.
Chromium(III)
===============
A large number of chromium(III) compounds are known, such as
chromium(III) nitrate, chromium(III) acetate, and chromium(III) oxide.
Chromium(III) can be obtained by dissolving elemental chromium in
acids like hydrochloric acid or sulfuric acid, but it can also be
formed through the reduction of chromium(VI) by cytochrome c7. The
ion has a similar radius (63 pm) to (radius 50 pm), and they can
replace each other in some compounds, such as in chrome alum and alum.
Chromium(III) tends to form octahedral complexes. Commercially
available chromium(III) chloride hydrate is the dark green complex
[CrCl2(H2O)4]Cl. Closely related compounds are the pale green
[CrCl(H2O)5]Cl2 and violet [Cr(H2O)6]Cl3. If anhydrous violet
chromium(III) chloride is dissolved in water, the violet solution
turns green after some time as the chloride in the inner coordination
sphere is replaced by water. This kind of reaction is also observed
with solutions of chrome alum and other water-soluble chromium(III)
salts. A tetrahedral coordination of chromium(III) has been reported
for the Cr-centered Keggin anion [α-CrW12O40]5-.
Chromium(III) hydroxide (Cr(OH)3) is amphoteric, dissolving in acidic
solutions to form [Cr(H2O)6]3+, and in basic solutions to form . It is
dehydrated by heating to form the green chromium(III) oxide (Cr2O3), a
stable oxide with a crystal structure identical to that of corundum.
Chromium(VI)
==============
Chromium(VI) compounds are oxidants at low or neutral pH. Chromate
anions () and dichromate (Cr2O72−) anions are the principal ions at
this oxidation state. They exist at an equilibrium, determined by pH:
:2 [CrO4]2− + 2 H+ [Cr2O7]2− + H2O
Chromium(VI) oxyhalides are known also and include chromyl fluoride
(CrO2F2) and chromyl chloride (). However, despite several erroneous
claims, chromium hexafluoride (as well as all higher hexahalides)
remains unknown, as of 2020.
Sodium chromate is produced industrially by the oxidative roasting of
chromite ore with sodium carbonate. The change in equilibrium is
visible by a change from yellow (chromate) to orange (dichromate),
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:
: + 14 + 6 e− → 2 + 21 (ε0 = 1.33 V)
They are, however, only moderately oxidizing at high pH:
: + 4 + 3 e− → + 5 (ε0 = −0.13 V)
Chromium(VI) compounds in solution can be detected by adding an acidic
hydrogen peroxide solution. The unstable dark blue chromium(VI)
peroxide (CrO5) is formed, which can be stabilized as an ether adduct
.
Chromic acid has the hypothetical formula . It is a vaguely described
chemical, despite many well-defined chromates and dichromates being
known. The dark red chromium(VI) oxide , 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.
Other oxidation states
========================
Compounds of chromium(V) are rather rare; 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 (CrF5). This red solid has a melting
point of 30 °C and a boiling point of 117 °C. It can be prepared 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 peroxochromate (K3[Cr(O2)4]) 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 () with the corresponding halogen at
elevated temperatures. Such compounds are susceptible to
disproportionation reactions and are not stable in water. Organic
compounds containing Cr(IV) state such as chromium tetra 't'-butoxide
are also known.
Most chromium(I) compounds are obtained solely by oxidation of
electron-rich, octahedral chromium(0) complexes. Other chromium(I)
complexes contain cyclopentadienyl ligands. As verified by X-ray
diffraction, a Cr-Cr quintuple bond (length 183.51(4) pm) has also
been described. Extremely bulky monodentate ligands stabilize this
compound by shielding the quintuple bond from further reactions.
Occurrence
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Chromium is the 21st most abundant element in Earth's crust with an
average concentration of 100 ppm. Chromium compounds are found in the
environment from the erosion of chromium-containing rocks, and can be
redistributed by volcanic eruptions. Typical background concentrations
of chromium in environmental media are: atmosphere <10 ng/m3; soil
<500 mg/kg; vegetation <0.5 mg/kg; freshwater <10 μg/L;
seawater <1 μg/L; sediment <80 mg/kg. Chromium is mined as
chromite (FeCr2O4) ore.
About two-fifths of the chromite ores and concentrates in the world
are produced in South Africa, about a third in Kazakhstan, while
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
diamonds.
The relation between Cr(III) and Cr(VI) strongly depends on pH and
oxidative properties of the location. In most cases, Cr(III) is the
dominating species, but in some areas, the ground water can contain up
to 39 μg/L of total chromium, of which 30 μg/L is Cr(VI).
Early applications
====================
Chromium was not used anywhere else until the experiments of French
pharmacist and chemist Louis Nicolas Vauquelin (1763-1829) in the late
1790s.
From the 1970s, until 2019, it was widely believed that the technique
of chromium plating to prevent metal corrosion had been invented in
ancient China. This technique was thought to have been used, for
example, to protect bronze artifacts such as arrowheads and sword
blades buried as grave goods in the mausoleum of the First Emperor of
Qin. However, a detailed scientific investigation in 2019 revealed
that the chromium found on these artifacts originated naturally from
the lacquer applied to them. Scientists now believe that the excellent
preservation of the artifacts was not due to intentional chromium
plating, but rather to the burial environment: the soil was
fine-grained and alkaline, which limited aeration and the growth of
organic matter, thereby creating optimal conditions for metal
preservation.
Chromium minerals as pigments came to the attention of the west in the
eighteenth 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 was
discovered to possess useful properties as a pigment in paints. After
Pallas, the use of Siberian red lead as a paint pigment began to
develop rapidly throughout the region. Crocoite would be the principal
source of chromium in pigments until the discovery of chromite many
years later.
In 1794, Louis Nicolas Vauquelin received samples of crocoite ore. He
produced chromium trioxide (CrO3) by mixing crocoite with hydrochloric
acid. In 1797, Vauquelin discovered that he could isolate metallic
chromium by heating the oxide in a charcoal oven, for which he is
credited as the one who truly discovered the element. Vauquelin was
also able to detect traces of chromium in precious gemstones, such as
ruby and emerald.
During the nineteenth century, chromium was primarily used not only as
a component of paints, but in tanning salts as well. For quite some
time, the crocoite found in Russia was the main source for such
tanning materials. In 1827, a larger chromite deposit was discovered
near Baltimore, United States, which quickly met the demand for
tanning salts much more adequately than the crocoite that had been
used previously. This made the United States the largest producer of
chromium products until the year 1848, when larger deposits of
chromite were uncovered near the city of Bursa, Turkey. With the
development of metallurgy and chemical industries in the Western
world, the need for chromium increased.
Chromium is also famous for its reflective, metallic 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.
Production
======================================================================
Approximately 28.8 million metric tons (Mt) of marketable chromite ore
was produced in 2013, and converted into 7.5 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 in 2013 have been South Africa
(48%), Kazakhstan (13%), Turkey (11%), and India (10%), with several
other countries producing the rest of about 18% 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
(FeCr2O4) 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 must 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
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The creation of metal alloys account for 85% of the available
chromium's usage. The remainder of chromium is used in the chemical,
refractory, and foundry industries.
Metallurgy
============
The strengthening effect of forming stable metal carbides at grain
boundaries, and the strong increase in corrosion resistance made
chromium an important alloying material for steel. High-speed tool
steels contain 3-5% chromium. Stainless steel, the primary
corrosion-resistant metal alloy, is formed when chromium is introduced
to iron in concentrations above 11%. For stainless steel's formation,
ferrochromium is added to the molten iron. Also, nickel-based alloys
have increased 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. ASTM B163
relies on chromium for condenser and heat-exchanger tubes, while
castings with high strength at elevated temperatures that contain
chromium are standardised with ASTM A567. AISI type 332 is used where
high temperature would normally cause carburization, oxidation or
corrosion. Incoloy 800 "is capable of remaining stable and maintaining
its austenitic structure even after long time exposures to high
temperatures". Nichrome is used as resistance wire for heating
elements in things like toasters and space heaters. These uses make
chromium a strategic material. Consequently, during World War II, U.S.
road engineers were instructed to avoid chromium in yellow road paint,
as it "may become a critical material during the emergency". The
United States likewise considered chromium "essential for the German
war industry" and made intense diplomatic efforts to keep it out of
the hands of Nazi Germany.
The high hardness and corrosion resistance of unalloyed chromium makes
it a reliable metal for surface coating; it is still the most popular
metal for sheet coating, with its above-average durability, compared
to other coating metals. A layer of chromium is deposited on
pretreated metallic surfaces by electroplating techniques. There are
two deposition methods: thin, and thick. Thin deposition involves a
layer of chromium below 1 μm thickness deposited by chrome plating,
and is used for decorative surfaces. Thicker chromium layers are
deposited if wear-resistant surfaces are needed. Both methods use
acidic chromate or dichromate solutions. To prevent the
energy-consuming change in oxidation state, the use of chromium(III)
sulfate is under development; for most applications of chromium, the
previously 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 of 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 methods are under
development.
Chromic acid anodizing (or Type I anodizing) of aluminium is another
electrochemical process that does not lead to the deposition of
chromium, but uses chromic acid as an 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.
Pigment
=========
The mineral crocoite (which is also lead chromate PbCrO4) 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 United States and for the postal
services (for example, the Deutsche Post) in Europe. The use of chrome
yellow has since declined due to environmental and safety concerns and
was replaced by organic pigments or other alternatives that are free
from lead and chromium. Other pigments that are based around chromium
are, for example, the deep shade of red pigment chrome red, which is
simply lead chromate with lead(II) hydroxide (PbCrO4·Pb(OH)2). 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
pre-treating 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 pigment in the field of
glassmaking and also as a glaze for ceramics. Green chromium oxide is
extremely lightfast and as such is used in cladding coatings. It is
also the main ingredient in infrared reflecting paints, used by the
armed forces to paint vehicles and to give them the same infrared
reflectance as green leaves.
Other uses
============
Chromium(III) ions present in corundum crystals (aluminium oxide)
cause them to be colored red; when corundum appears as such, it is
known as a ruby. If the corundum is lacking in chromium(III) ions, it
is known as a sapphire. 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. Ruby has a laser transition at 694.3
nanometers, in a deep red color.
Chromium(VI) salts are used for the preservation of wood. For example,
chromated copper arsenate (CCA) 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 were used in 1996.
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 4-5% 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. Recovery and reuse, direct/indirect
recycling, and "chrome-less" or "chrome-free" tanning are practiced to
better manage chromium usage.
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).
Several chromium compounds are used as catalysts for processing
hydrocarbons. For example, the Phillips catalyst, prepared from
chromium oxides, is used for the production of about half the world's
polyethylene. Fe-Cr mixed oxides are employed as high-temperature
catalysts for the water gas shift reaction. Copper chromite is a
useful hydrogenation catalyst.
Uses of compounds
===================
* Chromium(IV) oxide (CrO2) is a magnetic compound. Its ideal shape
anisotropy, which imparts high coercivity and remnant magnetization,
made it a compound superior to γ-Fe2O3. Chromium(IV) oxide is used to
manufacture magnetic tape used in high-performance audio tape and
standard audio cassettes.
* Chromium(III) oxide (Cr2O3) 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 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 (50 g/L
versus 200 g/L respectively). 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.
* Chromates are added to drilling muds to prevent corrosion of steel
under wet conditions.
* Chrome alum is Chromium(III) potassium sulfate and is used as a
mordant (i.e., a fixing agent) for dyes in fabric and in tanning.
* Zinc chromate is used as an anticorrosive agent for aluminium,
especially in the aerospace industry.
Biological role
======================================================================
The possible nutritional value of chromium(III) is unproven. Although
chromium is regarded as a trace element and dietary mineral, its
suspected roles in the action of insulin – a hormone that mediates the
metabolism and storage of carbohydrate, fat, and protein – have not
been adequately established. The mechanism of its actions in the body
is undefined, leaving in doubt whether chromium has a biological role
in healthy people.
In contrast, hexavalent chromium (Cr(VI) or Cr6+) is highly toxic and
mutagenic. Ingestion of chromium(VI) in water has been linked to
stomach tumors, and it may also 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 not
accepted as a medical condition, as it has no symptoms and healthy
people do not require chromium supplementation. Some studies suggest
that the biologically active form of chromium(III) is transported in
the body via an oligopeptide called low-molecular-weight
chromium-binding substance (chromodulin), which might play a role in
the insulin signaling pathway.
The chromium content of common foods is generally low (1-13 micrograms
per serving). The chromium content of food varies widely, due to
differences in soil mineral content, growing season, plant cultivar,
and contamination during processing. Chromium (and nickel) leach into
food cooked in stainless steel, with the effect being largest when the
cookware is new. Acidic foods that are cooked for many hours also
exacerbate this effect.
Dietary recommendations
=========================
There is disagreement on chromium's status as an essential nutrient.
Governmental departments from Australia, New Zealand, India, and Japan
consider chromium as essential, while the United States and European
Food Safety Authority of the European Union do not.
The U.S. National Academy of Medicine (NAM) updated the Estimated
Average Requirements (EARs) and the Recommended Dietary Allowances
(RDAs) for chromium in 2001. For chromium, there was insufficient
information to set EARs and RDAs, so its needs are described as
estimates for Adequate Intake (AI). From a 2001 assessment, AI of
chromium for women ages 14 through 50 is 25 μg/day, and the AI for
women ages 50 and above is 20 μg/day. The AIs for women who are
pregnant are 30 μg/day, and for women who are lactating, the set AI is
45 μg/day. The AI for men ages 14 through 50 is 35 μg/day, and the AI
for men ages 50 and above is 30 μg/day. For children ages 1 through
13, the AI increases with age from 0.2 μg/day up to 25 μg/day. As for
safety, the NAM sets Tolerable Upper Intake Levels (ULs) for vitamins
and minerals when the evidence is sufficient. In the case of chromium,
there is not yet enough information, hence no UL has been established.
Collectively, the EARs, RDAs, AIs, and ULs are the parameters for the
nutrition recommendation system known as Dietary Reference Intake
(DRI).
Australia and New Zealand consider chromium to be an essential
nutrient, with an AI of 35 μg/day for men, 25 μg/day for women, 30
μg/day for women who are pregnant, and 45 μg/day for women who are
lactating. A UL has not been set due to the lack of sufficient data.
India considers chromium to be an essential nutrient, with an adult
recommended intake of 33 μg/day. Japan also considers chromium to be
an essential nutrient, with an AI of 10 μg/day for adults, including
women who are pregnant or lactating. A UL has not been set.
The EFSA does not consider chromium to be an essential nutrient.
Labeling
==========
For U.S. food and dietary supplement labeling purposes, the amount of
the substance in a serving is expressed as a percent of the Daily
Value (%DV). For chromium labeling purposes, 100% of the Daily Value
was 120 μg. As of 27 May 2016, the percentage of daily value was
revised to 35 μg to bring the chromium intake into a consensus with
the official Recommended Dietary Allowance. A table of the old and new
adult daily values in the United States is provided at Reference Daily
Intake.
After evaluation of research on the potential nutritional value of
chromium, the European Food Safety Authority concluded that there was
no evidence of benefit by dietary chromium in healthy people, thereby
declining to establish recommendations in Europe for dietary intake of
chromium.
Food sources
==============
Food composition databases such as those maintained by the U.S.
Department of Agriculture do not contain information on the chromium
content of foods. A wide variety of animal and vegetable foods contain
chromium. Content per serving is influenced by the chromium content of
the soil in which the plants are grown, by foodstuffs fed to animals,
and by processing methods, as chromium is leached into foods if
processed or cooked in stainless steel equipment.
One diet analysis study conducted in Mexico reported an average daily
chromium intake of 30 micrograms. An estimated 31% of adults in the
United States consume multi-vitamin/mineral dietary supplements, which
often contain 25 to 60 micrograms of chromium.
Supplementation
=================
Chromium is an ingredient in total parenteral nutrition (TPN), because
deficiency can occur after months of intravenous feeding with
chromium-free TPN. It is also added to nutritional products for
preterm infants. Although the mechanism of action in biological roles
for chromium is unclear, in the United States chromium-containing
products are sold as non-prescription dietary supplements in amounts
ranging from 50 to 1,000 μg. Lower amounts of chromium are also often
incorporated into multi-vitamin/mineral supplements consumed by an
estimated 31% of adults in the United States. Chemical compounds used
in dietary supplements include chromium chloride, chromium citrate,
chromium(III) picolinate, chromium(III) polynicotinate, and other
chemical compositions. The benefit of supplements has not been proven.
Initiation of research on glucose
===================================
The notion of chromium as a potential regulator of glucose metabolism
began in the 1950s when scientists performed a series of experiments
controlling the diet of rats. The experimenters subjected the rats to
a chromium deficient diet, and witnessed an inability to respond
effectively to increased levels of blood glucose. A chromium-rich
Brewer's yeast was provided in the diet, enabling the rats to
effectively metabolize glucose, and so giving evidence that chromium
may have a role in glucose management.
Approved and disapproved health claims
========================================
In 2005, the U.S. Food and Drug Administration had approved a
qualified health claim for chromium picolinate with a requirement for
specific label wording:
:'"One small study suggests that chromium picolinate may reduce the
risk of insulin resistance, and therefore possibly may reduce the risk
of type 2 diabetes. FDA concludes, however, that the existence of such
a relationship between chromium picolinate and either insulin
resistance or type 2 diabetes is highly uncertain."'
In other parts of the petition, the FDA rejected claims for chromium
picolinate and cardiovascular disease, retinopathy or kidney disease
caused by abnormally high blood sugar levels. As of March 2024, this
ruling on chromium remains in effect.
In 2010, chromium(III) picolinate was approved by Health Canada to be
used in dietary supplements. Approved labeling statements include: a
factor in the maintenance of good health, provides support for healthy
glucose metabolism, helps the body to metabolize carbohydrates and
helps the body to metabolize fats. The European Food Safety Authority
approved claims in 2010 that chromium contributed to normal
macronutrient metabolism and maintenance of normal blood glucose
concentration, but rejected claims for maintenance or achievement of a
normal body weight, or reduction of tiredness or fatigue.
However, in a 2014 reassessment of studies to determine whether a
Dietary Reference Intake value could be established for chromium, EFSA
stated:
:'"The Panel concludes that no Average Requirement and no Population
Reference Intake for chromium for the performance of physiological
functions can be defined."' and
:'"The Panel considered that there is no evidence of beneficial
effects associated with chromium intake in healthy subjects. The Panel
concludes that the setting of an Adequate Intake for chromium is also
not appropriate."'
Diabetes
==========
Given the evidence for chromium deficiency causing problems with
glucose management in the context of intravenous nutrition products
formulated without chromium, research interest turned to whether
chromium supplementation would benefit people who have type 2 diabetes
but are not chromium deficient. Looking at the results from four
meta-analyses, one reported a statistically significant decrease in
fasting plasma glucose levels and a non-significant trend in lower
hemoglobin A1C. A second reported the same, a third reported
significant decreases for both measures, while a fourth reported no
benefit for either. A review published in 2016 listed 53 randomized
clinical trials that were included in one or more of six
meta-analyses. It concluded that whereas there may be modest decreases
in fasting blood glucose and/or HbA1C that achieve statistical
significance in some of these meta-analyses, few of the trials
achieved decreases large enough to be expected to be relevant to
clinical outcome.
Body weight
=============
Two systematic reviews looked at chromium supplements as a mean of
managing body weight in overweight and obese people. One, limited to
chromium picolinate, a common supplement ingredient, reported a
statistically significant −1.1 kg (2.4 lb) weight loss in trials
longer than 12 weeks. The other included all chromium compounds and
reported a statistically significant −0.50 kg (1.1 lb) weight change.
Change in percent body fat did not reach statistical significance.
Authors of both reviews considered the clinical relevance of this
modest weight loss as uncertain/unreliable. The European Food Safety
Authority reviewed the literature and concluded that there was
insufficient evidence to support a claim.
Sports
========
Chromium is promoted as a sports performance dietary supplement, based
on the theory that it potentiates insulin activity, with anticipated
results of increased muscle mass, and faster recovery of glycogen
storage during post-exercise recovery. A review of clinical trials
reported that chromium supplementation did not improve exercise
performance or increase muscle strength. The International Olympic
Committee reviewed dietary supplements for high-performance athletes
in 2018 and concluded there was no need to increase chromium intake
for athletes, nor support for claims of losing body fat.
Fresh-water fish
==================
Irrigation water standards for chromium are 0.1 mg/L, but some rivers
in Bangladesh are more than five times that amount. The standard for
fish for human consumption is less than 1 mg/kg, but many tested
samples were more than five times that amount. Chromium, especially
hexavalent chromium, is highly toxic to fish because it is easily
absorbed across the gills, readily enters blood circulation, crosses
cell membranes and bioconcentrates up the food chain. In contrast, the
toxicity of trivalent chromium is very low, attributed to poor
membrane permeability and little biomagnification.
Acute and chronic exposure to chromium(VI) affects fish behavior,
physiology, reproduction and survival. Hyperactivity and erratic
swimming have been reported in contaminated environments. Egg hatching
and fingerling survival are affected. In adult fish there are reports
of histopathological damage to liver, kidney, muscle, intestines, and
gills. Mechanisms include mutagenic gene damage and disruptions of
enzyme functions.
There is evidence that fish may not require chromium, but benefit from
a measured amount in diet. In one study, juvenile fish gained weight
on a zero chromium diet, but the addition of 500 μg of chromium in the
form of chromium chloride or other supplement types, per kilogram of
food (dry weight), increased weight gain. At 2,000 μg/kg the weight
gain was no better than with the zero chromium diet, and there were
increased DNA strand breaks.
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. Acute oral toxicity ranges between 50
and 150 mg/kg. A 2008 review suggested that moderate uptake of
chromium(III) through dietary supplements poses no genetic-toxic risk.
In the US, the Occupational Safety and Health Administration (OSHA)
has designated an air permissible exposure limit (PEL) in the
workplace as a time-weighted average (TWA) of 1 mg/m3. The National
Institute for Occupational Safety and Health (NIOSH) has set a
recommended exposure limit (REL) of 0.5 mg/m3, time-weighted average.
The IDLH (immediately dangerous to life and health) value is 250
mg/m3.
Chromium(VI) toxicity
=======================
The acute oral toxicity for chromium(VI) ranges between 1.5 and 3.3
mg/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 oxidant properties. After it reaches
the blood stream, it damages the kidneys, the liver and blood cells
through oxidation reactions. Hemolysis, renal, and liver failure
result. Aggressive dialysis can be therapeutic.
The carcinogenity of chromate dust has been 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 (chromates) 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
======================
Because chromium compounds were used in dyes, paints, and leather
tanning compounds, these compounds are often found in soil and
groundwater at active and abandoned industrial sites, needing
environmental cleanup and remediation. 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 in the first nationwide study. 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.
The more toxic hexavalent chromium form can be reduced to the less
soluble trivalent oxidation state in soils by organic matter, ferrous
iron, sulfides, and other reducing agents, with the rates of such
reduction being faster under more acidic conditions than under more
alkaline ones. In contrast, trivalent chromium can be oxidized to
hexavalent chromium in soils by manganese oxides, such as Mn(III) and
Mn(IV) compounds. Since the solubility and toxicity of chromium (VI)
are greater than those of chromium (III), the oxidation-reduction
conversions between the two oxidation states have implications for
movement and bioavailability of chromium in soils, groundwater, and
plants.
External links
======================================================================
* [
https://www.atsdr.cdc.gov/csem/chromium/cover-page.html ATSDR Case
Studies in Environmental Medicine: Chromium Toxicity] U.S. Department
of Health and Human Services
*
[
https://web.archive.org/web/20040701090041/http://www-cie.iarc.fr/htdocs/monographs/vol49/chromium.html
IARC Monograph "Chromium and Chromium compounds"]
* [
http://education.jlab.org/itselemental/ele024.html It's Elemental -
The Element Chromium]
* [
http://www.merck.com/mmpe/sec01/ch005/ch005b.html The Merck Manual
- Mineral Deficiency and Toxicity]
* [
https://www.cdc.gov/niosh/topics/chromium/ National Institute for
Occupational Safety and Health - Chromium Page]
* [
http://www.periodicvideos.com/videos/024.htm Chromium] at 'The
Periodic Table of Videos' (University of Nottingham)
*
License
=========
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http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Chromium
