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Chromium
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Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable. The name of the element is derived from the Greek word "chroma" (???µa) meaning color, because many of its compounds are intensely colored. It was discovered by Louis Nicolas Vauquelin in the mineral crocoite (lead chromate) in 1797.
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Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable. The name of the element is derived from the Greek word "chroma" (???µa) meaning color, because many of its compounds are intensely colored. It was discovered by Louis Nicolas Vauquelin in the mineral crocoite (lead chromate) in 1797. Crocoite was used as a pigment, and after the discovery that the mineral chromite also contains chromium this latter mineral was used to produce pigments as well.
Chromium was regarded with great interest 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 chromium and nickel to form stainless steel. This application and electroplating are currently the highest-volume uses of the metal. Chromium and ferrochromium are produced from the single commercially viable ore —chromite— by silicothermic or aluminothermic reaction or by roasting and leaching processes. Although trivalent chromium (Cr(III)) is required in trace amounts for sugar and lipid metabolism in humans and its deficiency may cause a disease called chromium deficiency, hexavalent chromium (Cr(VI)) is a toxin and a carcinogen and at abandoned chromium production sites environmental cleanup is necessary.
Characteristics
Occurrence
Chromium is th 21st most abundant element in Earth's crust with an average concentration of 100 ppm. Chromium compounds are found in the enviroment, due to erosion of chromium containing rocks and can be distributed by vulcanic eruptions. The concentrations range normaly in soil is between and 1 and 3000 mg/kg in sea water 5 to 800 µg/liter and in rivers and lakes 26 µg/liter to 5.2 mg/liter. although in some areas the ground water can contain upto 39 µg of total chromium of which 30µg is present as Cr(VI).
Chromium is mined as chromite (FeCr2O4) 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.
Though native chromium deposits are rare, some native chromium metal has been discovered. The Udachnaya Mine 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. (See also chromium minerals)
Isotopes
Naturally occurring chromium is composed of three stable isotopes; 52Cr, 53Cr, and 54Cr with 52Cr being the most abundant (83.789% natural abundance). Nineteen radioisotopes have been characterized with the most stable being 50Cr with a half-life of (more than) 1.8x1017 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 isotope systematics 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 (43Cr) to 67 u (67Cr). The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.
Chemistry
Chromium is a member of the transition metals, in group 6. Chromium(0) has an electronic configuration of 4s13d5, due to the lower energy of the high spin configuration. Chromium exhibits a wide range of possible oxidation states. The most common oxidation states of chromium are +2, +3, and +6, with +3 being the most stable. +1, +4 and +5 are rare. Chromium compounds of oxidation state +6 are powerful oxidants. All (except the chromiumhexafluoride) stable chromium compounds of the oxidation state +6 contain oxygen as ligand, for example the chromate (CrO42-) and Chromyl chloride (CrO2Cl2).
Chemistry
The oxidation state 3+ is the most stable one and therefore a large number of chromium(III) compounds is known.
The second very stable oxidation state is 6+, for example the chromate which is produced in large scale by oxidative roasting of chromite ore with sodium carbonate. Chromate and dichromate are in an equilibrium, which is influenced by the pH of the solution.
- 2 CrO42- + 2 H3O+ ? Cr2O72- + 2 H2O
The change in equilibrium is also visible by a change from yellow (chromate) to orange (dichromate )if a acid is added to a neutral solution of potassium chromate. At lower pH further condensation to more complex chromates is possible. The chromate and dichromate are strong oxidizing reagents at low pH
- Cr2O72- + 14 H3O+ + 6 e- ? 2 Cr3+ + 21 H2O e0 = 1.33V
but only moderate ones at high pH
- CrO42- + 4 H2O + 3 e- ? Cr(OH)3+ + 5 OH- e0 = - 0.13V
The dark red chromium(VI) oxide, which can be produced by mixing sulfuric acid with dichromate, is a extremely strong oxidizing agent. The detection of chromium(VI) compounds in solution can be done by adding acidic hydrogen peroxide solution. A dark blue instable chromium(VI) peroxide (CrO5) is formed which can be stabilized by adding ether (OR2), which leads to the formation of the ether adduct CrO5· OR2.
The oxidation state 5+ is only realized in few compounds. The only binary compound is the highly volatile chromium(V) fluoride (CrF5). The red solid with a melting point of 30°C and a boiling point of 117°C can be synthesized by reacting fluorine at 400°C and 200 bar pressure with chromium. The peroxochromate(V) is another example for the oxidation state 5+. For example potassium peroxochromate (K3[Cr(O2)]4) is made by reacting potassium chromate with hydrogen peroxide at low temperatures. The red brown compound is stable at room temperature but decomposes spontaneously at higher temperatures. The chromium(IV) compounds (4+) are slightly more stable than the chromium(V) compounds and the halogen compounds CrF4, CrCl4 and CrBr4 can be produced by the reaction of the trihalogens with additional elementary halogenes at elevated temperatures. Most of the compounds are susceptible to disproportation reactions and therefore are not stable in water. An example for a Chromium(II) compounds (2+)is the water stable chromium(II) chloride which can be produced by reduction of chromium(III) chloride with zink. The light blue solutions are only stable at neutral pH when the solution is very pure.
Passivation
Chromium is passivated by oxygen, forming a thin protective oxide surface layer with another element such as nickel or iron. This layer is a spinel structure only a few atoms thick and is very dense, preventing diffusion of oxygen into the underlying material. (In iron or plain carbon steels the oxygen migrates into the underlying material.) Chromium is usually plated on top of a nickel layer which may first have been copper plated. Chromium, unlike metals such as iron and nickel, does not suffer from hydrogen embrittlement. It does suffer from nitrogen embrittlement and hence no straight chromium alloy has ever been developed. Below, the Pourbaix diagram can be seen. It is important to understand that the diagram only displays the thermodynamic data and it does not display any details of the rates of reaction. The passivation can be increased by short contact with oxidating acids like nitric acid. The passivated chromium is stable against acids. The contrary effect can be achieved if a strong reducing reactant destroys the oxide protection layer on the metal, a metal treated in this way readily dissolves in weak acids.
Quintuple bond
Chromium is notable for its ability to form quintuple covalent bonds. The synthesis of a compound of chromium(I) and a hydrocarbon radical was shown via X-ray diffraction to contain a quintuple bond of length 183.51(4) pm (1.835 angstroms) joining the two central chromium atoms. This was accomplished through the use of an extremely bulky monodentate ligand
History
Weapons found in burial pits dating from the late 3rd century BC 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 no sign of corrosion, because the bronze was coated with chromium.
Chromium came to the attention of westerners in the 18th century. On 26 July 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains which he named Siberian red lead. Though misidentified as a lead compound with selenium and iron components, the mineral was Crocoite (lead chromate) with a formula of PbCrO4, now known as the mineral crocoite.
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 developed rapidly. A bright yellow made from crocoite also became fashionable.
In 1797, Louis Nicolas Vauquelin received samples of crocoite ore. He produced chromium oxide (CrO3) by mixing crocoite with hydrochloric acid. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He 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 large chromite deposit was discovered near Baltimore United States. This made the United states the larges producer of chromium products till 1848 when large deposits of chromite where 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 4.4 million metric tons of marketable chromite ore were produced in 2000, and converted into approximately 3.3 million tons of ferro-chrome with an approximate market value of 2.5 billion United States dollars.The largest producers have been South Africa (44%) India (18%), Kazakhstan (16%) Zimbabwe (5%), Finland (4%) Iran (4%) and Brazil (2%) 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 (FeCr2O4) is reduced with either aluminium or silicon in a 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 unsoluble iron oxide. The chromate is converted by sulfuric acid into the dichromate.
- 4FeCr2O4 + 8 Na2CO3 + 7 O2 ? 8 Na2CrO4 + 2 Fe2O3 + 8 CO2
- Na2CrO4 + H2SO4 ? Na2Cr2O4 + Na2SO4 + H2O
The dichromate is converted to the chromium(III) oxide by reduction with carbon and than reduced in a aluminothermic reaction to chromium.
- Na2Cr2O4 . H2O + 2 C ? Cr2O3 + Na2CO3 + 2 H2O + CO
- Cr2O3 + 2 Al ? Al2O3 + Cr
Compounds
Chromic acid has the hypothetical structure H2CrO4. Neither chromic nor dichromic acid is found in nature, but their anions are found in a variety of compounds. Chromium trioxide, CrO3, the acid anhydride of chromic acid, is sold industrially as "chromic acid".
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 (5 g/100 ml vs. 20 g/100 ml respectively). It is not advisable to store the prepared mixture, especially if it has been used - the oxidation of the dissolved organic compounds can continue and may cause a capped bottle to over-pressurize.
Chrome green is the green oxide of chromium, Cr2O3, used in enamel painting, and glass staining. Chrome yellow is a brilliant yellow pigment, PbCrO4, used by painters.
Applications
Metallurgy
The strengthening effect on steel by forming stable carbide grains 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 % of chromium. A important stainless steel is 18/10 stainless, made from iron with 10% nickel and 18% chromium, is widely used for cookware and cutlery. For these applications ferrochromium is added to the molten iron. Also nickel based alloys increase in strength due to the formation stable carbide grains at the grain boundaries, for example Inconel 718 contains 18.6 % chromium. Due to the excellent heat stability of these nickel superalloys they are used in jet engines and gas turbines in large quantities.
The relative high hardness and corrosion resistance of unalloyed chromium makes it a good surface coating. A thin layer of chromium is deposited on pretreated metallic surfaces by electroplating techniques. There are two methods used to do this coating. Thin, below 1 µm thickness, layers are deposited by chrome plating are used for decorative surfaces. If wear-resistant surfaces are needed thicker chromium layers of up to mm thickness are deposited.
Both methods normally uses acidic chromate or dichromate solutions. To prevent the energy consuming change in oxidation state from 6+ to O the use of Chromium(III) sulfate is under development, but for most of the applications the established process is used.
In the chromate conversion coating process is a type of conversion coating applied to passivate aluminium, zinc, cadmium, to slow corrosion.
Another electrochemical process which does not lead to the deposition of chromium, but uses chromic acid as electrolyte in the solution is anodizing of aluminium. During anodizing a oxide layer is formed on the aluminium. The use of chromic acid instead of the sulfuric acid normally used leads to a slight difference of these oxide layers.
The high toxicity of Cr(VI) compounds used in the established chromium electroplating process and subsequent increase in safety and environmental regulations made a search for substitutes for chromium or at least a change to less toxic chromium(III) compounds is necessary.
Dye and Pigment
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The mineral crocoite (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 is light stable and has a strong color. The signaling effect of yellow was used for schoolbuses in the United States and for Postal Service (for example Deutsche Post) in Europe. While yellow is stil used the use of chrome yellow declined due to environmental and safety concerns and was subsituted by organic pigments or other lead free alternatives. Other pigments based on chromium are for example, the bright red pigment Chrome red, which is a basic lead chromate (PbCrO4•Pb(OH)2). Chrome green is a mixture of Prussian blue and chrome yellow, while the Chrome oxide green is Chromium(III) oxide.
Glass is colored green by the edition of chromium(III) oxide, this is similar to true emerald, which is also colored by chromium. A red color is acchieved if the chromium(III) a small amount of aluminium(III) in the crystal of corundum, which is than called ruby. Therefore chromium is used in producing synthetic rubies.
The toxicity of chromium(VI) salts is used in the preservation of wood, for example Chromated copper arsenate (CCA) is used in timber treatment to prevent 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 within the leather. Chromium tanned leather can contains between 4 and 5% of chromium, which is tightly bound to the proteins.
Refractory material
Other use
- As a catalyst.
- Potassium dichromate is a chemical reagent, used in cleaning laboratory glassware and as a titrating agent. It is also used as a mordant (i.e., a fixing agent) for dyes in fabric.
- Chromium(IV) oxide (CrO2) is used to manufacture magnetic tape, where its higher coercivity than iron oxide tapes gives better performance.
- In well drilling muds as an anti-corrosive.
- In medicine, as a dietary supplement or slimming aid, usually as chromium(III) chloride, chromium(III) picolinate, chromium(III) polynicotinate or as an amino acid chelate, such as chromium(III) D-phenylalanine.
- Chromium hexacarbonyl (Cr(CO)6) is used as a gasoline additive.
- Chromium boride (CrB) is used as a high-temperature electrical conductor.
- Chromium(III) oxide is a metal polish known as green rouge.
Biological role
Trivalent chromium (Cr(III) or Cr3+) is required in trace amounts for sugar and lipid metabolism in humans and its deficiency may cause a disease called chromium deficiency. In contrast, hexavalent chromium (Cr(VI) or Cr6+) is very toxic and mutagenic when inhaled. Cr(VI) has not been established as a carcinogen when in solution, though it may cause allergic contact dermatitis (ACD).
The use of chromium containing dietary supplements is controversial due to the complex effects of the used supplements. 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 from 50-200 µg for an adult to 35 µg (adult male) and to 25 µg (adult female).
Precautions
Chromium metal and chromium(III) compounds are not usually considered health hazards; chromium is an essential trace mineral. However, hexavalent chromium (chromium VI) compounds can be toxic if ingested or inhaled. The lethal dose of poisonous chromium (VI) compounds is about one half teaspoon of material. Most chromium (VI) compounds are irritating to eyes, skin and mucous membranes. Chronic exposure to chromium (VI) compounds can cause permanent eye injury, unless properly treated. Chromium(VI) is an established human carcinogen. An actual investigation into hexavalent chromium release into drinking water was used as the plot-basis of the motion picture Erin Brockovich.
World Health Organization recommended maximum allowable concentration in drinking water for chromium (VI) is 0.05 milligrams per liter. Hexavalent chromium is also one of the substances whose use is restricted by the European Restriction of Hazardous Substances Directive.
In some parts of Russia, pentavalent chromium was reported as one of the factors of incidence of premature senility.
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 leads 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.
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.
See also
Chromium minerals
External links
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