Chromium

History

On 26 July 1761, crocoite.

In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red "lead" mineral that had very useful properties as a yellow made from crocoite became a color in fashion.

In 1797, emerald. Later that year he successfully isolated chromium atoms.

During the 1800s chromium was primarily used as a component of paints and in tanning salts but now metal alloys account for 85% of the use of chromium. The remainder is used in the foundry industries.

Chromium was named after the Greek word "Chrôma" meaning color, because of the many colorful compounds made from it.

Occurrence and production

  Chromium is mined as chromite (FeCr₂O₄) ore. About two-fifths of the chromite ores and concentrates in the world are produced in South Africa. Kazakhstan, India, Russia and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.

Approximately 15 million tons of marketable chromite ore were produced in 2000, and converted into approximately 4 million tons of ferro-chrome with an approximate market value of 2.5 billion United States dollars.

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 chromium minerals)

Chromium is obtained commercially by heating the ore in the presence of aluminium or silicon.

Chemical properties

  Chromium is a member of the 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.

Chromium is passivated by oxygen, forming a thin (usually a few atoms thick being transparent because of thickness) protective oxide surface layer with another element such as nickel, and/or iron. It forms a compound called a spinal structure which, being very dense, prevents diffusion of oxygen into the underlying layer. (In iron or plain carbon steels the oxygen actually migrates into the underlying material.) Chromium is usually plated on top of a nickel layer which may or may not have been copper plated first. Chromium as opposite to most other metals such as iron and nickel does not suffer from hydrogen embrittlement. It does though 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.

 

Compounds

Main article: Chromium compounds

Potassium dichromate is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any trace organics. It is used as a saturated solution in concentrated sulfuric acid for washing the apparatus. For this purpose, however, sodium dichromate is sometimes used because of its higher solubility (5 g/100 ml vs. 20 g/100 ml respectively). Chrome green is the green oxide of chromium, Cr₂O₃, used in Chrome yellow is a brilliant yellow pigment, PbCrO₄, used by painters.

anhydride of chromic acid, is sold industrially as "chromic acid".

Chromium and the quintuple bond

  Chromium is notable for its ability to form quintuple quintuple bonds have been observed.

Applications

Uses of chromium:

• In metallurgy, to impart corrosion resistance and a shiny finish:
  • as an stainless steel in cutlery
  • in chrome plating,
  • in ruby.
• As dyes and paints :
  • Chromium(III) oxide is a metal polish known as green rouge.
  • Chromium salts color glass an emerald green.
  • Chromium is what makes a ruby red, and therefore is used in producing synthetic rubies.
  • also makes a brilliant yellow for painting
• As a catalyst.
• bricks.
• Chromium salts are used in the tanning of leather.
• 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.
• 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 or chromium(III) picolinate.
• Chromium hexacarbonyl (Cr(CO)₆) is used as a gasoline additive.
• Chromium boride (CrB) is used as a high-temperature electrical conductor.
• Chromium (III) sulfate (Cr₂(SO₄)₃) is used as a green chrome plating.
• Chromium (VI) is used in the post Ballard preparation of Gravure (rotogravure) printing Forme Cylinders. By electroplating the metal onto the second coat of copper (after the Ballard skin), the longevity of the printing cylinder is increased.
• Micronutrient, in "health" aware drinks, known to improve the amount of energy you get from food.

Biological role

Trivalent chromium (Cr(III), or Cr³⁺) is required in trace amounts for sugar mutagenic when inhaled as publicized by the film Erin Brockovich. Cr(VI) has not been established as a carcinogen when not inhaled but in solution it is well established as a cause of allergic contact dermatitis (ACD).^[3]

Recently it was shown that 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).^[4]

Isotopes

Main article: Isotopes of chromium

Naturally occurring chromium is composed of three stable 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 Pd 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 beta decay.

Precautions

Chromium metal and chromium(III) compounds are not usually considered health hazards; chromium is an essential trace mineral.^[5] However, carcinogen. An investigation into hexavalent chromium release into drinking water formed the plot 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.

As chromium compounds were used in 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 compounds
• Chromium minerals
• Chromium VI

References

• Los Alamos National Laboratory - Chromium
• WebElements.com – Chromium