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CHEMISTRY : Metal Cation Identification


Information on CHROMIUM



  1. General Information

  2. Occurence Uses and Properties

  3. History of the Metal

  4. Compounds
  5. Back to Main Metal List



General Information


Chromium (Cr), chemical element of Group VIb of the periodic table, a hard, steel-gray metal that takes a high polish and is used in alloys to increase strength and corrosion resistance. Chromium was discovered (1797) by the French chemist Nicolas-Louis Vauquelin and isolated as the metal a year later; it was named for its multicoloured compounds. The green colour of emerald, serpentine, and chrome mica and the red colour of ruby are due to chromium.

The name of the element chromium (from Greek chromos, "colour") connotes the pronounced and varied colorations of chromium compounds. The red colour of ruby and the green colour of emerald, to mention only gems, are due to small amounts of chromium. The free metal is never found in nature; most ores consist of the mineral chromite, the ideal formula of which is FeCr2O4. Natural deposits are usually contaminated with magnesium, aluminum, and silica and their chromium content varies from 42 to 56 percent. One of the chief uses of chromium is in ferrous alloys, for which the pure metal is not required. Accordingly, chromite is often reduced with carbon in a furnace, producing the alloy ferrochromium, which contains iron and chromium in an atom ratio of approximately 1 to 2. To obtain pure chromium, chromite is first treated with molten alkali and oxygen, converting all of the chromium to the alkali chromate, and the latter is dissolved in water and eventually precipitated as sodium dichromate, Na2Cr2O7. The dichromate is then reduced with carbon to chromium(III) oxide, Cr2O3, and that oxide in turn reduced with aluminum to give the chromium metal.

The metal is white, hard, lustrous, and brittle and is extremely resistant to ordinary corrosive reagents; this resistance accounts for its extensive use as an electroplated protective coating. Chromium metal dissolves rather readily in nonoxidizing mineral acids--for example, hydrochloric or sulfuric acids; it does not dissolve in cold aqua regia or in nitric acid because of a curious phenomenon known as passivation, in which the metal presumably becomes covered with a thin unreactive coating. At elevated temperatures chromium unites directly with the halogens or with sulfur, silicon, boron, nitrogen, carbon, or oxygen.


Occurrence, uses, and properties.


Chromium is a relatively abundant element in the Earth's crust. It is widely dispersed in natural deposits, where it is always combined with other elements, especially oxygen. Chromite (FeCr2O4) is the only important commercial mineral.

Chromium is added to iron and nickel in the form of ferrochromium (about 70 percent chromium) to produce alloys specially characterized by their high resistance to corrosion and oxidation. Used in small amounts, chromium hardens steel. Stainless steels are alloys of chromium and iron in which the chromium content varies from 10 to 26 percent. Chromium alloys are used to fabriCATe such products as oil tubing, automobile trim, and cutlery. Chromite is used as a refractory and as a raw material for the production of chromium chemicals.

For additional treatment of chromium metal and its production, see Industries, Extraction and Processing: Chromium.

Natural chromium consists of a mixture of four stable isotopes: chromium-52 (83.76 percent), chromium-53 (9.55 percent), chromium-50 (4.31 percent), and chromium-54 (2.38 percent). The metal is paramagnetic (weakly attracted to a magnet). It exists in two forms: body-centred cubic (alpha) and hexagonal close-packed (beta). At room temperature, chromium slowly dissolves in hydrochloric and dilute sulfuric acids. Certain oxidizing agents produce a thin oxide layer on the metal, rendering it passive also to dilute mineral acids, such as sulfuric. At ordinary temperatures the metal shows no reaction to seawater or to wet or dry air.

History


Chromium is unusual among metals in that its ores and chemical compounds were used extensively long before the pure metal was prepared. As early as 1800, they were used to make pigments and chemicals for leather tanning, and in 1879 they were successfully used as refractories for the lining of steelmaking furnaces.

Chromium metal was discovered by the French chemist Louis-Nicolas Vauquelin in 1797; the following year he isolated the metal by the carbon reduction of crocoite, or red lead, a chromate mineral whose brilliant hue inspired Vauquelin to give the metal its current name (from Greek chromos, "colour"). Iron containing chromium was first produced in the mid-19th century, and the first use of chromium as an alloying agent in the manufacture of steel took place in France in the 1860s. In 1893 Henri Moissan smelted chromium ore and carbon in an electric furnace and produced ferrochromium; this has remained the basis of the modern commercial method of producing the alloy even while that method has continuously evolved under the influence of changing markets, technology, and raw materials.

In 1898 Hans Goldschmidt, a German chemist, produced pure chromium by the aluminothermic reduction of chromium oxide; the silicothermic process for producing low-carbon ferrochromium was developed in 1907. Chromium metal was produced by electrolysis in 1854, but this method did not find wide commercial acceptance until a century later.


Chemical compounds


The most common oxidation states of chromium are +6, +3, and +2. A few stable compounds of the +5, +4, and +1 states, however, are known.

In the hexavalent state, the most important species formed by chromium are the chromate(VI), CrO42-, and dichromate(VI), Cr2O72-, ions. These ions form the basis for a series of industrially important salts. Among them are sodium chromate, NaCrO4, and sodium dichromate, Na2Cr2O7, which are used in leather tanning, in metal surface treatment, and as CATalysts in various industrial processes.

Chromium forms several commercially valuable oxygen compounds, the most important of which is chromium(VI) oxide, commonly called chromium trioxide or chromic acid, CrO3. An orange-red crystalline solid, chromatic acid liquefies gradually when exposed to moist air. It is usually produced by treatment of sodium dichromate with sulfuric acid. Chromic acid is used chiefly for chromium plating but is also employed as a colorant in ceramics. It is a powerful oxidant and may react violently with some organic materials, but such solutions are often utilized by controlled oxidations in organic synthesis.

Another significant oxygen compound is chromium(III) oxide, also known as chromium sesquioxide or chromic oxide, Cr2O3. It is prepared by calcining sodium dichromate in the presence of carbon or sulfur. Chromium oxide consists of a green powder and is employed extensively as a pigment; its hydrate form, known as Guignet's green, is used when chemical and heat resistance are required. For comparative statistical data on chromium production and reserves, see mining (table).

atomic number 24 atomic weight 51.996 melting point 1,890 C (3,434 F) boiling point 2,482 C (4,500 F) specific gravity 7.20 (28 C) valence 2, 3, 6 electronic config. 2-8-13-1 or (Ar)3d54s1



Reference: Encyclopędia Britannica, Inc. 1994-2000 ©



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