Chemical elements
  Chromium
    Isotopes
    Energy
    Occurrence
    Preparation
    Physical Properties
    Chemical Properties
    Alloys
    Amalgams
    Compounds
      Chromous Fluoride
      Chromic Fluoride
      Chromyl Fluoride
      Chromous Chloride
      Chromic Chloride
      Oxychlorides
      Chromyl Chloride
      Trichromyl Chloride
      Chromium Chlorate
      Chromium Perchlorate
      Chromous Bromide
      Chromic Bromide
      Complex Halogen-halides
      Chromous Iodide
      Chromic Iodide
      Chromium Iodate
      Chromous Oxide
      Chromo-chromic Oxides
      Chromic Oxide
      Chromic Hydroxide
      Barium Chromite
      Cadmium Chromite
      Calcium Chromite
      Cobalt Chromite
      Cupric Chromite
      Cuprous Chromite
      Ferrous Chromite
      Lithium Chromite
      Magnesium Chromite
      Manganese Chromite
      Zinc Chromite
      Chromium Dioxide
      Chromium Trioxide
      Aluminium Chromate
      Ammonium Chromate
      Ammonium Lithium Chromate
      Ammonium Potassium Chromate
      Ammonium Sodium Chromate
      Ammonium Dichromate
      Ammonium Fluochromate
      Barium Chromate
      Barium Dichromate
      Barium Potassium Trichromate
      Beryllium Chromate
      Bismuth Chromate
      Bismuth Potassium Chromates
      Cadmium Chromate
      Cadmium Dichromate
      Cadmium Trichromate
      Caesium Chromate
      Caesium Dichromate
      Calcium Chromate
      Calcium Dichromate
      Cobalt Chromate
      Cobalt Dichromate
      Copper Chromates
      Cupric Dichromate
      Gold Chromates
      Iron Chromates
      Ferric Chromate
      Ferric Ammonium Chromate
      Lead Chromate
      Basic Lead Chromates
      Lead Dichromate
      Lithium Chromate
      Lithium Chlorochromate
      Magnesium Chromates
      Manganese Chromates
      Mercuric Chromate
      Mercuric Dichromate
      Nickel Chromate
      Nickel Dichromate
      Potassium Chromate
      Potassium Dichromate
      Potassium Trichromate
      Potassium Tetrachromate
      Potassium Fluochromate
      Potassium Chlorochromate
      Rubidium Dichromate
      Silver Chromate
      Silver Dichromate
      Sodium Chromate
      Sodium Dichromate
      Sodium Trichromate
      Sodium Chlorochromate
      Stannic Chromate
      Strontium Chromate
      Strontium Dichromate
      Strontium Trichromate
      Thallous Chromate
      Thallic Chromate
      Thallous Dichromate
      Thallous Trichromate
      Thallous Chlorochromate
      Uranyl Chromate
      Zinc Chromate
      Zinc Dichromate
      Zinc Trichromate
      Perchromic Acid
      Chromium Tetroxide Triammine
      Chromous Sulphide
      Chromium Tetrasulphide
      Chromic Sulphide
      Sodium Thiochromite
      Potassium Thiodichromite
      Chromic Sulphite
      Chromous Sulphate
      Chromic Sulphate
      Lithium Chromic Sulphate
      Sodium Chromic Sulphates
      Potassium Chromic Sulphates
      Potassium Chromium Alum
      Ammonium Chromium Alum
      Hydrazine Chromium Alum
      Sulphochromic Acid
      Chromous Selenide
      Chromic Selenide
      Chromic Selenite
      Chromium Nitrides
      Chromium Azide
      Chromic Nitrate
      Chromium Monophosphide
      Chromium Sesquiphosphide
      Chromic Hypophosphite
      Chromous Orthophosphate
      Chromic Orthophosphates
      Chromous Metaphosphate
      Chromic Metaphosphate
      Chromic Pyrophosphate
      Ammonium Chromi-pyrophosphate
      Potassium Chromi-pyrophosphate
      Sodium Chromi-pyrophosphate
      Chromous Thiophosphite
      Chromous Thiopyrophosphite
      Chromous Thiopyrophosphate
      Chromous Arsenide
      Chromium Sesqui-arsenide
      Chromic Arsenite
      Chromic Arsenate
      Chromium Pyroarsenate
      Chromic Thioarsenite
      Chromium Chlorantimonate
      Chromium Orthoantimonichloride
      Tetrachromium Carbide
      Pentachromium Dicarbide
      Tetrachromium Dicarbide
      Chromium Tungsten Carbide
      Chromous Carbonate
      Ammonium Chromous Carbonate
      Potassium Chromous Carbonate
      Sodium Chromous Carbonate
      Chromic Carbonates
      Chromium Thiocarbonate
      Chromous Cyanide
      Chromic Cyanide
      Potassium Chromocyanide
      Hydrogen Chromicyanide
      Ammonium Chromicyanide
      Lithium Chromicyanide
      Sodium Chromicyanide
      Potassium Chromicyanide
      Cobaltous Chromicyanide
      Cupric Chromicyanide
      Lead Chromicyanide
      Manganous Chromicyanide
      Mercury Chromicyanide
      Nickel Chromicyanide
      Silver Chromicyanide
      Zinc Chromicyanide
      Chromous Thiocyanate
      Chromic Thiocyanate
      Chromithiocyanic Acid
      Ammonium Chromithiocyanate
      Sodium Chromithiocyanate
      Potassium Chromithiocyanate
      Barium Chromithiocyanate
      Silver Chromithiocyanate
      Lead Chromithiocyanate
      Chromium Ferrocyanide
      Trichromium Silicide
      Dichromium Silicide
      Trichromium Disilicide
      Chromium Disilicide
      Chromium Aluminium Silicide
      Sodium Chromisilicates
      Chromium Silicofluoride
      Chromium Boride
      Trichromium Diboride
      Chromous Borate
      Chromic Borate
    Detection
    Estimation
    PDB 1huo-9icc

Chromic Oxide, Cr2O3






Chromic Oxide, Chromium Sesquioxide, Cr2O3, which occurs naturally in chrome ochre, exists in both amorphous and crystalline forms. The amorphous form may be obtained by calcination of a mixture of three parts of potassium chromate with two parts of sal-ammoniac; by heating potassium dichromate with sulphur:

K2Cr2O7 + S = K2SO4 + Cr2O3,

or with starch,7 and extraction of the residue with water; by gentle ignition of mercurous chromate or ammonium chromate; by heating finely divided chromium in oxygen; by ignition of chromic hydroxide, Cr(OH)3; or by heating chromic chloride in air. The substance can be obtained in the crystalline condition by passing chromyl chloride vapour through a red-hot porcelain tube when the following reaction occurs:

4CrO2Cl2 = 2Cr2O3 + O2 + 4Cl2;

by heating to bright redness a mixture of potassium chromate or dichromate with common salt; by heating potassium chromate to redness in a current of chlorine, hydrogen chloride, or hydrogen; by heating to redness potassium dichromate alone; by fusion under suitable conditions of the amorphous oxide; and by various other methods.

Amorphous chromium sesquioxide is a green powder, the tint of which depends upon the method of preparation; the colour becomes brownish on heating. The crystalline oxide forms very dark green hard crystals belonging to the trigonal system (ditrigonal scalenohedral):

a:c = 1:1.3770; α = 85° 22',

and isomorphous with aluminium and ferric oxides. Its density is 5.04 and the mean values for its specific heat are as follows:

Temperature, ° C.Specific Heat.
-191 to -80.30.0711
-76.5 to 00.1474
+2.4 to +49.30.1805


It possesses magnetic properties.

The crystal structure has been investigated by the Hull powder method of obtaining X-ray diffraction patterns. The molecule is hexahedral, consisting of three atoms of oxygen at the corners of an equilateral triangle with two chromium atoms immediately above and below the centre of the triangle, three such molecules forming a unit prism.

Though difficult to fuse by ordinary methods, chromium sesquioxide melts readily in the electric furnace. The substance is undecomposed by heat, and is not reduced by heating in hydrogen or carbon monoxide. It is only reduced by carbon when an intimate mixture is very strongly heated; it is reduced by magnesium, aluminium, and the alkali metals at high temperatures. On heating the unignited oxide at 440° C. in oxygen, Moissan obtained the "dioxide," CrO2. The sesquioxide is readily attacked and oxidised by fused potassium nitrate, chlorate, hydrogen sulphate, or permanganate; by any suitable base in presence of oxygen; or by lead dioxide or manganese dioxide in presence of sulphuric acid. Heated in dry air with potassium chloride, some chlorine is evolved; if heated alone in a current of chlorine, chromic chloride is formed, while if hydrates are present, chromyl chloride is also formed. It also yields chromic chloride when mixed with carbon and heated in a current of chlorine, or by heating with phosphorus trichloride, or by interaction with sulphur chloride.

Anhydrous chromic oxide, if it has not been strongly heated, though insoluble in water, is soluble in acids, with the formation of chromic salts; if heated to redness, a glow passes over the mass at 500° to 610° C. and the substance is generally stated to become, like the crystalline form, insoluble in water, alkalies, and acids. It is, however, soluble in acids to a certain extent. For analytical purposes, the oxide, especially after strong ignition or when in the form of chromite, is best fused with a mixture of sodium carbonate (2 parts) and potassium nitrate (1 part) for ten minutes, the resulting mass extracted with water and the insoluble residue again fused with potassium pyrophosphate.


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