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

Chromium Trioxide, CrO3






Chromium Trioxide, Chromic Anhydride, CrO3, was first obtained by Unverdorben by the interaction of chromyl fluoride and water. It is usually prepared by the action of concentrated sulphuric acid on solutions of a chromate, usually of potassium dichromate. Zettnow recommends that 300 grams of potassium dichromate be warmed until dissolved with 500 c.c. of water after the addition of 420 c.c. of concentrated sulphuric acid; after standing for ten to twelve hours, the liquid is poured away from the potassium hydrogen sulphate crystals that have separated, heated to 80° to 90° C., and 150 c.c. of concentrated sulphuric acid added, then water, a few drops at a time, until the chromium trioxide crystals which separate at first are just redissolved. Crystals are allowed to deposit during twelve hours, and subsequently, after concentration, they are collected upon a platinum, asbestos, or pumice-stone filter, and washed with pure nitric acid of density 1.46. The crystals are then mixed with a little concentrated nitric acid, and dried upon a porous plate, the process being repeated until the product is quite free from potassium sulphate and sulphuric acid. The potassium sulphate may also be removed as potassium alum by addition of aluminium sulphate, and the sulphuric acid separated by fusion. The crystals are freed from nitric acid by warming, preferably in a current of dry air in a tube at 60° to 80° C. Other methods of formation are by the action of concentrated sulphuric acid upon lead chromate; from barium and strontium chromates by the action of sulphuric acid or nitric acid; by the action of chlorine, or of hydrochloric acid upon silver chromate. Commercial "pure chromic acid" has been found to contain a certain amount of sandy material; it can be purified sufficiently highly even for atomic weight determinations by repeated recrystallisation, after filtration, from distilled water.

Chromium trioxide forms lustrous red needles, crystallising in the rhombic system (bipyramidal):

a:b:c = 0.692:1:0.628,

and having a mean density of 2.74. It melts at 196° C. with slight decomposition, the molten mass having a density of 2.8. It is considered to contain hexavalent chromium, being represented by the constitutional formula:



The anhydrous oxide is very soluble in water: the solubility in grams of solute per 100 grams of solution is as follows:

Temperature, °C.Grams CrO3 per 100 Grams Solution.
061.2
2062.5
4063.5
6064.65
8065.9
10067.4
12069.5


A saturated solution at 22° C., containing 62-23 per cent, of chromium trioxide, has a density of 1.7028. It is soluble in dilute alcohol, with slight decomposition in light or by heat, but is immediately decomposed by pure alcohol. Several esters of chromic acid have been described. Chromium trioxide decomposes at 330° C. with deflagration, evolving oxygen. Concentrated sulphuric acid dissolves it in the cold, probably with formation of compounds. A powerful oxidising agent, it is very easily reduced by hydrogen, hydrochloric, hydrobromic, and hydriodic acids, phosphorus, hydrogen sulphide, hydrazine, etc., and by electrolysis. Reduction is considered to take place in three stages: (a) CrVICrV; (b) CrVCrIV; (c) CrIVCrIII. With hydrogen peroxide, in presence of sulphuric acid, the reaction is probably represented by the equation:

4CrO3 + 8H2O2 + 6H2SO4 = 2Cr2(SO4)3 + 7O2 + 14H2O.

With chlorine, oxygen, ozone, or silicon, reaction is known to take place. The liberation of iodine from an acidified solution of potassium iodide can probably be represented (assuming orthochromic acid to exist in solution) by the equation:

Cr(OH)6 + 3HI = Cr(OH)3 + 3H2O + 3I.

The rates of solution of different metals in solutions of chromic acid in sulphuric acid has been studied.

Many organic compounds are readily oxidised by chromic acid.

No hydrates of CrO3 have been obtained in the solid state. In aqueous solution, however, they must be assumed to exist, and to possess the properties of acids. Numerous physico-chemical investigations have been carried out dealing with the nature of aqueous solutions of chromic acid, from which it appears that the acids H2CrO4 and H2Cr2O7 exist simultaneously in solution, dissociating into ions as follows:

H2CrO4H + HCrO4'

H2Cr2O7 ⇔ 2H + Cr2O7',

the concentration of the intermediate ion, HCr2O7', being inappreciable. The relative amounts of HCrO4' and Cr2O7' present depend upon the concentration, and upon the equilibrium constant of the reaction:

2HCrO4' ⇔ H2O + Cr2O7'.

In dilute aqueous solution the chief constituent is undoubtedly dichromic acid, H2Cr2O7, and a study of the molecular solution volumes and molecular refractivities of chromic acid, potassium chromate, and potassium dichromate affords further evidence in support of this formula for the acid.

The products of combination of chromic acid with bases are known as chromates. Besides normal chromates, several series of more complex salts, e.g. di-, tri-, and tetra-chromates, exist. These will be dealt with under the general heading.


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