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.
0	61.2
20	62.5
40	63.5
60	64.65
80	65.9
100	67.4
120	69.5

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) Cr^VI → Cr^V; (b) Cr^V → Cr^IV; (c) Cr^IV → Cr^III. 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:

H₂CrO₄ ⇔ H^• + 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.