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 -80.3	0.0711
-76.5 to 0	0.1474
+2.4 to +49.3	0.1805

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.