Thallic oxide, Tl₂O₃

Thallic oxide or thallium sesqui-oxide, Tl₂O₃, is produced by the action of air or oxygen upon thallium at temperatures between a dull red heat and 700°, and may be prepared by dehydrating thallic hydroxide. It is obtained in the crystalline form by fusing thallous sulphate, nitrate, or chromate with excess of potassium hydroxide at a dull red hea t and extracting the mass with water, or by heating thallous nitrate to 450°. The crystals have a density of 9.97.

When hydrogen peroxide is added to a cold solution of a thallous salt containing excess of alkali hydroxide, brown thallic oxide is precipitated, which soon becomes crystalline and has a density of 9.65 at 21°. When precipitation is effected at 80°, the oxide obtained is black and amorphous, and has a density of 10.19 at 22°. At temperatures above 100°, the brown variety becomes black; each form contains a little water which is not completely eliminated below 500°.

Thallic oxide is deposited upon the anode when a neutral solution of thallous sulphate or nitrate is electrolysed between platinum electrodes and the anodic potential difference (between electrode and electrolyte) exceeds 1.43 volts. When the potential difference at the anode exceeds 1.81 volts for thallous nitrate or 2.27 volts for thallous sulphate, thallic hydroxide is deposited.

Thallic oxide melts at 725° ± 10°, begins to decompose into thallous oxide' and oxygen at 800°, and decomposes rapidly at 1000°. It is reduced by hydrogen and by carbon monoxide at a red heat. It is insoluble in water and in alkalies; it reacts with hydrochloric acid, producing thallic and thallous chlorides and chlorine; it is scarcely affected by cold sulphuric acid, and is attacked by the hot acid with the production of thallous sulphate and oxygen.