Titanium Dioxide, TiO₂

Titanium Dioxide (Titanic Oxide), TiO₂, is naturally trimorphous, and occurs as the three minerals rutile, anatase, and brookite.

Rutile, the most important of the three, crystallises in tetragonal prisms, and has a density of 4.18 to 4.25. It is lustrous, of a reddish brown colour, has a hardness of 6 to 6.5 on Moh's scale, and is isomorphous with cassiterite (SnO₂), zircon, and thorianite.

Anatase is also tetragonal, but its crystals differ from those of rutile; it has a density of 3.82 to 3.95, a hardness of 5.5 to 6, and is brown or black in colour.

Brookite crystallises in rhombic prisms, having a density of 4.12 to 4.23 and a hardness of 5.5 to 6.

The three natural forms of titanic oxide are said to be isotrimorphous with stannic oxide. They can all be produced artificially. Rutile is formed when amorphous titanic oxide is fused in a bead of borax, microcosmic salt or potassium carbonate, or melted with stannic oxide; anatase is produced by the decomposition of titanium cyanonitride at red heat by water-vapour; brookite results when a mixture of titanium tetrachloride vapour, steam, and carbon dioxide is passed through a red-hot tube, and also when the vapour of titanium tetrachloride is decomposed by heated lime. It has been shown by Hautefeuille that anatase is the stable form of titanic oxide below 860° C., brookite between 860° C. and 1040° C., rutile above 1040° C.

Amorphous titanium dioxide may be obtained by precipitating an aqueous solution of titanic chloride with ammonia; by fusing finely powdered rutile or titanic iron ore with potassium carbonate, decomposing the resulting titanate with hydrofluoric acid in a platinum dish, separating and recrystallising the potassium titanifluoride formed, and decomposing its hot aqueous solution with ammonia; or by igniting titanic iron ore in a current of chlorine and hydrochloric acid, when the iron is eliminated as volatile ferric chloride and titanic oxide remains, thus:

2FeTiO₃ + 4HCl + Cl₂ = 2FeCl₃ + 2TiO₂ + 2H₂O.

Amorphous titanium dioxide is a white powder which, like stannic oxide, turns yellow and then brown when heated, and fuses in the oxyhydrogen flame. Its heat of formation when the metal is oxidised by sodium peroxide is:

Ti + O₂ = TiO₂ (amorphous) + 215,600 calories;

and when the finely divided metal is burnt in oxygen, 218,400 calories.

Titanic oxide is insoluble in water and dilute acids, but dissolves when heated with sulphuric acid, potassium hydrogen sulphate, or pyrosulphate, forming a sulphate of quadrivalent titanium. It produces titanates when fused with alkalis or alkali carbonates. Thus titanic oxide possesses both basic and acidic properties, and so differs from silica, which is not basic.