Titanates

It has already been seen that titanic hydroxide is amphoteric, dissolving in strong acids to form salts of these acids, and in strong alkalis to form titanates. Titanic hydroxide is thus an acid of the weakest description, and is unable to displace carbonic acid from aqueous solutions of carbonates. Alkali titanate, however, is formed when titanic oxide is fused with alkali carbonate, and the reaction is reversible when the fusion is carried out in an atmosphere of carbon dioxide, the conditions of equilibrium at 900°-1100° C. and 1 atmosphere CO₂ being represented by the following equations:

0.29 Na₂CO₃ + 0.29TiO₂ ⇔ 0.71 Na₂TiO₃ + xCO₂
0.65 K₂CO₃ + 0.65TiO₂ ⇔ 0.35 K₂TiO₃ + xCO₂.

Titanic oxide resembles silica in its power of displacing carbon dioxide, and this power diminishes through zirconium to thorium with increase in the basic power of the dioxide.

Certain mineral titanates are known, and can be prepared artificially.

Perofskite is calcium metatitanate, CaTiO₃, containing small quantities of ferrous, manganese, and magnesium oxides; it occurs in the Urals and Arkansas in yellow to iron-black rhombic crystals, having a density of 4.0 and hardness 5.5 on Moh's scale, and can be produced artificially by the strong ignition of a mixture of titanic oxide, lime, and potassium carbonate.

Ilmenite or titanic iron is ferrous metatitanate, FeTiO₃; it occurs in Norway, Canada, and elsewhere in black rhombohedral crystals, having a sub-metallic lustre, density 4.5 to 5, and hardness 5 to 6 (Moh's scale).

Pyrophanite, isomorphous with ilmenite, is MnTiO₃, and geikielite is MgTiO₅.

Pseudobrookite is ferric orthotitanate, Fe₄(TiO₄)₃; it consists of brown or black orthorhombic crystals of density 4.39 and hardness 6 (Moh's scale).

Titanite or sphene, calcium titanisilicate, CaTiSiO₅, may be regarded as the calcium salt of dimetasilicic acid, H₂Si₂O₅, in which a molecule of silica has been exchanged for one of titanic oxide. It occurs, embedded in granite, gneiss, and mica-schist, in yellow, green, red, grey, brown, or black monoclinic crystals of density 3.54 and hardness 5 to 5.5 (Moh's scale). It has been obtained artificially by fusing together a mixture of silica, titanic oxide, and calcium chloride.

Titanic oxide is known partially to replace silica in such minerals as biotite, augite, and olivine; and, according to Groth, the oxides ZrO₂, ThO₂, and SnO₂ may also replace silica isomorphously. Neither titanic oxide nor any of these other oxides is known, however, to form such complex acids as are formed by silica.

Alkali titanates may be prepared by fusion, by boiling titanic acid with aqueous alkali, or by precipitating a hydrochloric acid solution of titanic acid with alkali carbonate. Thus potassium metatitanate, K₂TiO₃, is obtained as a yellow, fibrous mass when titanic oxide is fused with potassium carbonate; the hydrated salt K₂TiO₃.4H₂O crystallises on evaporating a solution of titanic acid in caustic potash; the trititanate K₂Ti₃O₇.2H₂O is precipitated by potassium carbonate from a hydrochloric acid solution of titanic acid, whilst K₂Ti₃O₇.3H₂O is formed when K₂TiO₃ is boiled with water, and the hexatitanate K₂Ti₆O13.2H₂O is produced by the action of hydrochloric acid on the dihydrated trititanate.

The three sodium salts, Na₂Ti₂O₅, Na₂Ti₃O₇, Na₄Ti₃O₈, all of which are crystalline but insoluble in water, are produced by the fusion of titanic oxide with sodium carbonate, together with sodium tungstate, to promote crystallisation, and treatment of the cooled product with water.

In addition to these alkali salts and artificial minerals, a number of meta-, ortho-, and poly-titanates of other metals have been prepared, generally by fusing titanic oxide with the' carbonate, chloride, or sulphate of the metal. They are as follow: Sr₂Ti₃O₈, Ba₂Ti₃O₈, Mg₂TiO₄, MgTiO₃, Zn₂TiO₄, ZnTiO₃, ZnTi₃O₇, Zn₃Ti₂O₇, Zn₄Ti₅O14, Mn₂TiO₄, MnTiO₃, CoTiO₃, NiTiO₃, Fe₂TiO₄, FeTiO₃.