Metatitanic Acid, TiO(OH)₂

Metatitanic Acid, TiO(OH)₂, insoluble in hydrochloric acid, is obtained by various means from hot solutions. Thus when an aqueous solution of titanic chloride is boiled the meta-acid is gradually precipitated; this precipitation takes place the more readily in presence of sulphuric acid or a soluble sulphate owing to the inferior stability of titanic sulphate; similarly, the soluble product of fusion of a titanic mineral with potassium hydrogen sulphate yields metatitanic acid when boiled with water. The hydrolysis of a slightly acidified solution of titanic sulphate which occurs on boiling is quantitative, and is employed for the estimation of titanium.

Metatitanic acid is also formed when a hot acid solution of a titanic compound is precipitated by alkali, as well as by the action of nitric acid of density 1.25 on metallic titanium. Although practically insoluble in dilute acids, metatitanic acid forms an opalescent, colloidal suspension with water, whence it is precipitated by acids and salt solutions.

When ignited, the meta-acid passes into the anhydride without evolving light.

The assumption of the colloidal state by metatitanic acid suggests that the molecules of this substance are complex. Colloidal titanic acid has been obtained in other ways. On adding hydrochloric acid to the mass obtained by fusing titanic oxide with alkali, Rose obtained a jelly consisting of the hydrated oxide; whilst Knop found that the white precipitate formed on adding ammonia to a solution of titaniferous magnetite, in presence of tartaric acid added to keep the iron in solution, swelled to a transparent jelly when washed with water. Graham, also, obtained colloidal titanic acid as a hydrosol, by the dialysis of a hydrochloric acid solution of the ortho-acid, and this on concentration yields a hydrogel. Thus it appears that in its colloidal properties titanic acid closely resembles silicic acid; moreover, the question arises whether the simple formulae Ti(OH)₄ and TiO(OH)₂ can properly be applied to the two hydrates of titanic oxide which are respectively soluble and insoluble in hydrochloric acid, or whether the molecules of these two substances, being very complex, approximate to rather than correspond exactly in composition with these formulae. According to Carnelley and Walker, who have examined the influence of the gradual increase of temperature upon these and other hydrates, the regular form of the curves obtained with titanic as well as silicic hydrate is against the existence of definite hydrates of any evident degree of stability.