With aqueous hydrofluoric acid it forms the complex H₂TiF₆, as shown by conductivity measurements, and the fact that only a slow and incomplete precipitation of the hydroxide is effected by addition of ammonia. The solution dissolves metallic oxides and carbonates, forming titanofluorides, which are for the most part isomorphous with the corresponding silicofluorides, stannofluorides, and zirconofluorides. They are very stable crystalline salts, of the general formula R´₂TiF₆; many salts of the types R´´TiF₆, R´₃TiF₇, etc., have been prepared. The most important is the potassium salt, K₂TiF₆, which crystallises from acid solutions in monoclinic tablets; from aqueous solution it separates as the monohydrate, K₂TiF₆,H₂O, isomorphous with the compounds K₂CbOF₅,H₂O and K₂WO₂F₄,H₂O. The hydrate loses its water at 100°, and melts at a white heat without decomposition. It is moderately soluble in hot, very sparingly soluble in cold water, and hence is readily recrystallised.
The tetrachloride, TiCl₄, is important, on account of its low boiling-point, for the separation and purification of titanium compounds. In physical as well as chemical properties, it resembles the chloride of a non-metallic element rather than a normal salt, and is distinguished by the ease with which it combines or reacts with the most widely differing organic compounds. It is prepared by the action of chlorine upon the element, the carbide, or a mixture of the dioxide with carbon, and by the action of chloroform or carbon tetrachloride upon the dioxide at a bright red heat. It is a colourless, transparent liquid, of density 1·76 at 0°; it freezes at -23°, and boils at 136° under atmospheric pressure. In moist air it fumes excessively, yielding hydrogen chloride by hydrolysis: TiCl₄ + H₂O = TiOCl₂ + 2HCl, and is decomposed by water with separation of the hydrated oxide. If the compound be added slowly to a large quantity of cold water, and the clear solution warmed, the oxide formed by hydrolysis remains in colloidal solution.
The chloride dissolves in fuming hydrochloric acid, forming a deep yellow solution, which becomes colourless when diluted. The solution appears to contain the unstable complex acid H₂TiCl₆, or its ions; by addition of ammonia, or organic bases, salts of the type (NH₄)₂TiCl₆ can be obtained as yellow crystalline solids. An interesting property of the chloride is its ability to form stable additive compounds with the chlorides of negative elements. A long series of these are known, of which the compounds TiCl₄,PCl₃, TiCl₄,PCl₅, TiCl₄,POCl₃, and TiCl₄,2POCl₃ may be considered examples; for the most part, they can be distilled without decomposition. A very long series of compounds, partly additive and partly condensation products, with all kinds of organic substances, is also known.
A series of oxychlorides, or basic chlorides, TiCl₃(OH), TiCl₂(OH)₂, and TiCl(OH)₃, has been obtained by addition of hydrochloric acid, in certain quantities and concentrations, to the chloride; they are amorphous solids, of which little is known.
The tetrabromide, TiBr₄, is a yellow crystalline solid, melting at 39° and boiling at 230°. Its solutions in concentrated hydrobromic acid are of a blood-red colour, and by treatment with ammonia and organic bases yield deep red crystalline salts of the type (NH₄)₂TiBr₆. The tetraiodide, TiI₄, is a reddish-brown metallic-looking solid, melting at 150°, and boiling at 360°; no complex salts are known.
The sulphates.—Many compounds of doubtful composition and individuality have been described as titanium sulphates, but relatively little is known with certainty of this class of derivatives. The most stable seems to be the titanyl sulphate, TiOSO₄, obtained as a white powder, which is slowly hydrolysed by water, by evaporating a solution of the dioxide in concentrated sulphuric acid. Under suitable conditions, e.g. when separated from acid or alcoholic solutions, it is said to form hydrated compounds; the mono-, di- and penta-hydrate have been described. When solutions of this compound in concentrated sulphuric acid are treated with concentrated aqueous solutions of alkali sulphates, salts of the formulæ (NH₄)₂TiO(SO₄)₂,H₂O and K₄(TiO)₃(SO₄)₅,10H₂O, are obtained. By treating solutions of the dioxide in a large excess of concentrated acid with solutions of calcium or strontium sulphate in sulphuric acid, salts of the type R´´Ti(SO₄)₃ are obtained; the barium salt has the formula 3Ti(SO₄)₂,2BaSO₄. All these compounds are rapidly hydrolysed by water.
Phosphoric Acid Derivatives.—Solutions of titanium compounds are completely precipitated by the addition of phosphoric acid, or soluble phosphates, even in presence of a large excess of mineral acid, but the composition of the precipitate obtained is unknown. By heating the oxide with orthophosphoric acid, a crystalline compound, TiO₂,P₂O₅, is obtained, and various alkali double phosphates may be prepared by suitable fusions.
Concentrated aqueous oxalic acid solutions readily dissolve one equivalent of titanium dioxide, forming greenish-yellow solutions which contain titanyl oxalate, TiO(C₂O₄). From alcoholic solution, this substance can be precipitated by ether as the alcoholate, TiO(C₂O₄),C₂H₅OH, a micro-crystalline precipitate soluble in water and alcohol. Titanyloxalic acid, TiO(HC₂O₄)₂,H₂O, and its salts are stable compounds; the latter are obtained by dissolving the dioxide in alkali binoxalate, the acid itself being obtained by treatment of the sparingly soluble barium salt with sulphuric acid.
Complex acids are also formed with tartaric acid, and other organic hydroxy-acids; from its solutions in these acids, the dioxide cannot be again precipitated by boiling, or by addition of alkalies.
Titanates and Pertitanates.—On account of the weakly acid character of the dioxide, stable titanates can be prepared only in the dry way. The dioxide resembles silica in the conditions under which it forms salts, and in the nature, and generally the crystallographic properties, of the products obtained. The commonest salts are the metatitanates of the formulae R´₂TiO₃ and R´´TiO₃, which are obtained by fusing the dioxide with metallic oxides and carbonates, sometimes with addition of a suitable agent to act as a crystallising medium, e.g. sodium tungstate, calcium chloride, magnesium chloride, etc. Calcium metatitanate, CaTiO₃, prepared by heating titanium dioxide with calcium carbonate in presence of calcium chloride, is identical in properties with the naturally occurring compound, [Perovskite] (q.v.). Orthotitanates of divalent metals only are known; these have the general formula R´´₂TiO₄, and are prepared by similar methods. The iron compound FeTiO₃ is also identical in properties with the mineral ilmenite, and isomorphous with the sesquioxides Fe₂O₃, Ti₂O₃. Magnesium titanates of both the ortho type (Mg₂TiO₄) and the meta type (MgTiO₃) have been prepared in the laboratory; the latter is identical with the mineral [Geikielite] (q.v.).