Titanium Trichloride, TiCl₃

Titanium Trichloride, TiCl₃, was first obtained in the anhydrous state by Ebelmen, who passed a mixture of the vapour of the tetrachloride and hydrogen through a red-hot tube and found the trichloride deposited in the cold part of the tube. Large quantities of titanium trichloride have been prepared by Goerges and Stahler according to this reaction, by the employment of a porcelain tube heated electrically by means of a wire spiral within it and cooled externally by water. The reaction began above 600° C., at 785° C. the percentage decomposition was 21.7, at 1200° C. it was 94.3. The trichloride is also formed when the tetrachloride is heated in a sealed tube with reduced silver at 180°-200° C.:

TiCl₄+Ag ⇔ TiCl₃+AgCl,

when the same substance is heated with mercury, and when electric sparks are passed through a mixture of the vaporised tetrachloride and hydrogen. Titanium trichloride is produced in solution by reducing a hydrochloric acid solution of the tetrachloride with zinc, or electrolytically. The anhydrous trichloride forms dark violet scales which are not volatile, but when heated to 440° C. decompose into the tetrachloride which is vaporised, and the dichloride which remains behind. By passing the silent electric discharge through a mixture of titanium tetrachloride vapour and hydrogen, Bock and Moser obtained a brown substance, which appeared to be a labile, allotropic form of the trichloride, since it changed into the ordinary form of the latter when heated in a vacuum to 150°-200° C. The reaction is not reversible, the brown and violet trichlorides being monotropic modifications. When the trichloride is heated in air the tetrachloride vaporises, and the residue then consists of the dioxide. The trichloride is deliquescent, and forms a reddish violet solution with water, whilst its alcoholic solution is green.

From hydrochloric acid solution titanium trichloride separates in violet crystals of the hexahydrate TiCl₃.6H₂O; it is thus obtained by the electrolytic reduction of the tetrachloride. When a concentrated aqueous solution of the violet hydrate is covered with a layer of ether, and saturated with hydrogen chloride while it is kept cool, an unstable green hydrated form, TiCl₃.6H₂O, separates in crystals, which are reconverted into the violet form when the excess of hydrogen chloride is washed out with ether. Thus titanium trichloride resembles the trichlorides of chromium and vanadium in existing in two forms.

When hydrogen chloride is passed through concentrated aqueous solutions of titanium trichloride with the corresponding alkali chlorides, the double salts, TiCl₃.2RbCl.H₂O and TiCl₃.2CsCl.H₂O, separate. They are green, but yield violet solutions owing to dissociation into their component salts.

It was observed by Ebelmen that titanium trichloride is a powerful reducing agent, as shown by its action on salts of copper and iron. The capabilities of this compound as a reducing agent have been exhaustively studied by Knecht and Hibbert, who have devised a number of volumetric processes depending on its use. The reagent is prepared by dissolving the metal in hydrochloric acid, or by metallic or electrolytic reduction of the tetrachloride; it may now be purchased in 20 per cent, aqueous solution. When a dilute aqueous solution of titanium trichloride, which is violet, is exposed to the air its colour fades, and titanic hydroxide is gradually precipitated; the presence of sufficient hydrochloric acid, however, prevents this precipitation. Titanium trichloride is a more powerful reducing agent than stannous chloride, and the following reductions are effected by this substance: concentrated nitric acid, with violent action, to nitrous fumes and ammonia; dilute nitric acid, in presence of hydrochloric acid, to a stage intermediate between nitric and nitrous oxides; chlorates and perchlorates to chlorides; hydrogen peroxide to water; persulphates to sulphates; sulphurous acid to hyposulphurous acid (H₂S₂O₄), which is thus conveniently prepared; ferric salts to ferrous salts; cupric salts to cuprous salts and copper; mercuric chloride to mercurous chloride, only on boiling; chromic and permanganic acids to chromic and manganous salts. The use of titanium trichloride in qualitative analysis, on account of its characteristic reactions, is recommended by Monnier. Titanium trichloride reacts with a gold solution like stannous chloride, producing colloidal gold, analogous to purple of Cassius. This reaction will detect one part of gold in twenty million parts of water. Many organic substances also undergo interesting reductions in presence of titanium trichloride.

For volumetric analysis the commercial solution of titanium trichloride is diluted twenty times and stored under hydrogen. It is standardised by means of ferric solution, with which it reacts quantitatively thus:

TiCl₃ + FeCl₃ = TiCl₄ + FeCl₂.

The end of the reaction is shown by sulphocyanide or methylene-blue solution used as an external indicator. Titanium trichloride solution may be employed for estimating not only iron, but also copper, tin, chromium, hydrogen peroxide, chlorates, perchlorates, nitrates, hydroxyl- amine, hyposulphites (hydrosulphites), and numerous organic compounds, including dye-stuffs.