The dichloride, TiCl₂, is obtained in the impure state as a black powder by decomposition of the trichloride at a red heat: the tetrachloride is formed at the same time, and volatilises.[444] According to v. d. Pfordten,[445] it is obtained by reduction of the tetrachloride by sulphuretted hydrogen or sodium amalgam in the cold. The latter author states that it dissolves in alcohol or water in absence of air to a dark brown solution; Friedel and Guérin, however, state that it acts energetically on these solvents with evolution of hydrogen, forming a yellow solution. When heated in the air it burns, evolving fumes of the tetrachloride and leaving a residue of the dioxide. The iodide, TiI₂, has been obtained by Defacq and Copaux[446] by reduction of the tetraiodide with silver or mercury, as a black, lustrous, infusible sublimate. It is insoluble in organic solvents, but reacts with water and aqueous alkalies, and is readily attacked by acids. Hydrogen at a bright red heat reduces it to amorphous titanium.
[444] Friedel and Guérin, Compt. rend. 1875, 81, 889; 1876, 82, 509, 872.
[445] Loc. cit.
[446] Compt. rend. 1908, 147, 65.
Compounds of Trivalent Titanium.[447]
[447] Compounds of trivalent titanium are frequently referred to in English chemical and technical literature as ‘Titanous Compounds,’ the salts of the tetravalent element being tacitly recognised as ‘Titanic Compounds.’ In view of the existence of compounds of divalent titanium, to which the name ‘Titanous Compounds’ might be more logically applied, the former nomenclature cannot be regarded as altogether satisfactory, and it is therefore not adopted here.
These salts are obtained when the element is dissolved in hydrochloric and sulphuric acids, and by reduction of the compounds of tetravalent titanium in solution by means of zinc and hydrochloric acid, or by electrolysis. According to Diethelm and Forster[448] the reduction may also be effected by hydrogen in presence of finely divided platinum. The salts have strong reducing properties, transforming nitro-bodies to amines and decolourising azo-derivatives very rapidly; they reduce unsaturated bodies, and reduce dyes to the leuco-bases; they reduce sulphurous acid to sulphur, precipitate gold, silver and mercury from their salts, and reduce cupric and ferric salts to cuprous and ferrous compounds respectively. The salts are green or violet in solution, showing the phenomenon of hydrate-isomerism which is exhibited by the chromic salts; they are to some extent hydrolysed in aqueous solution, as shown by the acid reaction of the chloride. They resemble the salts of ferric iron and aluminium in giving precipitates of basic salts when boiled with sodium acetate or sodium formate, and in giving no precipitate with alkalies in the presence of organic hydroxy-acids. Ferrocyanide and ferricyanide give brown precipitates.
[448] Zeitsch. physikal. Chem. 1908, 62, 129.
The hydroxide, Ti(OH)₃,xH₂O, is thrown down as a dark precipitate with strong reducing properties; it attacks water with evolution of hydrogen, forming the dioxide; when an aqueous suspension is shaken with air, autoxidation occurs, hydrogen peroxide being formed. The sesquioxide, Ti₂O₃, has been prepared by Friedel and Guérin[449] by heating the dioxide to a white heat in a current of hydrogen and titanium tetrachloride; it forms black lustrous crystals, isomorphous with hæmatite. The sulphide, Ti₂S₃, is best obtained by reduction of the disulphide, at a moderate temperature, in a stream of hydrogen or nitrogen, but is also prepared by the action of a mixture of carbon disulphide and sulphuretted hydrogen on the dioxide at a high temperature. It is a dark grey metallic powder, stable towards air, water, alkalies and dilute acids.
[449] Loc. cit.