Titanium Nitride, TiN, is obtained in all reduction processes in which titanium compounds are used, if air or nitrogen is admitted; it is formed when the element is heated in nitrogen, and by the action of ammonia on the chloride. It forms lustrous, bronze-coloured leaflets, which appear blue or violet when powdered. It is extremely hard, and very stable, but is attacked by alkalies with evolution of ammonia. It reduces the oxides of copper and lead in the fused state. Ruff and Eisner have shown that it is a true nitride of the trivalent element, and that only one nitride exists.[450]
[450] Ber. 1905, 38, 742; 1908, 41, 2250.
The fluoride, TiF₃, has been obtained as an insoluble violet powder by reduction of potassium titanofluoride, K₂TiF₆, with hydrogen. From a solution it may be obtained by reduction of the same salt with zinc and hydrochloric acid, or sodium amalgam. It forms complex salts with alkali or ammonium fluoride, of which the compound (NH₄)₃TiF₆ is an example; this salt appears to be isomorphous with the analogous compounds, (NH₄)₃VF₆, (NH₄)₃CrF₆, and (NH₄)₃FeF₆. By autoxidation in the air, the solutions form fluoroxypertitanates. The complex salts appear to exist in two forms, a violet insoluble form and a green soluble modification.
The chloride, TiCl₃, is obtained anhydrous by reduction of the tetrachloride—mercury, silver, and hydrogen being the most suitable agents. Heated in hydrogen, it breaks up, forming the tetrachloride and the dichloride; heated in air it burns, evolving the tetrachloride and leaving a residue of dioxide. In solution, in combination with alkali chlorides, and as the solid hydrate, it exists in the green and violet forms. Concentrated aqueous solutions deposit the violet hexahydrate, TiCl₃,6H₂O. If such a solution be covered with ether, and saturated at 0° with hydrogen chloride, the green modification is formed, and may be extracted by the ether; it is stable only in the presence of hydrochloric acid. In the violet form, all the chlorine is in the ionic condition, and can be removed by silver nitrate; similar determinations have not been made with the green form, but it is most probable, as in the case of the analogous chromic salts, that only part of the chlorine content can be removed by silver nitrate. Böck and Moser[451] have recently described a brown substance, obtained by the action of the silent electric discharge on a mixture of hydrogen and the vapour of titanium tetrachloride at the ordinary temperature, which they believe to be a monotropic modification of the ordinary violet trichloride; the change of this brown form to the violet form is irreversible.
[451] Monats. 1912, 33, 971; 1913, 34, 1825.
The bromide and iodide resemble the chloride, but are very unstable.
The sulphate, Ti₂(SO₄)₃, is obtained as a green crystalline powder by heating with sulphuric acid the violet solution obtained by reduction of a solution of the dioxide in sulphuric acid. It dissolves in dilute acids, forming violet solutions. With alkali sulphates it forms titanium alums, which can be recrystallised from dilute sulphuric acid, and have the general formulae, properties, and crystal form of the other alums. An acid sulphate, 3Ti₂(SO₄)₃,H₂SO₄,25H₂O, is obtained by electrolytic reduction of a strongly acid solution of the dioxide in sulphuric acid, or by treating the chloride repeatedly with hot dilute sulphuric acid. It forms a crystalline violet powder, with silky lustre, insoluble in alcohol, ether, and 60 per cent, sulphuric acid; it dissolves slowly in water, forming a violet solution. When the aqueous solution is treated with excess (21⁄2 molecules) of alkali sulphate, it forms sparingly soluble double sulphates, which separate in bright blue crystals; the compounds Ti₃(NH₄)(SO₄)₅,9H₂O, and Ti₃Rb(SO₄)₅,12H₂O, have been obtained in this way.
The Use of Salts of Trivalent Titanium in Volumetric Analysis.—Owing to their powerful reducing properties, these salts have been proposed as very convenient reagents in volumetric analysis,[452] the chloride being most useful in this respect. The estimations must be carried out in absence of air, to avoid atmospheric oxidation; generally the apparatus is filled with carbon dioxide. The titanium solutions for use must also be preserved from the oxidising action of the air.
[452] See Knecht, Ber. 1903, 36, 166; Knecht and Hibbert, ibid. 1903, 36, 1549; 1905, 38, 3318; 1907, 40, 3819.
For estimation of ferric salts, an aliquot quantity is titrated directly with the titanium solution, ammonium thiocyanate being used as indicator. Ferrous salts and ferric salts in the same solution are easily estimated by titrating the former with permanganate solution, or better with hydrogen peroxide, and then estimating the total ferric salt with the titanium solution. Oxidising agents like nitrates and chlorates can be estimated in acid solution by treatment with an excess of a ferrous salt, and estimation of the ferric compound formed by means of titanium. Azo-bodies and organic dyes can be titrated directly, if soluble in hydrochloric acid, the disappearance of colour marking the end of the reaction; nitroso-compounds can also be estimated in this way. If the compound is insoluble, it may be reduced in hydrochloric acid suspension with excess of the titanium salt, and the excess then determined by means of ferric iron. Insoluble dyes may also be converted into soluble sulphonic acids, and estimated directly in solution. Ammonium persulphate may be estimated by reduction with excess of the chloride, and back titration of the excess with ferric iron. Hydrogen peroxide may be estimated directly, the disappearance of the yellow colour formed at the first addition marking the end of the reaction. Tin may be estimated by addition of an excess of a ferric salt, and estimation of the excess by titanium in the usual way. Cupric salts also may be estimated directly,[453] the end point being reached when the bluish-green solution becomes colourless.