Chemical Properties of Titanium

The relation of titanium to other elements of the fourth and other groups has been fully discussed in the introductory chapter. It will be useful to set forth here the types of compound formed by this element, since these are varied, in accordance with the fact that titanium may show bi-, ter- or quadri-valency.

	TiII	TiIII	TiIV
Halides	TiCl2	TiF3 TiCl3	TiF4; K2TiF6; TiCl4; (NH4)2TiCl6; TiCl3OH; TiCl2(OH)2TiCl(OH)3; Ti2O3Cl2
		TiBr3	TiBr4; (NH4)2TiBr6; TiCl2Br2; TiClBr3
	TiI2	TiI3	TiI4
Oxides and hydroxides	TiO; Ti(OH)2?	Ti2O3; Ti(OH)3.xH2O	TiO2; Ti(OH)4; TiO(OH)2; Titanates
Sulphides	TiS	Ti2S3	TiS2
Sulphates	?	Ti2(SO4)3; acid and double salts, including an alum	Ti(SO4)2; acid and double salts
Nitrides	-	TiN	Ti3N4
Superoxide - TiO3aq., TiO3.3H2O or Ti(OH)4.H2O2 Pertitanates - e.g. (NH4)2O2.TiO3.H2O2.

Titanium forms mono-, sesqui-, and di-oxides, TiO, Ti₂O₃, and TiO₂ respectively. To the monoxide there possibly corresponds a sulphate; the sesquioxide is basic, giving rise to salts; the dioxide is acidic, forming titanates, but also possesses feebly basic properties and forms some salts.

In addition to these, several mixed or saline oxides have been described. The oxide Ti₃O₄ or TiO.Ti₂O₃, which may be called titano-magnetite, was obtained by Rose, and by Piccini and Marino by the dry reduction of titanic oxide, TiO₂; whilst Ti₃O₅(Ti₂O₃.TiO₂) is said to be formed when titanic oxide is heated in a reducing atmosphere with hydrogen chloride. It has been alleged that Ti₇O₁₂ is the final product of the reduction of titanic oxide by hydrogen, but the latter may be Ti₂O₃. Whether these supposed saline oxides are single substances or mixtures must be regarded as doubtful.

A characteristic of titanium is its proneness to combine with nitrogen to form a nitride. Such a compound was obtained many years ago by heating TiCl₄.4NH₃ alone or in a current of ammonia; it was copper- coloured and was mistaken for the element until Wohler proved that it contained nitrogen and assigned to it the formula Ti₃N₄. Wohler also obtained a dark blue substance with a coppery lustre, to which he attributed the formula TiN₂, by igniting titanic oxide in a current of ammonia. Schneider, however, showed that the supposed nitride Ti₃N₄ contained oxygen; and he was unable to obtain Wohler's dinitride, whose existence had previously been denied by Friedel and Guerin. Until recently, therefore, the existence of only one nitride of titanium, TiN or Ti₂N₂, was recognised; but titanic nitride, Ti₃N₄, has now been obtained by Ruff and Treidel.

Titanium is detected in the dry way by the borax or phosphate bead reaction. The bead is not coloured when heated in the outer flame, but in the inner flame shows a yellow colour while hot, and the characteristic violet colour of titanous compounds when cold. The violet colour appears more quickly when a particle of zinc or tin is introduced as a reducing agent. If a small quantity of an iron salt is fused into the titanium bead a red colour is produced in the inner flame. Titanium compounds do not colour the Bunsen flame, but the arc or spark spectrum of this element contains a large number of lines, chiefly in the blue and green.

The titanium present in minerals such as rutile is best brought into solution by fusion with potassium pyrosulphate, when the following reaction takes place -

TiO₂ + 2K₂S₂O₇ = Ti(SO₄)₂ + 2K₂SO₄.

The fused mass can then be dissolved in cold water. Fusion with sodium carbonate produces sodium metatitanate, insoluble in water, but soluble in acids.

From a titanic solution, the hydroxide,Ti(OH)₄, easily soluble in acids, is precipitated by potassium hydroxide in the cold. From the hot solution alkali precipitates less soluble TiO(OH)₂. Alkali acetates on boiling cause the precipitation of TiO(OH)₂ from titanium tetrachloride solution, owing to hydrolysis. Moreover, all titanic salts are hydrolysed when their dilute solutions are boiled; consequently titanium can be separated from aluminium, iron, and chromium by fusion with potassium pyrosulphate, K₂S₂O₇, dissolving the melt in much cold water and then heating to boiling. Tartaric and citric acids prevent this hydrolysis by the formation of complex salts. Boiling with sodium thiosulphate causes a similar hydrolysis, the thiosulphuric acid formed decomposing with separation of sulphur, thus - TiCl₄ + 2Na₂S₂O₃ + 3H₂O = TiO(OH)₂ + 4NaCl + 2H₂SO₃ + 2S.

This reaction also serves to separate titanium from iron. Potassium ferrocyanide gives a brown precipitate in slightly acid titanic solutions.

Hydrogen peroxide, when added to a slightly acid solution of a titanic salt, produces a yellow or orange-red colour according to concentration, owing to the formation of TiO₃. This is a very delicate reaction for titanium, but vanadic acid gives a similar colour with hydrogen peroxide.

When a solution of a titanic compound in hydrochloric acid is reduced by tin or zinc a violet solution of titanium trichloride is produced which yields a violet precipitate, turning white by oxidation. The violet solution becomes pink on dilution. This is also a delicate colour reaction for titanium. Quadrivalent titanium in moderately dilute solution gives with dihydroxymaleic acid a brilliant orange colour; tervalent titanium gives a dull yellowish brown colour, changed to orange by oxidation.

Titanic oxide is distinguished and separated from silica by heating it with a mixture of hydrofluoric and sulphuric acids. Under these conditions the silica is changed into gaseous silicon tetrafluoride, and is thus eliminated, whilst titanic oxide remains. Indeed, titanium tetrafluoride is quantitatively changed into titanic oxide by evaporation with" sulphuric acid.

Titanium is estimated gravimetrically in the form of its dioxide, TiO₂. This is precipitated in the hydrated form by adding ammonia to a titanic solution, or by the hydrolysis of the acetate or sulphate, as explained above.

It may also be estimated colorimetrically, by means of the hydrogen peroxide reaction; this is a method suitable for the determination of small quantities of the element occurring in rocks. The titanium is brought into solution as sulphate by fusion with potassium pyrosulphate, K₂S₂O₇, and extraction with water; then to a suitably diluted volume of this solution, in presence of free sulphuric acid, hydrogen peroxide is added, and the colour produced is matched with standard titanic sulphate solution. Hydrofluoric acid interferes with this reaction, but small quantities of iron do not; chromic, molybdic, and vanadic acids must, however, be absent. A solution of thymol in concentrated sulphuric acid is a more delicate colorimetric reagent for titanium than hydrogen peroxide.

Lastly, titanium may be estimated volumetrically by reducing its acid solution to the tervalent condition by means of zinc, and titrating with standard permanganate solution, with standard ferric chloride solution in presence of potassium thiocyanate, or with standard methylene blue.