CHAPTER XV

THE GROUP IVA ELEMENTS--TITANIUM

The oxides zirconia and thoria were generally classed among the rare earths by the earlier chemists. This view was based partly upon the mode of occurrence of the oxides, which are very generally associated in nature with rare earths, and were believed to be equally sparingly distributed, and partly on fallacious chemical analogies. Thus Berzelius regarded thoria as a monoxide, ThO, and classed it with the other earth oxides, magnesia, lime, ceria, lanthana, etc., to all of which the general formula RO was assigned. Zirconia was regarded as a sesquioxide, Zr₂O₃, analogous to alumina, Al₂O₃, which in turn showed many points of resemblance to the rare earths. The introduction of the periodic classification, and a wider knowledge of the chemical properties of the oxides, have gradually altered the older conceptions, and zirconia and thoria are now only classed under the head ‘Rare Earths’ when that term is used in its widest sense. More generally, the term is restricted to the oxides of the cerium and yttrium elements, which, whilst they cannot all be placed in Group III of the table, yet constitute a series with properties which entitle them to be considered in that relation.

The elements which fall into group IVA of Mendelejeff’s classification are titanium, zirconium, cerium, and thorium; the elements of lower atomic weight, carbon and silicon, are placed by some authors in Group IVB, by others in Group IVA. It is a feature of the periodic system that the members of the A and B families show great differences in the end groups, I and VII, II and VI, which disappear as the middle groups are approached; in group IV the families A and B show only slight differences in properties, corresponding to their amphoteric character and electrochemical indifference, so that the elements carbon and silicon may be placed as well in the one as in the other. Generally they are placed in family B.

In its tetravalent condition, titanium shows a close relationship to silicon; the similarity is manifested by the ease with which the dioxide replaces silica in many minerals, and the isomorphism of many titanates with corresponding silicates. Yet the strengthening of electropositive character, which always accompanies the change in atomic weight in descending a vertical column of the table, is very apparent in the case of titanium, and its ability to form salts in the tetravalent state is a very important property. This strengthening of the electropositive character is still more marked in the case of the succeeding elements. The salts of zirconium are highly hydrolysed in solution, but they are considerably more stable than those of tetravalent titanium; the ceric salts show the same change, whilst thorium salts are comparatively stable in solution, and can be recrystallised from water without change. Zirconium hydroxide will not dissolve in alkalies, though zirconates may be obtained in the dry way; thorium hydroxide shows no acidic properties whatever.

The change in electrochemical character is accompanied by corresponding changes in physical properties of the elements and their compounds. With the exception of cerium, which has a very low melting-point (623°), the elements fuse only at high temperatures; titanium is the most refractory, zirconium melts at over 1500°, and thorium at about 1450°. The boiling-points of the chlorides rise as the series is descended; titanium tetrachloride boils at 136°, zirconium and thorium chlorides at 400°-450° and 950° respectively; zirconium chloride partly sublimes, whilst ceric chloride decomposes when heated.

The elements of Group IVA are distinguished from the rare earth elements by their much less strongly marked electropositive character. This is apparent not only in the amphoteric nature of the oxides, and in the ease with which the salts are hydrolysed in solution, but in the more pronounced tendency to the formation of complex salts. The complex fluorides of the type K₂RF₆ are peculiarly characteristic, and in the case of titanium and zirconium have been very important for purposes of analysis and atomic weight determination. The solubility of zirconium and thorium salts in excess of alkali oxalate or carbonate is also in harmony with the less pronounced electropositive character of these elements. The sulphates of titanium and zirconium appear to be of complex constitution, whilst their neutral chlorides cannot be obtained from solution. As is to be expected from its high atomic weight, thorium approaches most nearly to the rare earths in chemical properties; thus it forms stable double nitrates of the type R₂Th(NO₃)₆ and its salts, especially the sulphate, resemble those of the rare earth elements in their solubility relations.

The elements titanium, zirconium, and thorium are distinguished also by the fact that they form no definite hydroxides. The precipitates thrown down from solutions of the salts, on addition of alkali, are hydrated oxides, which lose water continuously when dried, giving rise to no definite chemical individuals until constant weight is reached with the anhydrous oxides. The hydroxides have the further characteristic, common also to the other members of Group IV, of readily forming colloidal solutions and gels, a property possessed to some extent also by the elements themselves, and particularly by zirconium, which, when reduced from its compounds, shows a great tendency to go into colloidal solution merely on washing. Highly characteristic also is the property of forming ‘meta’-oxides (acids) and ‘meta’-salts, which is common to all the Group IV elements which have solid oxides.

In presence of hydrogen peroxide, alkalies throw down characteristic hydrated peroxides, which have definite acidic properties in the case of titanium: the zirconium compound is less strongly acidic, the cerium compound shows no tendency to salt formation, whilst if hydrogen peroxide be added to a neutral or faintly acid solution of a thorium salt, the precipitate is a peroxy-salt, containing some acid grouping, _e.g._ SO₄,NO₃.

With regard to valency, the elements in the typical compounds are tetravalent. Titanium forms three series of salts, in which the element is respectively di-, tri-, and tetravalent; salts of the first two series have powerful reducing properties, and the compounds in which the metal is tetravalent are most stable. Zirconium is always, with the doubtful exception of its peroxy-compounds and the lower oxides, tetravalent. Cerium, as already described, can form two series of compounds, in which it is respectively tri- and tetravalent; thorium, like zirconium, is always tetravalent.

~Titanium~, Ti = 48·1

Though generally classed among the rare elements, titanium is probably at least as widely distributed in nature as most of the common metals. It occurs as the dioxide in small quantities in all the common silicate rocks and minerals, and in traces in the animal and vegetable kingdoms; the element has been identified in the sun and in many stars, and has been found in meteorites. Probably the commonest mineral in which the element occurs in quantity is ilmenite, or titaniferous ironstone, which occurs in enormous quantities in many parts of the world (see p. 57). The pure dioxide occurs in the three forms Rutile, Brookite, and Anatase (_q.v._), in which it is said to be isotrimorphous with tin dioxide. Other important titanium minerals are Perovskite, Titanite or Sphene, the Euxenite series, and other minerals of the tantalo-columbate group (see Part I).

The commercial sources of titanium compounds are the minerals rutile and ilmenite. These may be opened up by fusion with alkali or alkali carbonate; the residue after extraction with water is dissolved in acid, and precipitated with ammonia; the mixture of iron and titanium oxides thrown down may be separated by one of the methods outlined on p. 339. Fusion with potassium bisulphate has also been employed. A very satisfactory method is that of Stähler,[437] in which the ore is fused with carbon in the electric furnace. The carbides so obtained are heated in a stream of chlorine, when the volatile titanium tetrachloride distils over, and may be obtained quite pure by redistillation; by appropriate methods, the required compounds may be obtained from this. (See also pp. 326-7.)

[437] _Ber._ 1904, ~37~, 4405; 1906, ~38~, 2619.

_The Metal._--The difficulty of isolating metallic titanium in the pure state is very great, on account of its great affinity for nitrogen, oxygen, hydrogen, carbon, etc., the ease with which it forms alloys with all the common metals, and the extremely high melting-point; in consequence, it is only within recent times that the element has been obtained in a state approximately approaching purity, and the accounts given of its physical properties vary very widely.

Berzelius prepared an impure titanium (Ti = 86 per cent.) by reduction of potassium titanofluoride with potassium; the method was modified by Wöhler, who heated a tube containing two boats, of which one was filled with the fluoride, the other with sodium, reduction being effected by the sodium vapour. Many authors have attempted the reduction of titanium tetrachloride by means of hydrogen. By heating the tetrachloride with sodium in a cast iron bomb, Nilson and Pettersson obtained a product containing 95 per cent. of the element. Reduction of the dioxide by means of sodium, magnesium, silicon, or aluminium has not been found to yield good results, by reason of the ease with which titanium alloys with these elements. Reduction of the dioxide with carbon yields good results only when precautions are taken to avoid the formation of the compound which the element so readily forms with carbon and nitrogen. Moissan[438] found that if temperatures high enough to decompose this compound were used, the product contained as the only impurity carbon, which could be partly removed by fusing with the dioxide; the product then contained 98 per cent. of titanium.

[438] _Compt. rend._ 1895, ~120~, 290.

The element has been obtained in the fused condition by Weiss and Kayser,[439] who pressed the amorphous form into sticks, under a pressure of 70,000 atmospheres, and employed these as pencils for the electric arc _in vacuo_; the metal fused, forming globules on the ends of the electrodes, which were detached after the apparatus had been allowed to cool.

[439] _Zeitsch. anorg. Chem._ 1910, ~65~, 388.

The amorphous element is a dark powder, resembling finely divided iron (Ferrum reductum), of density 3·5-3·6. The specific heat rises rapidly with the temperature, so that the atomic heat has the values 5·40 between 0° and 100°, 6·18 between 0° and 210°, 7·13 between 0° and 300°, and 7·77 between 0° and 440°. The amorphous element is said to be paramagnetic.

The fused carbonaceous product of Moissan formed an extremely brittle mass, with a shining white lustre on the fractured surface, sufficiently hard to scratch quartz and steel; its density was determined as 4·87. The product of Weiss and Kayser was also extremely hard and brittle; when rubbed against steel, it gave bright sparks. Its density was found to be 5·174, and the heat of combustion for the gram-atom, 97·79 K.

The amorphous variety is fairly stable in air, but burns vigorously when heated in air, oxygen, or halogens. Heated in nitrogen or ammonia, it reacts vigorously, forming the nitride TiN; if carbon is present, a peculiar substance of uncertain composition, known as _titanium cyanonitride_, is formed. This substance is also obtained when air is passed over a heated mixture of the dioxide with coke, and is found in blast-furnaces in which ores containing small quantities of titanium are worked; it forms brilliant red cubes, which are extremely hard and resistant to acids. This substance, as well as the nitride itself, yields ammonia when heated in steam, and has been proposed as a medium for ‘fixing’ atmospheric nitrogen (see p. 337).

The amorphous element also absorbs hydrogen, when heated in the gas, but no definite hydride is known. It combines when heated with almost all the known non-metals, and forms alloys with all the common metals. Moissan[440] claims to have prepared a compound as hard as diamond by heating titanium with boron in the electric furnace. The element attacks steam at a red heat.

[440] _Loc. cit._

The element is fairly resistant to acids in the cold, but is readily attacked, with evolution of hydrogen, on warming. Hot dilute hydrochloric acid gives the trichloride; but dilute sulphuric acid is variously reported to give the di- and tri-salt. Hot nitric acid oxidises it readily, forming the so-called metatitanic acid. Hydrofluoric acid attacks it very readily, forming the tetrafluoride.

COMPOUNDS OF DIVALENT TITANIUM.

The compounds of divalent titanium show resemblances to those of divalent iron, chromium and vanadium, but on account of the great difficulty of preparing them and protecting them from oxidation, little is known of their properties and behaviour; even the colour of the salts in solution is not known with certainty. In its divalent state; the element does not appear to act as a strongly positive metal; the salts in solution are said to show an acid reaction, whilst the precipitates thrown down with alkali oxalates and acetates are soluble in excess of the precipitant, forming deeply coloured solutions. With sodium phosphate the soluble salts give a bluish-black precipitate, with potassium ferrocyanide and ferricyanide, dark brown and reddish-brown precipitates respectively. They are distinguished from salts of the higher oxides of titanium by the brown colouration produced by potassium thiocyanate in presence of hydrochloric acid.[441]

[441] v. d. Pfordten, _Annalen_, 1886, ~234~, 257; 1887, ~237~, 201; see also _Ber._ 1889, ~22~, 1485.

The _hydroxide_ is thrown down from solutions by addition of alkali, alkali carbonate, alkali cyanide, or ammonium sulphide, as a black precipitate. It cannot be transformed to the corresponding oxide by drying, since it attacks the water with evolution of hydrogen, forming the dioxide. The _monoxide_, TiO, has probably never been obtained in the pure state; it is formed by reduction of the dioxide with zinc or magnesium. Moissan[442] obtained it in the form of black prismatic crystals by treating the dioxide with the calculated amount of charcoal in the electric furnace. The _sulphide_, TiS, is an extremely stable compound; it can be prepared by heating the higher sulphides in a stream of hydrogen to a very high temperature, and then forms pseudomorphs after these.[443] It is a dark red metallic mass, which reacts in the air only when heated, forming the dioxide; dilute acids and alkalies have no action on it, concentrated nitric acid oxidises it slowly.

[442] _Loc. cit._

[443] See v. d. Pfordten (_loc. cit._); Thorpe, _Chem. News_, 1885, ~51~, 260.

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,_x_H₂O, 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._

_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.

[453] Moser, _Chem. Zeitg._ 1912, ~36~, 1126.

COMPOUNDS OF TETRAVALENT TITANIUM

The compounds of tetravalent titanium are much more stable than the compounds in which the element has a lower valency, and are very readily formed from them. The dioxide is amphoteric in character, and acts as a weak acid as well as a weak base; the salts it forms with acids as well as those it forms with bases are very easily hydrolysed, with separation of the hydrated oxide. Titanium salts, therefore, can only be held in solution by a considerable excess of acid. The tendency to the formation of complex compounds is very pronounced, particularly in the case of the fluoride, oxalate and tartrate.

The _hydroxide_, or _hydrated oxide_, is capable of existing in two modifications, according to the conditions under which it is thrown down, though the two can hardly be said to be very definitely differentiated. The α or ortho form is obtained as a voluminous white precipitate by the addition of ammonia or alkali hydroxide in the cold to a freshly prepared solution of a titanium salt. It is insoluble in water and alcohol, but dissolves readily in dilute mineral acids, and to some extent also in dilute alkalies. The water content is very variable, and no definite hydrate or hydroxide can be prepared; when the substance is heated, it loses water continuously, and at a definite temperature glows, doubtless by reason of some polymeric change. If it be maintained for some time at a temperature somewhat below the normal temperature of glowing, this phenomenon no longer occurs when the temperature is further raised.

The β modification, or metatitanic acid, as it is called, is obtained by hydrolysis of the salts by boiling, or by addition of alkali at 100°, as a fine white precipitate. It is almost insoluble in dilute acids and alkalies, but dissolves in water to a colloidal solution; when heated it does not glow. The β form is also obtained when the metatitanates are treated with water; these compounds hydrolyse very readily, but the precipitated dioxide carries down alkali by adsorption.

The _dioxide_, TiO₂, occurs crystalline in nature in the three forms Rutile, Brookite, and Anatase, all of which can be prepared by laboratory methods;[454] the amorphous form is obtained by ignition of the hydrated oxide, and of suitable salts. The oxide melts at 1560°, forming a mobile (?) liquid of density 4·1; for the physical properties, see the accounts of the naturally occurring forms in Chapter V. When heated in a current of hydrogen or carbon monoxide, it gives rise to intermediate oxides, Ti₃O₄, Ti₇O₁₂, etc., which are not very well known, and are of doubtful individuality. It reacts when heated in chlorine, and with many non-metallic chlorides, forming the tetrachloride; with carbon disulphide at high temperatures it gives the disulphide, ammonia at a red heat forms the nitride. It is exceedingly resistant to acids, but is attacked slowly by boiling sulphuric acid, more quickly by fused bisulphate.

[454] See p. 79; also Hautefeuille, _Ann. chim. phys._ 1863, [iv.], ~4~, 129.

_Titanium disulphide_, TiS₂, is obtained in the pure state when a mixture of the vapour of the tetrachloride and sulphuretted hydrogen is led through a strongly heated porcelain tube. It is a fairly stable substance, forming metallic crystals which yield the dioxide when heated in air. When heated in a stream of hydrogen or nitrogen it yields one or other of the lower sulphides according to the temperature employed. It is not attacked by water, but dissolves in acids, and is decomposed by boiling potash, forming a titanate; it is insoluble in alkali sulphides.

The _carbide_, TiC, was prepared by Moissan by heating the oxide with carbon in the electric furnace; any excess of carbon separates on cooling as graphite. It has the density 4·25, and resembles the fused element in appearance. It dissolves in nitric but not in hydrochloric acid.

_Titanium tetrafluoride_, TiF₄, is obtained by the action of fluorine on the element or the carbide, and by the action of anhydrous hydrofluoric acid on the element or the tetrachloride. It is a white powder, and boils at 284°; it is very hygroscopic, and dissolves easily in alcohol and water, showing little tendency to form basic salts. From the concentrated aqueous solution it separates as the dihydrate, TiF₄,2H₂O; basic salts are obtained only by repeated evaporation with water. The anhydrous compound forms additive products with ammonia and with pyridine.

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._).

The compounds prepared in this way are all insoluble in water, doubtless by reason of the slowness with which such compact solids can be attacked; they dissolve easily in dilute acids. The weakly acid character of titanium dioxide is shown by the fact that if the fusion with metallic carbonates be carried out in vessels so adjusted that the carbon dioxide exerts a pressure of one atmosphere, a condition of equilibrium is reached, in which a considerable part of the carbonate remains unattacked. In the presence of hydrogen peroxide, however, the acidic properties are considerably strengthened, and the per-salts can be obtained in the wet way.

Addition of hydrogen peroxide to a neutral or acid solution of a titanium compound gives a yellow colour, due to the formation of a peroxide, TiO₃,aq. Such solutions have the same oxidising powers as hydrogen peroxide, but do not give the blue colouration with chromium salts. By treatment of the solution with dilute alkalies, an hydrated peroxide is thrown down, which, when dried over phosphoric anhydride, has the formula TiO₃,3H₂O, and forms a yellow, horny mass. The freshly precipitated peroxide dissolves in acids and alkalies; from the latter solutions, by addition of hydrogen peroxide and alcohol, pertitanates of various composition can be obtained, of which the following are examples : Na₂O₂,TiO₃,3H₂O; (NH₄)₂O₂,TiO₃,H₂O₂; BaO₂,TiO₃,5H₂O; K₂O₄,K₂O₂,TiO₃,10H₂O, etc. These salts lose hydrogen peroxide when treated with dilute acids, and their constitutions are unknown.

An interesting series of fluoroxypertitanates has been prepared by oxidation of the solution of titanium dioxide in hydrofluoric acid with hydrogen peroxide, and addition of metallic fluorides. The ammonium compound, (NH₄)₃TiO₂F₅, crystallises in yellow octahedra, isomorphous with the salts ZrF₄,3NH₄F and CbOF₃,3NH₄F. The potassium salt, K₂TiO₂F₄, crystallises well from water, and is easily obtained in the pure state; various barium salts are known. Similar compounds with oxalic acid have also been prepared.

~Atomic Weight of Titanium.~--The first reliable determinations of this constant were carried out by H. Rose in 1829. He determined the ratio TiCl₄ : 4AgCl, by dissolving the pure tetrachloride, weighed in sealed glass bulbs, in water in closed flasks, precipitating the dioxide by ammonia, and weighing the silver chloride obtained by adding silver nitrate to the filtered and acidified solution. He obtained the values 48·27 and 48·13, which agree very well with the accepted value, 48·1. In the same year, Mosander, using a method not specified, obtained the value 47·15. Determinations carried out by Pierre (1847) and Demoly (1849) led to widely varying results. A series of determinations carried out by Thorpe during the years 1883-1885 gave consistent results. The method used was the precipitation of silver halide from the tetrachloride and tetrabromide, and the mean value of seven series of determinations gave the number 48·08. The International Committee have adopted this result as the basis of the accepted value, 48·1.

~Detection.~--The specific reactions for the element are frequently masked by the presence of other metals, especially of iron, columbium, tantalum, and vanadium, which most frequently accompany it in nature, and from which a quantitative separation is frequently very difficult (see p. 338). The most characteristic reactions are the following:

(1) Reduction in acid solution by means of tin or zinc gives an intense violet colour, due to the formation of trivalent titanium salts. Various colours are given by vanadium, columbium, and tungsten, so that the test is not decisive if these are present.

(2) Hydrogen peroxide in acid solution gives a reddish-yellow colour, which is very delicate, and is used in quantitative estimation; vanadium compounds interfere.

(3) In sulphuric acid solution, characteristic colours are obtained with many phenolic compounds; thymol gives a blood-red colour which is exceedingly intense.

(4) A very characteristic and intense colour is given in acid solution on the addition of 1:8-dihydroxynaphthalene-2:4-disulphonic acid (chromotropic acid).

The methods for the estimation of titanium are given in Chapter XXII.