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

CHAPTER XVI
THE GROUP IVA ELEMENTS (continued)—ZIRCONIUM AND THORIUM

Zirconium, Zr = 90·6

The oxide zirconia was isolated as a new earth from zircon from Ceylon by Klaproth in 1789; six years later the new earth was obtained also from hyacinth, the gem-variety of the same mineral. The new oxide was examined in 1818 by Berzelius, who pointed out its resemblance to alumina, and gave it the formula Zr₂O₃; during the next two decades he thoroughly investigated its properties, preparing the element itself, and determining its equivalent. In 1857 a determination of the vapour density of the chloride, by Deville and Troost, showed that the element is really tetravalent, and that the formula ZrO₂ must be assigned to the oxide; this formula was shown to accord with the isomorphism of rutile (TiO₂) and zircon (ZrO₂,SiO₂) by Rose in 1859, whilst in the following year Marignac observed the isomorphism between the fluozirconates of zinc and nickel and the fluosilicates, fluotitanates, and fluostannates of these metals. The homogeneity of the oxide has been questioned; Svanberg in 1845 considered it to be a mixture of at least three earths, whilst Sorby and Forbes in 1869 claimed to have discovered in it a new oxide, ‘Jargonia.’ These claims, however, have been shown to have been founded on inaccurate experimental work, and the individuality of the element is at the present time considered to be well established.

Zirconium is fairly widely distributed in nature, but generally in very small quantities, and can be rightly classed as one of the rarer elements. It occurs in some silicates, and in small quantities in almost all the rare earth minerals. The most important source of the element and its compounds was until quite recently the mineral Zircon, with its gem-varieties Hyacinth and Jargon, and the large number of secondary altered zircon minerals. Since its discovery in 1892, however, the naturally occurring oxide, Baddeleyite,[455] has become increasingly important for the extraction of zirconium compounds, especially for the preparation of the pure oxide for fire-resistant materials.

The minerals may be treated by any of the usual methods. Zircon may be fused with alkali or alkali carbonate; the cooled melt is extracted with water, and the insoluble alkali zirconate decomposed by dilute acids; from the solution, zirconia is thrown down by alkalies. Potassium hydrogen fluoride and potassium hydrogen sulphate may be used for the treatment either of zircon or of baddeleyite; in the first case, the potassium fluozirconate formed may be dissolved by boiling with dilute hydrofluoric acid, and separates out readily on cooling, whilst the fluosilicate formed is not dissolved; the second treatment yields the sulphate, which may also be dissolved out by dilute acid. A very convenient method consists in reducing with carbon, either alone or in presence of lime, at the temperature of the electric arc; the infusible zirconium carbide is formed, whilst silica, if present, is reduced to the carbide, which is volatile at that temperature and is therefore driven off. The zirconium carbide may be dissolved in warm aqua regia.

In all these methods the compounds obtained are contaminated with iron, which clings to zirconium very tenaciously. Many methods have been devised for its removal. A very suitable method is the thiosulphate precipitation. Zirconia is thrown down quantitatively, mixed with sulphur, from a not too strongly acid solution by addition of sodium thiosulphate at the boiling-point, sulphur dioxide being at the same time evolved, by decomposition of the potential thiosulphuric acid formed by hydrolysis. Thorium and titanium accompany the zirconium, but iron, aluminium, and the rare earths remain in solution. Another method depends on the fact that zirconium is not precipitated from alkaline solution by ammonium sulphide in the presence of tartaric acid, whereas this reagent does not inhibit the precipitation of ferrous sulphide. Iron may also be removed from a solution in concentrated hydrochloric acid by means of ether, in which medium ferric chloride is easily soluble. Zirconium compounds may be obtained free from iron by repeated crystallisations of the oxychloride.

Zirconium forms only one series of compounds, in which the metal is tetravalent. Its chemical behaviour accords well with its position in the periodic classification. It is somewhat more electropositive than titanium, as shown by the fact that the hydroxide will not dissolve in alkalies, though zirconates may be obtained by the fusion methods; the oxide, however, is still a weak base, and the salts are to a large extent hydrolysed in solution. The formation of a stable oxychloride, which can be recrystallised without change in composition, shows clearly the strengthening of the electropositive character. It has still, however, in a high degree, the property of forming complex salts, which is characteristic of the less electropositive metals.

The group relations are borne out by the isomorphism of many related salts. The hydroxide and oxide show polymeric modifications, and the former has the usual tendency of compounds of this group to form colloidal solutions, a tendency which extends to the element itself. The metal resembles titanium in the eagerness with which it combines with other elements, especially with oxygen, nitrogen, and carbon, whilst the chloride closely resembles titanium tetrachloride in general properties, and in the ease with which it forms addition and condensation products with other substances.

The Metal.—All the difficulties which attend the attempts to prepare metallic titanium in the pure state have to be encountered in the preparation of metallic zirconium. The attempts which have been made have used the same methods, and obtained much the same kind of result as those employed in the case of titanium.[456] The reduction of potassium fluozirconate by metallic potassium, first employed by Berzelius, gives an amorphous product of unknown metal-content; it certainly contains a considerable percentage of oxygen. The monoxide is obtained when zirconia is reduced by magnesium (Winkler’s method). The reduction of the fluozirconates of potassium by means of sodium gives better results if the reaction is carried out in presence of sodium chloride in a sealed iron bomb; the product after careful washing contains 97-98 per cent. of the metal. Reduction with aluminium leads to the formation of alloys; Weiss and Neumann[457] have used these in the form of pencils as electrodes between which they pass the electric arc in vacuo, and so obtain an almost pure zirconium. The 97-98 per cent. amorphous product obtained by the sodium reduction also yields the practically pure metal when treated in this way (compare Titanium, p. 223). A very pure zirconium has been obtained by Wedekind[458] by heating the oxide with fine calcium turnings in an evacuated iron tube; the powdered product is washed, in absence of air, and heated in an evacuated porcelain tube to 800°-1000°, at which temperature the powder sinters into lumps which take a brilliant polish and contain 99·1 per cent. of the metal. Attempts to prepare a purer product from this by the method of Weiss and Neumann were unsuccessful.

The amorphous metal is a dark powder, which when washed with water on the filter paper passes through as a dark blue colloidal solution; it burns readily when heated in the air. According to Wedekind and Lewis,[459] amorphous zirconium is really the colloidal form of the metal. The fused metal is very hard (7-8, Mohs’ scale—it scratches quartz but not topaz) and very brittle; it has the density 6·4, and is of a whitish colour, with good metallic lustre on freshly broken surfaces. The atomic heat is abnormally high, being approximately 7·3; the element is paramagnetic. The melting-point was given by Wedekind and Lewis[460] as 2330°-2380°, but later work of the former author[461] gives the much lower value of 1530°, which seems more probable in view of the fact that the element cannot be employed for electric lamp filaments (see p. 322).

Metallic zirconium is highly resistant to acids; it is attacked only by hydrofluoric acid and by aqua regia. In the compact form it burns in the air only at very high temperatures, though when powdered it glows in the air at a red heat, forming probably a mixture of lower oxides. It is attacked by chlorine and by hydrogen chloride at a red heat, with formation of the chloride; fused potash also oxidises it, with evolution of hydrogen. When heated in a current of hydrogen at a red heat, it forms the hydride, ZrH₂,[462] as a velvet-black powder, which burns with an intense bluish flame in oxygen, forming the sesquioxide, Zr₂O₃. When heated in nitrogen or ammonia, amorphous zirconium yields nitrides, which are also obtained when any attempt is made to reduce zirconium compounds to the metal in air. The most definite is the compound Zr₂N₃,[462] which forms a bronze-coloured powder, resistant to all mineral acids except hydrofluoric acid. Chlorine and bromine transform this to the halide.

The hydroxide is of doubtful individuality, since on drying it loses water progressively as the temperature is raised, no definite stable compound being known; in this respect zirconium resembles titanium. When heated to 100°, its composition corresponds approximately with that required by the formula ZrO₂,H₂O, but the percentage of water varies with the history of the specimen. When precipitated by alkalies in the cold, it forms the so-called α or ortho modification, which, like the analogous titanium compound, is readily soluble in dilute acids, and glows when heated. By precipitation at the boiling point, the β form is obtained; this is less soluble in acids, and does not glow when heated. The differences between the two forms are by no means sharply marked; they are rather the limiting forms of a continuously varying series than distinct chemical individuals, and the properties of any hydroxide precipitate depend very largely on the conditions under which it is thrown down.

The hydroxide is insoluble in water, but can be obtained in colloidal solution after it has been repeatedly heated with dilute acids, which serve to break down the molecular complexes; it can be also readily obtained in colloidal solution by dialysis of the nitrate, chloride, or acetate. In these solutions it is positively charged; electrolytes precipitate it with great ease. The gel has a very high power of forming adsorption products. When thrown down from solution by soda or potash, it carries down considerable quantities of alkali, to which it clings so tenaciously that the most careful washing cannot entirely remove them. If the gel be placed in contact with an ammoniacal solution of a cupric compound, it removes the cuprammonium complex entirely from the solution, becoming itself deep blue in colour, and leaving the liquid quite clear and colourless. In colloidal solution it forms adsorption compounds with negatively charged colloids, especially metals, the gels obtained from such solutions containing both colloids.

In the presence of hydrogen peroxide, ammonia throws down an hydrated peroxide, which is also obtained[463] by electrolysis of a brine solution in which the hydroxide is suspended, oxidation being effected by the sodium hypochlorite formed. This reaction is expressed by the equation:

Zr(OH)₄ + NaOCl = Zr(OOH)(OH)₃ + NaCl

It is an endothermic compound, and is very unstable, losing oxygen on standing; by the action of acids it gives hydrogen peroxide. It dissolves in alkalies containing hydrogen peroxide; from such solutions, alcohol precipitates salts of the formula R´₄Zr₂O₁₁,9H₂O.

Zirconium oxide, ZrO₂, occurs in nature; it can be obtained in the laboratory as a voluminous white powder by ignition of the hydroxide or a suitable salt. The physical properties are described under the mineral Baddeleyite (p. 75) and in Chapter XXI (p. 323), in which an account of its technical applications is given. The melting-point is probably about 2700°; at 3000° it begins to volatilise. It dissolves readily in mineral acids, unless previously ignited very strongly; all specimens dissolve easily in hydrofluoric acid, and are readily converted by concentrated sulphuric acid into the sulphate.

When fused with metallic oxides or carbonates, it gives crystalline zirconates, of which a large number have been prepared; the calcium compound, CaZrO₃, is said to be isomorphous with perovskite, CaTiO₃.

A suboxide, ZrO, of somewhat doubtful individuality,[464] is said to be obtained when the dioxide is reduced with magnesium; it forms a dry black powder, which is not attacked by acids, and when heated glows, forming the dioxide. A sesquioxide, Zr₂O₃, is obtained as a greenish powder when the hydride is burnt in oxygen; when heated in the air, it oxidises very slowly, forming the dioxide.

An oxysulphide, ZrOS, is obtained when the anhydrous sulphate is heated in a current of sulphuretted hydrogen; it is a bright yellow powder, which ignites spontaneously in the air. No disulphide is known. The carbide, ZrC, is obtained, according to Moissan and Lengfeld,[465] when the oxide is heated with carbon in any proportions, excess of carbon separating on cooling as graphite; the process is hastened by addition of lime. It is a hard, dark-grey solid, and is a very good conductor of electricity. When heated in oxygen or nitrogen, it reacts readily, forming the oxide and nitride respectively; halogens attack it at quite low temperatures (250°-400°), forming the halide compounds, which are indeed best prepared in this way. Strong mineral acids, with the exception of hydrochloric acid, attack it, and fused alkalies dissolve it readily.

The fluoride, ZrF₄, is best obtained by the action of anhydrous hydrofluoric acid on the chloride. It forms a white crystalline mass, which readily sublimes, and is soluble in hydrofluoric acid; from the solution it crystallises as the trihydrate, ZrF₄,3H₂O. The anhydrous substance is very slightly soluble in water in the cold; when warmed, it hydrolyses, forming the hydroxide. The solution in hydrofluoric acid dissolves metallic carbonates and oxides, forming the numerous fluozirconates or zirconofluorides.

There are many types of these compounds, of which the potassium salt, K₂ZrF₆, is the most important. The solubility of this salt increases very rapidly with the temperature; 100 parts of water dissolve, at 15°, 1·41 parts, at 100°, 25 parts of the compound. It has been frequently used for the purification of zirconium compounds, for the preparation of the element, and for analytical determinations. Other potassium salts, K₃ZrF₇ and KZrF₅,H₂O, are obtained by using a large excess of potassium fluoride and zirconium fluoride respectively. The ammonium compounds are analogous in composition to the potassium salts, but the sodium salt, Na₅ZrF₉, is obtained from mixtures of the components in all proportions; on account of its very low solubility, it can be obtained by double decomposition of the potassium salt with sodium chloride. Of the salts with divalent metals, the types R´´ZrF₆,xH₂O and R´´₂ZrF₈,xH₂O, are the most common.

The chloride, ZrCl₄, is known, on account of the ease with which it hydrolyses, in the anhydrous state only. It can be obtained by all the usual methods, of which perhaps the action of chlorine on the carbide, and of carbon tetrachloride, or a mixture of chlorine and sulphur monochloride, on the oxide, are the most convenient; an interesting method consists in heating the oxide with phosphorus pentachloride in a closed tube at 190°. It forms a volatile white sublimate, which fumes strongly in air, and reacts vigorously with water; it is soluble in ether. It forms a series of addition compounds with ammonia and organic bases, as well as with the chlorides of non-metallic elements; warmed with phosphorus pentachloride, it forms a stable solid, 2ZrCl₄,PCl₅, which melts at 240°, and boils at 345°. With organic compounds, especially with esters, acids, and phenols, it forms a long series of addition and condensation products, of which the compounds ZrCl₄(C₆H₅·COOC₂H₅)₂ and ZrCl₂[O·C₆H₅·CHO]₂ may be taken as examples. By addition of organic bases to a solution of the chloride in alcoholic hydrogen chloride, double chlorides of the type (C₅H₅NH)₂ZrCl₆ are obtained.

The oxychloride, ZrOCl₂,8H₂O, separates in characteristic tetragonal prisms when the tetrachloride is dissolved in water or hydrochloric acid of any concentration. It is readily soluble in water and alcohol, but sparingly soluble in hydrochloric acid, from which therefore it is generally recrystallised. According to Chauvenet,[466] it effloresces in dry air, forming the hexahydrate, ZrOCl₂,6H₂O; when dried in a vacuum, it forms the hydrate, ZrOCl₂,3¹⁄₂H₂O, whilst the dihydrate, ZrOCl₂,2H₂O, is obtained by heating at 100°-105° in hydrogen chloride. When the dihydrate is heated to 230°, it forms another basic chloride, ZrOCl₂ZrO₂,[467] which is stable up to 600°; above this temperature, it breaks up, forming the volatile tetrachloride, and leaving a residue of the dioxide.

By repeated evaporation of the oxychloride with small quantities of water, a ‘metazirconium chloride’ is obtained, which dissolves in water to a colloidal solution, and on dialysis yields a colloidal solution of ‘metazirconic acid.’

The bromide, ZrBr₄, very closely resembles the chloride; when treated with water it forms the oxybromide, which separates from solution according to the conditions in various hydrated forms, of which the commonest is the octohydrate, ZrOBr₂,8H₂O. The iodide, ZrI₄, is a very reactive body, which closely resembles the preceding; it forms an oxyiodide, ZrOI₂,8H₂O.

Zirconyl chlorate, ZrO(ClO₃)₂,6H₂O, is obtained from the sulphate by double decomposition with barium chlorate; it forms very soluble colourless needles. Alkali iodates or iodic acid throw down a voluminous oxyiodate, very sparingly soluble, like the corresponding ceric and thorium salts, in water and acids.

The Sulphates.—When zirconium dioxide is dissolved in concentrated sulphuric acid, and the excess of acid removed by heating to 400°, the ‘neutral’ sulphate, Zr(SO₄)₂, remains. The compound dissolves in dilute sulphuric acid to form solutions which contain various ‘complexes’ as shown by conductivity measurements, and the behaviour towards oxalic acid. Whilst solutions of the nitrate or chloride give immediate precipitates with this reagent, solutions of the ‘sulphate’ give no precipitate, or at most a very gradual one; moreover, addition of sulphuric acid or of alkali sulphates to other zirconium salts inhibits the oxalate precipitation. These facts are explained by regarding the ‘neutral’ sulphate, Zr(SO₄)₂,4H₂O, as zirconylsulphuric acid, ZrOSO₄,H₂SO₄,3H₂O, which in solution ionises to 2H^. and ZrOSO₄,SO₄´´. This conclusion is confirmed by the fact that whilst in solutions of the chloride in hydrochloric acid, zirconium goes on electrolysis to the cathode, on addition of sulphuric acid to the solution it travels to the anode. The anhydrous compound and the hydrate are extremely soluble in water, but much less readily soluble in dilute sulphuric acid. Probably in solution more complex salts are formed by further hydrolysis, for by addition of concentrated alkali sulphate solution in the cold, double salts of the formula Zr₂O₃(RSO₄)₂,8H₂O are obtained. When the solution is kept for some time at 39°-40°, a basic sulphate, 4ZrO₂,3SO₃,14H₂O, separates slowly. When concentrated solutions are boiled, a salt, 2ZrO₂,3SO₃,5H₂O, separates as a crystalline precipitate; in contact with water it slowly hydrates itself to the compound 2ZrO₂,3SO₃,14H₂O; when heated to 300°, it becomes anhydrous without further change. Various other basic, acid and complex salts have also been described.

The nitrate, Zr(NO₃)₄,5H₂O, separates from concentrated solutions of the oxide in nitric acid by evaporation over sulphuric acid and sodium hydroxide; it is believed to be a zirconylnitric acid, ZrO(NO₃)₂,2HNO₃,4H₂O by analogy with the sulphate. When its aqueous solutions are warmed, basic salts separate. Kolbe[468] has described an additive compound with antipyrine, Zr(NO₃)₄,6C₁₁H₁₂ON₂, which is soluble in water, and melts at 217°-218°.

When phosphoric acid or a soluble phosphate is added to a solution of a zirconium salt, zirconium phosphates of doubtful composition are thrown down; by fusion methods, various double phosphates have been prepared. A hypophosphate, Zr(PO₃)₂,H₂O, has recently been obtained by Hauser and Herzfeld[469] by precipitation. The same authors have prepared a hypophosphite, which is sensitive to light. When hypophosphorous acid, H₃PO₂, is added to a solution of zirconium nitrate, a precipitate is obtained, which dissolves in excess of the acid; by addition of alcohol to the clear solution, the hypophosphite, Zr(H₂PO₂)₄,H₂O, is thrown down in colourless, highly refracting prisms, which on exposure to sunlight for a short time become deep violet, without further perceptible change.

Zirconium carbonate has recently been obtained by Chauvenet.[470] Addition of sodium carbonate precipitates a basic orthocarbonate, ZrCO₄,ZrO₂,8H₂O, soluble in excess; when dried in vacuo, the precipitate loses water, forming the dihydrate, ZrCO₄,ZrO₂,2H₂O. When the latter compound is treated with carbon dioxide at a pressure of 30-40 atmospheres, the neutral orthocarbonate, ZrCO₄,2H₂O, is formed. When the compounds are heated, other basic salts are obtained.

Zirconyl oxalate, ZrO,C₂O₄, is obtained in the hydrated form when oxalic acid is added to a zirconium salt in the presence of hydrochloric or acetic acid. It is a white powder, soluble in oxalic acid, and easily hydrolysed by water. If an aqueous solution of oxalic acid be saturated with zirconium hydroxide, an acid oxalate, ZrOH(HC₂O₄)₃,7H₂O, is obtained on evaporation. Double oxalates are readily obtained by dissolving zirconium hydroxide in solutions of alkali hydrogen oxalates, the general form being Zr(C₂O₄R´)₄,xH₂O. The tartrate precipitated when tartaric acid is added to a zirconium salt in solution probably has the cyclic structure, Tartrate as shown by the great rise in the specific rotatory power of solutions of alkali oxalates on addition of zirconium compounds. The precipitate dissolves readily in alkalies, and various double alkali tartrates have been prepared; the potassium salt, ZrO(C₄H₄O₆K)₂,3H₂O, is analogous to the thorium alkali tartrates. The solubility in alkalies is of great importance for the separation of iron and zirconium.

Atomic Weight of Zirconium.

—The value of this constant is not very accurately known. The International Committee has adopted the value 90·6, but there is some uncertainty as to the value of the decimal fraction. Berzelius in 1825 employed the analytical sulphate method, and found Zr = 88·47. The numbers of Hermann (1844), obtained by the analysis of the oxychloride, 2ZrOCl₂,9H₂O,[471] were very discordant, the mean giving the value 89·56. Marignac in 1860 analysed the potassium salt, K₂ZrF₆; this he heated with strong sulphuric acid, the residue being ignited until all the zirconium sulphate was transformed to oxide; the weighed mixture was then freed from potassium sulphate, and the residual oxide dried and weighed. From the three ratios K₂ZrF₆ : ZrO₂, K₂ZrF₆ : H₂SO₄, and K₂SO₄ : ZrO₂, he obtained the mean values 90·02, 91·55, and 90·68 respectively. Weibull in 1881-1882 determined the ratios Zr(SO₄)₂ : ZrO₂ and Zr(SeO₄)₂ : ZrO₂ by ignition of the sulphate and selenate respectively; he obtained the values 89·55 and 90·81.

Bailey carried out a series of analytical sulphate determinations in 1890, obtaining the mean value 90·656. Brauner criticises the method on the ground that the preparation of the pure neutral anhydrous sulphate is almost impossible; the sulphate heated to 400° is not yet anhydrous, so that Bailey’s result, on this ground, is probably too low. Venable in 1898 analysed the oxychloride; he claimed to have obtained the compound ZrOCl₂,3H₂O, by heating the crystallised salt at 100°-125° in hydrogen chloride, a method which Chauvenet (loc. cit.) has found to lead to the dihydrate, ZrOCl₂,2H₂O. His value was 90·803.

Detection and Estimation.

—The following reactions may be employed to distinguish zirconium:

(1) The oxalate precipitated from neutral or faintly acid solution dissolves readily in excess of oxalic acid; the oxalates of thorium and of the rare earth elements are practically insoluble under these conditions. The fluoride also dissolves in excess of hydrofluoric acid or of alkali fluoride, behaviour characteristic of this element alone among the group.

(2) By fusion with sodium carbonate in the oxidising flame, a bead is obtained, which, when dissolved in boiling hydrochloric acid, forms a solution which gives a voluminous precipitate on addition of disodium hydrogen phosphate, if zirconium is present. Iron, aluminium, titanium, thorium, and rare earths have no influence on the test.[472]

(3) A solution of a zirconium salt in hydrochloric acid gives an orange colouration with curcuma paper. Ferric and titanium salts, if present, must be reduced by means of zinc before the test is applied.

The estimation of zirconium is complicated by the difficulty of separating it from the accompanying elements. The solubility of the oxalate in oxalic acid allows of a rapid and easy separation from thorium and rare earth elements, so that iron, aluminium, and chromium only remain to be removed. Iron may be separated by the thiosulphate method, or other processes mentioned on p. 338; when free from that element, zirconium may be separated from aluminium and chromium by precipitation with alkali iodate in presence of the least possible excess of acid. The precipitates in the thiosulphate and iodate methods may be washed, and ignited directly to the dioxide, which is weighed as such; if the zirconium is left after separation in solution, it may be precipitated with ammonia,[473] and after washing and drying, ignited and weighed as dioxide.

Thorium, Th = 232·4

The name Thoria (thorina) was proposed by Berzelius in 1817 for what appeared to be a new earth, but which in 1824 was recognised as a basic yttria phosphate. In 1828 a new mineral was discovered by Esmark near Brevig in Norway; to the oxide isolated from this, Berzelius gave the name thoria, from its resemblance to the substance he had obtained in 1817. The homogeneity of the new element was questioned by Bergmann in 1857, and also by Bahr in 1862, but the conclusions of those authors have been shown to be quite unfounded.

Thorium occurs in traces in a large number of common minerals, and in varying quantities in most of the uranium and rare earth minerals. Its occurrence in monazite, and the distribution of the latter mineral, have already been dealt with; the commercial treatment of monazite is described in Chapter XVIII. The oxide forms the chief constituent in Thorite, with its gem-variety Orangite, and the various secondary minerals, and in the mineral Thorianite, in which the only other important constituent is uranous oxide. The extraction from these minerals is a comparatively simple matter. Decomposition is easily effected by hydrochloric or sulphuric acid, thorianite dissolving easily also in nitric acid; the solutions obtained, after appropriate treatment to remove silica, excess of acid, etc., are treated with sulphuretted hydrogen, to remove lead, bismuth, and similar foreign metals, and freed from the rare earths by the carbonate, oxalate, or sulphate methods. The last depends on the fact that thorium sulphate and its hydrates are much less soluble than corresponding compounds of the rare earth elements; the first two on the fact that thorium salts dissolve readily in excess of alkali carbonates or oxalates, whilst the rare earth compounds are much less easily soluble.

Thorium, like zirconium, forms only one series of salts, in which the metal is tetravalent. The formula ThO was originally put forward by Berzelius for the oxide, from its resemblance to the ceria and yttria oxides, and its general occurrence with these. The true formula was deduced, when the valency of zirconium had been decided by the vapour density experiments of Troost and Deville, in 1857, from the isomorphism of zircon and thorite, and the close relationship between the compounds of the two elements, especially among the double fluorides, and was confirmed by a determination of the specific heat of the metal by Nilson in 1883.

In its chemical relations, the element resembles zirconium, though, as is to be expected from the high atomic weight, it shows a much more marked electropositive character, approaching in this respect the elements of the yttrium group. The oxide has no longer acid properties, and the neutral salts, though they hydrolyse readily and are therefore acid to indicators in solution, may be recrystallised unchanged from aqueous solution. The tendency to form double salts is still present, though diminished; the oxalate is soluble in a large excess of alkali oxalate, but not in oxalic acid, and the double fluorides are less numerous and varied than those of zirconium and titanium. On the other hand, it forms a well-crystallised and characteristic series of double nitrates, R´₂Th(NO₃)₆, isomorphous with the analogous ceric salts. In the behaviour of its sulphate it differs markedly from zirconium, and closely approaches the rare earth elements. The hydroxide has the characteristic tendency to form colloidal solutions and gels.

Thorium is peculiar, among the elements which have been considered, through its property of giving characteristic radiations, and disintegrating with formation of a whole family of new elements; or, as it is commonly expressed, through its radioactive properties.[474] The element has a half-life period of the order of 4 × 10¹⁰ years; in the course of decay, it gives rise to mesothorium 1, which is rayless, but decays to mesothorium 2, with its product radiothorium, both of which give powerful radiations. Mesothorium 1 of course occurs in all thorium-containing minerals, and may be separated from monazite by addition of a barium compound during the sulphuric acid decomposition; in consequence of the powerful radiating properties of its products, it is itself of considerable importance, and proposals for extracting it from monazite in the preparation of the thorium nitrate of commerce have been put forward (see p. 276).

Mesothorium appears to be chemically identical with radium; since monazite, like all other thorium-bearing minerals, contains uranium and radium, the latter element is separated with the mesothorium, and indeed, having a very much larger half-life period, constitutes by far the greater part of such ‘mesothorium’ preparations. On account of the great activity of the mesothorium products, the best preparations from monazite, though estimated to contain only 1 per cent. of mesothorium to 99 per cent. radium, are said to be four times as active as pure radium compounds. The chemical identity of the two products seems to preclude any possibility of determining the physical properties and constants of mesothorium.

The element radiothorium, which was discovered by Hahn in 1905, in the mineral thorianite, is chemically identical with the parent element thorium, but can be separated by means of the intermediate element, mesothorium 1. The latter is readily separated by the sulphate precipitation, and the radiothorium to which it gives rise may be separated by precipitation with ammonia. Thorium is also chemically identical with ionium, the parent of radium, and the thorium nitrate of commerce therefore contains important quantities of ionium—important that is, in view of the high radiating power of the latter element. The study of these relationships constitutes one of the most important and interesting fields in the province of radioactivity.

The Metal.—Elementary thorium has not yet been obtained in the pure state, owing to the ease with which it forms compounds and alloys with all the common elements, and to its great affinity for oxygen; the high melting-point also increases the difficulty of obtaining the pure metal. Berzelius attempted to reduce the alkali double fluorides and double chlorides with sodium or potassium; Nilson carried out the same reaction in a closed iron cylinder, but his product still contained 20 per cent. of thoria. Reduction of the oxide with magnesium is never complete, and the carbon method gives only a mixture of carbide and metal. Electrolytic methods give no better results, since the metal liberated at the cathode always encloses oxide and other impurities. Moissan and Hönigschmid in 1906, by heating the carefully purified anhydrous chloride with sodium in a sealed glass tube from which air and moisture had been removed, claim to have obtained a product containing only 3 per cent. of the oxide. The element has recently been prepared in leaf form by forcing the amorphous product into the bore of a copper tube, hammering into sheets, and removing the copper by dilute nitric acid.[475]

The amorphous impure metal is a dark grey powder, of specific gravity 11·3; the hammered and strongly heated leaf has the density 12·16. It burns readily in air with great brilliance, and when finely powdered ignites if crushed or rubbed. When heated in the electric furnace, it melts, according to von Bolton,[476] at about 1450°; von Wartenburg[477] found the melting-point to be about 1700°; the fused beads resemble platinum in physical properties. It is somewhat resistant to acids, dissolving easily only in aqua regia, and more slowly in fuming hydrochloric acid. It combines directly when heated in sulphur or halogens, and in nitrogen and hydrogen.

The hydride, ThH₄, is best obtained by heating the metal in hydrogen, an energetic reaction taking place at a red heat. Winkler observed that a mixture of the dioxide with magnesium absorbs hydrogen readily when heated. The hydride is a stable greyish-black powder, not attacked by water, but dissolving readily in hydrochloric acid, with evolution of hydrogen. The nitride, Th₃N₄, is prepared by heating the metal in the gas, or the carbide in a stream of ammonia. It is a brown powder, decomposed by water with evolution of ammonia and formation of the dioxide. The azide has been used for purposes of detection and estimation, since in boiling solution it is hydrolysed with separation of the hydroxide; zirconium and ceric salts also show this reaction, but the rare earth salts give no precipitate.

The hydroxide, Th(OH)₄,xH₂O, is precipitated from solutions of thorium salts by alkalies or ammonia, as a gelatinous white precipitate, insoluble in excess. It dissolves readily in mineral acids or in alkali carbonates. Hydrogen peroxide and ammonia throw down an hydrated peroxide, Th₂O₇; from neutral solutions hydrogen peroxide alone throws down peroxy-salts, which contain acid groups. This peroxide may also be obtained by the action of sodium hypochlorite or hydrogen peroxide on the hydroxide, as in the case of the zirconium compound. It readily gives up oxygen, passing into the more stable peroxide, ThO₃. Since in neutral or faintly acid solutions zirconium and the rare earths give no precipitate with hydrogen peroxide, the reaction is extremely useful in the detection and estimation of thorium.

Thorium dioxide, ThO₂, is obtained by the ignition of the hydroxide or of suitable salts as a white powder, of which the properties and appearance depend largely on the method and temperature used in its formation. Whilst the residue obtained by ignition of the nitrate is an extraordinarily voluminous and light flaky mass, the sulphate yields a dense thick powder; the nitrate was therefore always preferred in the manufacture of incandescent mantles (q.v.), as it was thought that the oxide obtained from it was the most suitable for illumination. In the crystalline form the oxide has been obtained in the laboratory by fusion with borax and with potassium phosphate. The first method gives tetragonal crystals, probably isomorphous with those of rutile and cassiterite; the phosphate fusion is said to give cubic crystals (see p. 74). The oxide is insoluble in acids, but can be transformed into the sulphate by evaporation with concentrated sulphuric acid, or fusion with alkali bisulphate. It does not liberate carbon dioxide when fused with alkali carbonates.

By repeated evaporation with small quantities of acids, thoria can be transformed into a gel soluble in water (thorium meta-oxide). The sol is an opalescent fluid, orange-red by transmitted light, and contains small quantities of the acid employed. The hydroxide may also be obtained in this form by carefully washing it, and boiling with small quantities of acids, or with thorium or other salts, or even by long continued washing with pure water; similarly, continued dialysis of thorium salts eventually yields such gels. The colloid is positively charged, and resembles the zirconium oxide gel in its relation to negatively charged colloids. The gel is easily precipitated by electrolytes.

Ignited thorium oxide has found considerable application in recent years a catalyst in the preparation of ketones by the contact method of Sabatier and Senderens.[478] By passing mixtures of the vapours of appropriate acids over the catalyst heated to the necessary temperature, good yields of the required ketones are obtained.[479]