(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]
[472] Biltz and Mecklenburg, Zeitsch. angew. Chem. 1912, 25, 2110.
(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.
[473] On account of the adsorption of alkalies by the zirconium oxide gel, potash or soda is never used for this precipitation.
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.