The formate and acetate can be obtained in the form of neutral salts by the action of the acids on the hydroxide; by double decomposition, amorphous precipitates of basic salts are obtained. With tartaric acid stable complex compounds are formed, as shown by the fact that alkalies will not precipitate the hydroxide from a solution in presence of that reagent, and by the elevation of the specific rotatory power. Many complex salts are known, the simplest having the composition ThO(C₄H₄O₆R´)₂,8H₂O, where R´ = K,Na,NH₄; these are obtained by dissolving thorium hydroxide in concentrated solutions of alkali hydrogen tartrates. Thorium acetylacetone, Th(C₅H₇O₂)₄, is precipitated by addition of ammonia to an aqueous solution of the nitrate mixed with acetylacetone dissolved in ammonia; the solid is recrystallised from alcohol, and melts at 171°.
Atomic Weight of Thorium.
—The value adopted by the International Committee (1914) is 232·4, but most of the determinations carried out within the last thirty years show considerable discrepancies. The earlier work of Berzelius (1829) and Chydenius (1861) led to very widely varying results, and for the same reason little reliance can be placed on the results of Delafontaine (1863) and Hermann (1864). In 1874 Cleve determined the constant by ignition of the sulphate, obtaining the mean values 234·03 and 233·97; the figure 234 based on these results was for many years accepted as the true atomic weight. A series of determinations carried out by Nilson in 1882 led to much lower results. He employed the sulphate ennea- and octohydrates, first dehydrating these, and then igniting to oxide, and showed that Cleve’s value must be too high on account chiefly of the hygroscopic nature of the ignited oxide, which increases in weight when kept; but his own values show considerable discrepancies. The ratio Th(SO₄)₂,9H₂O-ThO₂ : ThO₂ (enneahydrate converted to oxide) gave the figure (corrected to vacuo) 232·51, whilst the ratio ThO₂ : 2SO₃ (anhydrous sulphate to oxide) gave 232·16; the ratio Th(SO₄)₂ : 9H₂O (hydrate to anhydrous salt) gave, however, 233·75. The value obtained for the ratio ThO₂ : 2SO₃ for anhydrous sulphate prepared from the octohydrate was 232·49 (corrected to vacuo). Five years later, Krüss and Nilson prepared the anhydrous sulphate from the pure octohydrate, and ignited this to the oxide. The ratio ThO₂ : 2SO₃ gave as a mean of very concordant results the figure 232·49.
Brauner criticises these values on the ground that no details are given as to the temperature required to obtain the anhydrous salt from the hydrates, and that probably some traces of sulphate must be decomposed at the temperatures required (450°-500°) to drive off all the water. The results obtained from the enneahydrate are to a great extent invalidated by the doubts as to the purity of the hydrate, completeness of dehydration, etc., which arise from the discrepancies in the values deduced from the three ratios. He accepts, however, the figure 232·49 obtained by Nilson and by Krüss and Nilson from material separated as octohydrate, with some uncertainty as to the second decimal figure.
Brauner himself employed the oxalate method in 1898; the purified hexahydrate was used, the percentage of thoria being determined by ignition, and of (C₂O₃) by titration with permanganate. The ratio ThO₂ : 2C₂O₃ gave results varying from 232·21 to 232·29, but as the value rose continuously as purification was carried further and further, he did not feel justified in taking a mean value. In 1900 Urbain determined the constant with material purified by the acetylacetone method. He prepared the octohydrate, heated it for ten hours in a bath of sulphur vapour at 440°, and ignited the anhydrous salt so obtained at a white heat. The ratio ThO₂ : 2SO₃ gave the result (corrected to vacuo) Th = 233·67. Brauner criticises the value on the ground that the hydrated salt was heated in a vessel open to the air, and that at the high temperature obtained, traces of moisture gaining access to the sulphate caused hydrolysis, with loss of sulphuric acid; this would cause the results to be too high. In 1905 Meyer and Gumperz employed the same method, and obtained values varying from 232·2 to 232·7, with the mean 232·47. Finally Brauner carried out an extended investigation to disprove the heterogeneity of thorium which had been ‘discovered’ by Baskerville (1904), in the course of which he showed the atomic weight of the element to lie between the limits 232·34 and 232·52.
Detection of Thorium.
—The element is best detected in a mixture of earths by the following reactions:
(1) Precipitation with hydrogen peroxide from warm, faintly acid solution.
(2) Precipitation with sodium hypophosphate, Na₂H₂P₂O₆, in concentrated hydrochloric acid solution. On boiling, a perceptible precipitate is obtained if only traces of thorium are present; but ceric and zirconium salts and titanium must be absent. The latter element gives no precipitate under these conditions if hydrogen peroxide is present; ceric salts may be decomposed by boiling. The possible presence of zirconium renders it necessary to boil the hypophosphate precipitate with nitric acid; on addition of oxalic acid to the clear solution, thorium is precipitated, whilst zirconium remains in solution, and may be detected.
(3) Potassium azide, KN₃, throws down thorium hydroxide from boiling neutral or faintly acid solutions. Ceric salts if present must be previously reduced; zirconium must be previously removed by oxalic acid.