Chemical Properties of Thorium

Thorium appears to be strictly quadrivalent in all its compounds; and though it is by no means a powerful metal, on the whole, base-producing predominate over acid-producing properties.

The following qualitative reactions are characteristic of thorium -

Alkali hydroxide, ammonia, and ammonium sulphide solutions precipitate from thorium salt solutions the hydroxide Th(OH)₄, insoluble in excess of the precipitant. Alkali carbonates precipitate basic thorium carbonate, soluble in excess; potassium sulphate precipitates K₄Th(SO₄)₄, soluble with difficulty in water, and distinguished from the corresponding yttrium salt by insolubility in excess of the precipitant. Oxalic acid precipitates the oxalate, insoluble in excess of the precipitant (distinction from zirconium); ammonium oxalate also precipitates the oxalate, but redissolves it when added in excess, forming a solution not precipitated by dilution (distinction from cerium and yttrium); hydrochloric acid, however, reprecipitates the oxalate from this solution, since the latter is insoluble in oxalic acid (difference from zirconium). Sodium thiosulphate on boiling precipitates all the thorium from a solution (preferably of the chloride) as basic thorium thiosulphate (difference from cerium and yttrium metals). Potassium iodate precipitates thorium as the iodate; precipitation is complete even in a solution containing 40 per cent, by volume of concentrated nitric acid provided a moderate excess of precipitant is added (difference from rare earth metals except zirconium and quadrivalent cerium). Sodium hypophosphate precipitates the corresponding thorium salt, insoluble in dilute mineral acids. Sodium pyrophosphate precipitates the thorium salt, practically insoluble in dilute (0.5 N.) mineral acids (difference from rare earth metals except zirconium and quadrivalent cerium).

Thorium is always estimated gravimetrically as oxide formed by the ignition of the hydroxide, superoxide, or oxalate. The oxide should be perfectly white; traces of "didymium" oxide impart to it a pink or brownish tint. It is very hygroscopic.

There is little difficulty in separating thorium quantitatively from all other metals except those of the rare earths. For the separation from these metals numerous methods have been proposed.

The determination of thorium in monazite sand is an important one. Owing to the presence of large amounts of other rare earth metals and phosphoric acid, the estimation is rather difficult and tedious. The only methods which do not necessitate removal of the phosphoric acid are precipitation as iodate, hypophosphate or pyrophosphate; and all the methods available necessitate a second precipitation of the thorium to remove small amounts of other rare earths carried down with the first precipitate. The sand is weighed, " broken " by being heated with excess of concentrated sulphuric acid to 200° C. for several hours, cooled, and the product poured into a large volume of cold water. From the cold filtered solution the rare earth metals are precipitated by an excess of oxalic acid. The washed oxalate precipitate is decomposed by ignition, by boiling it with sodium hydroxide, or by heating it with a mixture of concentrated and fuming nitric acid. The oxide or hydroxide may then be dissolved in hydrochloric acid, the thorium separated by double precipitation as the basic thiosulphate, the precipitate dissolved in hydrochloric acid, the solution filtered from sulphur, precipitated as oxalate and ignited to oxide; or the nitrate solution may be neutralized with ammonia, precipitated with hydrogen peroxide as hydrated thorium peroxide, and the precipitate changed into thoria by ignition.

In acetic acid solution, thorium may be determined by titration with ammonium molybdate solution, using diphenylcarbazide as an external indicator. Tervalent rare earth elements do not interfere.