Thorium Oxalate, Th(C₂O₄)₂

Thorium Oxalate, Th(C₂O₄)₂, is precipitated when oxalic acid is added to a thorium salt solution. When obtained from hot solutions containing about 10 per cent, of hydrochloric acid the precipitate is dense and crystalline; from cold and nearly neutral solutions the oxalate separates as a rather bulky, amorphous precipitate which readily passes into colloidal solution when washed with water. The composition of the air- dried salt is expressed by the formula Th(C₂O₄)₂.6H₂O; when heated to 100° C. or dried over sulphuric acid the dihydrate Th(C₂O₄)₂.2H₂O is formed, and at 160° C. the hydrate 4Th(C₂O₄)₂.3H₂O is obtained.

Thorium oxalate is practically insoluble in water and aqueous oxalic acid, and is much less soluble in dilute mineral acids than the oxalates of the tervalent rare earth elements. The solubility in aqueous sulphuric acid at 25° C. is as follows:

Normality of H2SO4	1	3.2	4.56	6.4	6.94
Grams of Th(C2O4)2 per 100 grams of solution	0.019	0.070	0.154	0.233	0.276

With higher concentrations of sulphuric acid the oxalate is changed into sulphate. Hydrochloric acid in concentrations greater than about 20 per cent, converts the oxalate into an oxalochloride, Th₄(C₂O₄)₆Cl₄.2OH₂O. The solubility of thorium oxalate in dilute mineral acids is reduced to a negligible amount by the addition of a moderate excess of oxalic acid.

A characteristic property of thorium oxalate is its power to combine with alkali oxalates to form complex salts, and this furnishes a method of distinguishing and separating thorium from other metals of the rare earths. The following complex salts have been described:

(NH₄)₂Th(C₂O₄)₃.3H₂O; (NH₄)₄Th(C₂O₄)₄.7H₂O; (NH₄)₂Th₂(C₂O₄)₅.7H₂O; K₄Th(C₂O₄)₄.4H₂O; Na₄Th(C₂O₄)₄.6H₂O; and the acid salt H₂Th₂(C₂O₄)₅.9H₂O.

According to Hauser and Wirth, the salts (NH₄)₄Th(C₂O₄)₄.7H₂O and (NH₄)₄Th(C₂O₄)₄.4H₂O are labile phases in the system thorium oxalate - ammonium oxalate - water at 25° C., the stable phases, in addition to the single oxalates, being the other salts mentioned above. The heptahydrate, which crystallises from a concentrated solution of thorium oxalate in a large excess of ammonium oxalate, passes into the tetrahydrate in dry air, and the latter salt may be dehydrated over phosphoric oxide. At the ordinary temperature 100 parts of water dissolve 90.3 parts of the tetrahydrate, but an extra 9.3 parts of ammonium oxalate must be present in solution to prevent decomposition of the double salt by water. When a solution of this salt in water is diluted with much water it becomes turbid, and a colloidal precipitate, which slowly becomes crystalline, is produced. The precipitate is the salt (NH₄)₂Th₂(C₂O₅)₅.7H₂O. Further work on these double salts, from the physicochemical standpoint, is desirable, since the results of Hauser and Wirth are not in accordance with those of James, Whittemore, and Holden.