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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 H2SO413.24.566.46.94
Grams of Th(C2O4)2 per 100 grams of solution0.0190.0700.1540.2330.276

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.

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