Phosphites are known in a few cases only; arsenates and arsenites of lanthanum have been prepared. Vanadates of some of the rare earth elements have been described.
Chromates.
—The rare earth chromates are, as a rule, sparingly soluble in water, and show considerable differences of solubility amongst themselves; for this reason, they have been of some use in the separation of the cerium elements.[165] They are obtained by addition of potassium chromate to neutral solutions of rare earth salts as crystalline precipitates, of the general formula R₂(CrO₄)₃,8H₂O; with a large excess of alkali chromate, double chromates are obtained, which are more readily formed, and more soluble, in the yttrium series than in the cerium group. Addition of chromic acid or alkali bichromate to solutions of the soluble salts gives no precipitate, a fact which allows of the separation of zirconium and thorium, and of cerium in the tetravalent state, since the tetravalent elements are precipitated by both these reagents.
[165] Muthmann and Böhm, Ber. 1900, 33, 42; Böhm, Zeitsch. angew. Chem. 1904, 15, 372 and 1282.
Ammonium molybdate throws down from neutral solution of rare earth salts gelatinous precipitates of the molybdates; the formula La₂2(HMoO₄)₆ is assigned to the lanthanum compound obtained in this way. No precipitation occurs if the solution be strongly acid; on this fact a process has recently been based for the volumetric estimation of thorium, in presence of rare earth salts, by means of ammonium molybdate (see [p. 289]).
Various silicotungstates and double tungstates have been described.
Carbonates.
—The more pronounced electropositive character of the rare earth elements, as contrasted with other trivalent metals, is well illustrated by the fact that they form stable neutral carbonates of the formula R₂(CO₃)₃,xH₂O. These may be obtained by passing a current of carbon dioxide through an aqueous suspension of the hydroxides, or by addition of an alkali carbonate to neutral solutions of the salts. Basic carbonates are known in the case of the less positive yttrium elements only; both these and the neutral carbonates are insoluble in water.
In presence of a large excess of alkali carbonate, double carbonates are formed. The stability as well as the solubility of these compounds increases in passing from the cerium to the yttrium group, i.e. as the electropositive character becomes weaker. The double carbonates of the cerium elements are sparingly soluble, and are decomposed by water, especially on warming; they may, however, be recrystallised from alkali carbonate solution. The sodium and ammonium double salts are less soluble than the potassium compounds. The latter have the general formula R₂(CO₃)₃,K₂CO₃,12H₂O, and are of considerable importance in many processes of separation. The yttrium elements can be separated from the cerium metals, and the latter from one another, by taking advantage of the differences of solubility shown by the potassium double carbonates. If a concentrated solution of the salts in potassium carbonate solution be fractionally diluted with water, the cerium elements separate in the order: lanthanum, praseodymium, cerium, neodymium, and samarium; the more soluble yttrium compounds remain in the solution. Thorium forms double alkali carbonates which are very readily soluble in excess of alkali carbonate; this property is of great importance for the technical separation of the element.