Ceroceric hydrosulphate

When ceric oxide or hydroxide is heated with fairly concentrated sulphuric acid it is not quantitatively converted into ceric sulphate; oxygen is evolved and part of the ceric salt reduced to the cerous state. From the acid solution a beautiful red hexagonal crystalline salt may be readily obtained, and from the mother liquor normal ceric sulphate, Ce(SO₄)₂.4H₂O, may then be separated in pale yellow crystals.

In addition to the preceding method of preparation, the red crystals may be obtained by mixing cerous sulphate with an excess of ceric sulphate and crystallising from fairly concentrated sulphuric acid.

The hexagonal crystals appear dark orange-coloured when large crystals are viewed in the direction of the principal axis, and orange-yellow when fine needles are similarly examined. Viewed parallel to the basal plane, the crystals appear to have a beautiful red colour. They may be heated to 230° without losing anything but their water of crystallisation.

The substance contains both cerous and ceric sulphates as well as sulphuric acid, and the latter cannot be eliminated at 130° in vacuo. It may therefore be described as a ceroceric hydrosulphate. The cerous sulphate may be replaced by the sulphates of the other rare earth elements, and Brauner has prepared the lanthanum, praseodymium., and neodymium salts. They are hexagonal, and isomorphous with one another and with the cerous salt. According to Brauner, they are acid salts of a complex cerisulphuric acid, H₄.Ce^IV(SO₄)₄.12H₂O, of the type HM^III.Ce^IV(SO₄)₄.12H₂O (M^III = Ce, La, Pr, or Nd). It may be remarked that the acid sulphate of thorium has the composition Th(SO₄)₂. H₂SO₄ or H₂Th(SO₄)₃, and not H₄Th(SO₄)₄. On the other hand, the acid sulphates of the rare earth elements have formulae of the type M₂^III(SO₄)₃.3H₂SO₄ or H₃[M^III(SO₄)₃], the ceric salt of which would be Ce₃^IV[M^III(SO₄)₃]₄, or 3Ce^IV(SO₄)₂.2M₂^III(SO₄)₃, in harmony with the formula of Wyrouboff and Verneuil; it is then difficult, however, to account for the extra sulphuric acid present in the molecule.

If ceroceric hydrosulphate is a "complex" salt, it is not a very stable complex, for the addition of alkali leads to the initial precipitation of the violet ceroceric hydroxide.

It will be noticed that the formula assigned to ceroceric hydrosulphate by Meyer and Aufrecht only differs from Brauner's formula (1904) in the amount of water of crystallisation present. Too much significance should not, however, be attached to this agreement, for the analytical data upon which the formulae are based are of slender value, and at the present time the composition and constitution of this interesting compound must be regarded as undetermined.

Thus, even the atomic ratio Ce^IV: Ce^III is uncertain. The analytical data given by Wyrouboff and Verneuil agree very well with the result Ce^IV: Ce^III::3:4, and the data given by Meyer and Aufrecht and by Brauner agree at least as well with this ratio as with the ratio 1:1, which they adopt. Brauner's analyses of the lanthanum, praseodymium, and neodymium salts are also unsatisfactory, as Wyrouboff and Verneuil have justly remarked.

It is, perhaps, worth while to point out that the formula 3Ce(SO₄)₂.2Ce₂(SO₄)₃.2H₂SO₄.42H₂O agrees with the analytical data given by Brauner and by Wyrouboff and Verneuil at least as well as do their own formulae. This is readily seen from the following table of results: -

Per cent, of	Calculated from the Formula of			Found by
	Brauner.	W. and V.	H.F.V.L.	Brauner.		W. and V.
Ce2O3	37.25	37.75	37.27	36.57	36.15	37.57
Active oxygen.	0.91	0.79	0.78	0.82	0.84	0.79
SO3	36.31	35.63	36.29	...	36.78	36.16
H2O.	25.53	25.83	25.66	26.08	26.13	25.51

which serves to illustrate the difficulties attaching to the investigation of the composition of ceroceric hydrosulphate.

Finally, it may be mentioned that Brauner has described a ceroceric sulphate, Ce^III[Ce^IV(SO₄)₄]₃.44H₂O, and the corresponding lanthanoceric salt.