In the analytical method, a known weight of sulphate is ignited to the oxide, and weighed as such. This method is most suitable for the less basic members of the yttria earths, of which the sulphates can be completely decomposed without difficulty at a red heat. By the use of the microbalance, a sufficiently accurate determination can be carried out by either of these methods in little more than half an hour, as the chemical changes are exceedingly rapid where only small quantities are employed, and no time is required to allow the vessels and solids to cool. Using the microbalance, Brill[192] has carried out a series of experiments to determine the limits of temperature within which the various steps of the process should be carried out. He finds that a temperature of 400°-550° is required to decompose the last traces of acid sulphate, and give the pure neutral sulphate. Between the temperatures of 850° and 950°, basic salts are formed, from which the last trace of sulphuric anhydride is expelled at 900°-1150°; the precise temperature required in each case depends, of course, on the basic strength of the oxide in question.
[192] Zeitsch. anorg. Chem. 1905, 47, 464.
The determination of equivalents by means of the ratio R₂O₃ : R₂(C₂O₄)₃, has been brought to a high degree of accuracy by Brauner.[193] A weighed quantity of the carefully prepared oxalate is ignited, with suitable precautions, to the oxide, in a tarred platinum crucible. A second weighed specimen of the same oxalate preparation is dissolved in dilute sulphuric acid, and titrated at 60° with permanganate, which is standardised against pure ammonium oxalate.
[193] Ibid. 1903, 34, 103, 207.
Of the methods of volumetric analysis which have been proposed, that put forward by Feit and Przibylla appears to be the most suitable. A convenient quantity of oxide, which has been ignited until constant in weight, is dissolved by gently heating with a known excess of N 2 sulphuric acid, in a conical flask of Jena glass. The excess of acid is titrated with N10 sodium hydroxide, using methyl orange as indicator. This method, which has the advantages of ease and quickness, is very reliable, if suitable precautions are taken, in the case of the more strongly basic oxides; but with the least strongly basic members of the yttria group, the erbia and ytterbia oxides, the end point is not very sharp, whilst with the weakly basic scandia, the method breaks down entirely.[194]
[194] Zeitsch. anorg. Chem. 1905, 43, 202; 1906, 50, 249.
CHAPTER XI
THE CERIUM GROUP—CERIUM
The extraction of the rare earth elements from minerals, by which they are obtained in the form of the oxalates, and the methods of bringing these into solution, have already been described. From the solution, before any separation of the rare earths is attempted, thorium should be removed; for this purpose, any of the methods described under estimation of thorium (see [p. 286]) may be used, the most convenient being the peroxide precipitation of Wyrouboff and Verneuil.
The solution is then treated with potassium sulphate until the absorption bands of didymium (praseodymium and neodymium) can no longer be observed, or appear only very faintly, when a layer of the solution is examined with a spectroscope; the precipitate then consists of the potassium double sulphates of the cerium with some of the terbium elements. If the mixture is very rich in the cerium elements, and correspondingly poor in the yttrium elements—as, for example, the mixture of earths obtained from monazite—Drossbach[195] recommends a preliminary separation by means of the double carbonates; the double sulphate method may then be employed to remove the last of the yttrium and most of the terbium elements. The sparingly soluble double sulphates of the cerium metals may be transformed into the hydroxides by digestion with potassium hydroxide, and these taken into solution, after washing, by hydrochloric or nitric acid.