Radium Sulphate, RaSO₄

Radium Sulphate, RaSO₄, may be obtained by precipitation of a soluble radium salt by sulphuric acid or a sulphate. It is a very insoluble compound, its solubility at 25° C. being 2.1×10^-8 grm. per c.c., about 100 times less than that of barium sulphate. The presence of sulphuric acid up to 50 per cent, has no appreciable influence on the solubility, but at higher concentrations this increases rapidly, as observed in the case of the other alkaline earths. In 70 per cent, acid it is more than 12 times greater than in 60 per cent. acid. Increase of temperature also causes a large increase of solubility in dilute acid solutions.

Much attention has been directed to the behaviour of radium sulphate in the presence of barium sulphate. When barium sulphate is completely precipitated from a solution containing a radium salt, the latter is also carried down, although the solubility product C_Ra••×C_SO4'' may not be attained.

Paneth pointed out that radio-elements are readily adsorbed by analogous compounds which are sparingly soluble in the solvent. Germann found that the adsorption laws are strictly valid for radium salts in contact with barium sulphate, but if the barium sulphate is precipitated in a radium solution all the radium is adsorbed, because, the particles at first being of molecular dimensions, the adsorbing surface is enormous. This result, however, is not in agreement with the statement of Lind and his colleagues, that when a radium-barium solution is precipitated by sulphuric acid or a soluble sulphate the ratio of radium to barium in the precipitate is the same as in the original solution. This behaviour is comparable with that of isotopic elements, although radium and barium are not isotopic. This phenomenon they denoted by the term pseudo-isotopy. A suggested explanation is that radium and barium sulphates are isomorphous, and, therefore, form a solid solution.