On some regularities of metal oxide solubility in molten CsI at T = 973 K

Solubility products of CdO (pKs,CdO = 6.80 ± 0.2), ZnO (pKs,ZnO = 10.0 ± 0.5), NiO (pKs,NiO = 11.2 ± 0.2) and EuO (pKs,EuO = 13.1 ± 0.2) in molten CsI at T = 973 K are determined by potentiometric titration of (0.02 to 0.03) mol · kg−1 solutions of the corresponding metal chlorides by strong base (KOH) using a membrane oxygen electrode Pt(O2)|ZrO2(Y2O3) as an indicator. On the basis of pKs,MeO values, all the oxides studied are referred to practically insoluble in molten CsI. The values of the oxide solubility in CsI melt are lower than the corresponding values in molten alkali metal chlorides. This can be explained by ‘softer’ basic properties of I– as compared with Cl– in the frames of the Pearson ‘hard’ and ‘soft’ acid–base concept. In the oxide samples studied, the values of the solubility fall with the decreasing cation radius. The correlation between pKs,MeO and the polarizing action by Goldshmidt (ZrMe2+-2) of the cation is practically linear and may be proposed for estimation of the solubility of s- and d- element oxides in molten CsI on the basis of their cation radii.

► CdO, ZnO, NiO, and EuO are insoluble in CsI melt at 973 K.
► The oxide solubilities are lower than those in chloride melts.
► The oxide solubilities reduce with the cation radius.
► ZnO, NiO, and EuO are suitable for scavenging CsO from oxide ion traces.