Solubilities and free energies of transfer for lanthanoid iodates in dimethyl sulfoxide-water mixtures.

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The solubilities of lanthanoid iodates in dimethyl sulfoxide (DMSO)–water mixtures at 25 °C were measured. Thirteen lanthanoid iodate dihydrates Ln(IO3)3·2H2O (Ln=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were used. From the measured solubilities, the Gibbs free energies of the transfer of the iodates from water into aqueous DMSO solutions were calculated. The tetrad effect behavior reported by Peppard et al. was observed in both the variation with the lanthanoid atomic number of the solubility data and that with the atomic number of the free energies of transfer. The tetrad effect behavior seems to depend on a change in the solvation of the lanthanoid ions in variation with the crystal radii of the trivalent lanthanoid ions. In each system the logarithm of the solubility of the iodates decreases almost linearly with the reciprocal of the dielectric constant of the mixtures, as is expected from the Born equation or its modification when dealing with mixtures with narrow dielectric constant, ranging between 78.5 and 78.0.
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