Solvation numbers and hydration constant for thorium(IV) in ethanol-water medium

The solvation number and hydration constant of Th{sup 4+} in ethanol-water medium were determined at 25{degrees}C using UV-spectral and electrochemical measurements. A solvate formation equilibrium is demonstrated and characterized. Three molecules of ethanol (S) can bond to the metal cation with strengths comparable to that for H{sub 2}O to form ThS{sub 3}(H{sub 2}O){sub 3}{sup 4+}. Formation of thorium monochelate with lawsone (2-hydroxy-1.4-naphthoquinone) eliminates bonding with alcohol molecules. The dissociation constant of the chelating agent {sub s}K{sub a} and the formation contant of the monochelated metal ion {sub s}K{sub f}{sup *} that are essentially independent of the solution composition are evaluated. Hydration titrations involving thorium-lawsone monochlate are performed and the data obtained from the changes of pH with solvent composition are analyzed. The solution independent constant, {sub s}K{sub f}{sup *} for thorium-lawsone complex formation in mixed aqueous ethanol is given by log {sub x}K{sub f}{sup *}=vpK{sub a} + log{sub s}K{sub h} - log [LH] - vpH + 3 log v where vpK{sub a} is the dissociation constant of the chelating agent LH in the solvent system of v volume fraction of water and {sub s}K{sub h} is the solution-independent hydration constant of thorium (IV) in the solvent system. Log-values for the constants {sub s}K{sub h}, {sub s}K{sub f}{sup *} and {sub s}K{sub z}{sup *} are found to be 7.8 {plus_minus}0.02, 11.38{plus_minus}0.04 and -0.753, respectively.