Interaction of thorium(IV) with nitrate in aqueous solution: Medium effect or weak complexation?

Microcalorimetric titrations were performed to study the Th(iv)/nitrate interaction in aqueous solution. The results show the formation of a weak mononuclear complex of Th(iv) with nitrate and allow the determination of the complexation thermodynamic parameters at 298 K and ionic strength 1.0 mol dm -3. The reaction is endothermic and entropy driven. Data and comparison with similar actinide(IV) complexes allow to confirm the inner-sphere nature of the Th(NO 3) 3+ complex.