Application of LIBD to the determination of the solubility product of thorium(IV)-colloids

A new experimental method is presented for the determination of solubility data, which is based on the laser-induced breakdown detection (LIBD). The m ethod is capable of monitoring the initial colloid generation when the meta l ion concentration reaches or just exceeds the solubility at given pH. The application is made to determine the solubility of Th(IV) in acidic soluti ons at I = 0.5 M (NaCl) and 25 degreesC. The initial colloid formation is d etermined as a function the Hf concentration in a series of 2.8 x 10(-2)-8. 9 x 10(-5) M thorium solutions. The conditional solubility product (log K-s p' = -49.54+/-0.22) obtained in this study corresponds to an equilibrium be tween solution and colloidal thorium dioxide particles. The solubility prod uct at I = 0 (log K-sp(o) = -52.8+/-0.3) is calculated with the SIT coeffic ients of the NEA-TDB. It corresponds to the known value for crystalline ThO 2(cr), in particular if the small particle size of about 20 nm is taken int o account. The present results indicate that the high thorium solubilities measured in the previous studies for amorphous Th(IV) hydroxide or hydrous oxide are primarily caused by the inclusion of polynuclear species or Th(IV ) colloids of very small size.