Thermodynamic and spectroscopic properties of thorium-arsenazo III complexes at various ionic strength conditions

Determination of thermodynamic data of major actinides is one of the pending technical tasks to reduce the uncertainty of safety assessment of deep geological disposal facility for high level radioactive waste. The safety of deep geological disposal facility is decidedly dependent on chemical behavior of the radionuclides which are released from disposal facility, and most of the major radionuclides which dominate long-term radiotoxicity of high level radioactive waste are uranium, neptunium, plutonium, and americium. The oxidation state of those actinides are highly dependent on the redox potential of aquatic system, and the redox potential of deep groundwater is commonly low enough to allow uranium, neptunium, and plutonium ions to exist as tetravalent ions. Thorium has been generally regarded as a good chemical homologue of tetravalent actinides since thorium ion has only tetravalent oxidation state irrespective of redox potential of aquatic system. Quantitative determination of thorium using arsenazo III has been widely applied, however the thermodynamic data and influence of ionic strength on thorium-arsenazo III complexation have received little attention. In this research, stability constants of thorium-arsenazo III complexes at various ionic strength conditions were determined by utilizing the Gauss-Newton algorithm SQUAD, and the determined stability constants were extrapolated to zero ionic strength through linear regression based on the ionic strength correction model (SIT, Specific ion interaction theory) approach