A physicochemical study on the hydrolysis of Thorium(IV) ion and its analytical importance

Hydrolysis of Th(IV) occurs at pH ∼ 3 leading to the formation of species like (Th(OH))3+, (Th(OH)2)2+, (Th(OH)3)+, (Th(OH)4), some dimeric and polymeric species of Th4+ ion. To study electronic transition of these species, absorbance was measured in the concentration range of Th(IV) ions from 1x10-8M to 1x10-5M in the absorption wavelength range of 190 nm to 400 nm. Absorbance was observed only in a narrow concentration range of 1x10-6M to 6x10-6M Th(IV) ions at 203 nm having molar extinction coefficient (ε) of 1x106 L.mole-1.cm-1. Above and below this concentration range, no absorbance peak was observed. The mass spectra study shows that the molecular ion peak is appearing at m/z value 298.8 and it is close to molecular weight of Th(OH)4. Further to support experimental work, theoretical absorption spectra of all possible species of Th(IV) hydrolysis were calculated using Gaussian 03 programme. (author)