Revised hydrolysis constants for thallium(I) and thallium(III) and the environmental implications

The hydrolysis constants for Tl(I) and TI(III) have been determined by combining potentiometric measurement with the PKAS computer program. The log hydrolysis constants (Kh(i)) for TI(I)(OH)(i-1)(i) + H2O double left right arrow TI(I)(OH)(1)(1-i)+ H+ have been determined to be -11.7, and those for Tl(III)(OH)(i-1)(4-1) + H2O double left right arrow TI(III)(OH)(i)(3-i) + H+ are -2.69, -6.36, -7.42, and -8.78, respectively. The corresponding stepwise association constant for monovalent thallium is 10(2.3) for Tl(OH)(aq') and those for trivalent thallium are 10(2.3) for Tl(OH)(2+), 10(7.64) for TI(OH)(2)(+), 10(6.58) for Tl(OH)(3aq), and 10(5.22) for TI(OH)(4)(-). The solubility product (K-sp) for solid Tl(OH)(3,Solid) has been redetermined to be 10(-45.2), implying that it is one of the least soluble among metal hydroxides. Revised fields of predominance of Tl(I) and TI(III) hydroxides in aquatic environment are described, and the importance of Tl(III)/Tl(I) redox couple in the cycling of thallium is discussed.