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

The hydrolysis constants for Tl(I) and Tl(III) have been determined by combining potentiometric measurement with the PKAS computer program. The log hydrolysis constants (Khi) for Tl(I)(OH)i-1i + H2O<==>Tl(I) (OH)i1-i + H+ have been determined to be -11.7, and those for Tl(III)(OH)i-1(4-i) + H2O<==>Tl(III)(OH)i3-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(11.31) for Tl(OH)2+, 10(7.64) for Tl(OH)2+, 10(6.58) for Tl(OH)3aq, and 10(5.22) for Tl(OH)4-. The solubility product (Ksp) 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 Tl(III) hydroxides in aquatic environment are described, and the importance of Tl(III)/Tl(I) redox couple in the cycling of thallium is discussed.