TERNARY METAL-COMPLEXES .2. THE U(VI)-SO(4)(2-)-OH- SYSTEM

The complexes formed in the ternary SySteM U(VI)-SO42- - OH- have been investigated at 25.00-degrees-C using potentiometric titrations in a 0.500 M Na2SO4 + 2.00 M NaClO4 ionic medium. The conditional equilibri um constants logK(pq)(log*K2-2 = -8.39 +/- 0.08, log*K3-4 = -16.64 +/ - 0.09, and logK5-8 = -32.98 +/- 0.10), for the reactions pUO22+ + qH 2O + rSO42- half arrow right over half arrow left ''(UO2)p(OH)q'' + qH +, have been determined from these data.''(UO2)p(OH)q'' denotes all th e ternary complexes with the same stoichiometric coefficients p,q, and K(pq) = [''(UO2)p(OH)q'']/[UO22+]p[H+]q. A previous experimental stu dy by Peterson (Acta Chem. Scand. 15, 101 (1961)) has been reinterpret ed. This showed that the model proposed by Peterson is not the best on e, the model presented in the present study gives a much better agreem ent with the experimental data. The equilibrium constants in our reint erpretation are logK2-2 = -8.23 +/- 0.06, log*K3-4 = -16.06 +/- 0.06, and logK5-8 = -32.03 +/- 0.06. The magnitude of the equilibrium cons tant for the (2,-2) species is very different from the value found in perchlorate media (logbeta2-2 = -5.74 +/- 0.07), furthermore complexe s with (p,q) equal to (3,-4) and (5,-8) are not formed there. These fa cts indicate that ternary sulphate containing species are formed in su lphate media. From the magnitude of the equilibrium constants and the known structures and coordination geometries of uranium(VI) complexes we propose the following stoichiometry of the ternary complexes: (UO2) 2(OH)2(SO4)22-, (UO2)3(O)(SO4)3(OH)24- (or (UO2)3(O)(SO4)4(OH)26-) and (UO2)5(O)3(SO4)5(OH)210-. The experimental finding indicate that the main features of the oxide/hydroxide bridges are retained in the terna ry complexes, and that sulphate can be bonded both in a bridging and a terminal fashion. The bonding of sulphate is sufficiently strong to i nfluence the speciation of uranium in some ground and surface water sy stems.