Studies on metal carbonate equilibria. Part 10. A solubility study of the complex formation in the uranium(VI)–water–carbon dioxide (g) system at 25 °C

Abstract

The complex formation in the system U^VI–H₂O–CO₂(g) was studied by measuring the solubility of UO₂(CO₃)(s) as a function of the CO₂ pressure and the hydrogen-ion concentration. The measurements were made at 25 °C, by using two different ionic media, 0.5 and 3 mol dm^–3 NaClO₄, respectively. The equilibrium constants for the following reactions were determined. Interaction UO₂(CO₃)(s)⇌ UO₂ ²⁺+ CO₃ ^2– K_s,o UO₂ ²⁺+tCO₃ ^2–⇌ UO₂(CO₃)_t, t= 1–3 β_t,1 3UO₂ ²⁺+ 6CO₃ ^2–⇄[(UO₂)₃(CO₃)₆]^6–β_6,3 coefficients ε(i,j) for the various uranium species with ClO₄ ^–(or Na⁺) were determined by using the Brønsted, Guggenheim, Scatchard specific ion-interaction theory. By using these and equilibrium data from the literature, we propose the following set of ‘best’ estimates of equilibrium constants, referred to the pure water reference state: log K_s,o=–14.4 ± 0.1, log β_1,1= 9.5 ± 0.2, log β_2,1= 16.6 ± 0.2, log β_3,1= 21.3 ± 0.2, and log β_6,3= 53.4 ± 0.8.