Electrometric determination of stability constants of uranyl-chloride and uranyl-nitrate complexes with pCl-stat

An electrochemical pCl-stat was devised to determine the stability constants of UO₂Cl⁺ and UO₂NO₃⁺ ion complexes in the aqueous solutions of KNO₃ and NaNO₃-NaClO₄ containing minor amounts of Cl^- ion at 25℃. The stability constant K_c of UO₂Cl⁺ at various ionic strengths μ was estimated from the amount of UO₂Cl⁺ ion obtained by the integration of the current-time curve of the pCl-stat following the addition of a given amount of uranyl salt. As previous investigators have pointed out, the stability constant at higher ionic strengths is relatively small, e.g., K_c=2.42, 1.40 and 0.89 at μ=0.54, 0.82 and 1.06, respectively. However, the thermodynamic stability constant K_a at μ=0 obtained by extrapolation of the curve of K_c against μ, resulted in the remarkably high value of 15. The stability constants of UO₂NO₃⁺ were also determined by measuring the K_c values of UO₂Cl⁺ as a function of compositions at constant μ in mixed solutions of NaNO₃ and NaClO₄. The observed results were 0.37, 0.20 and 0.19 at μ=0.54, 0.82 and 1.06, respectively. (author)