Spectrophotometric study of the uranyl monobenzoate complex at moderate ionic strength

Abstract

The protonation of benzoate and the complexation of uranyl monobenzoate in NaClO4 solutions ranging from 0.1 to 0.5 M were quantitatively investigated by means of ultraviolet–visible (UV–Vis) spectrophotometry. The spectrophotometric study revealed that the uranyl monobenzoate species has a significantly enhanced absorption than that of free uranyl, especially in the UV region. The non-linear regression analysis of the UV–Vis absorption data was the first to calculate the protonation constant of benzoate and the stability constant of uranyl monobenzoate at moderate ionic strength. Based on the SIT (specific ion interaction theory), the protonation constants were extrapolated to the zero ionic strength as logβP° = 4.21 ± 0.01, and the ion interaction coefficient of the benzoate anion with the sodium cation was determined as ε(Ben-,Na+) = 0.17 ± 0.05 kg·mol−1. In the same manner, the stability constant of uranyl monobenzoate at zero ionic strength and the ion interaction coefficient of the uranyl monobenzoate cation with perchlorate anion were determined to be logβU° = 2.91 ± 0.01 and ε(UO2Ben+,ClO4-) = 0.22 ± 0.07 kg·mol−1, respectively.