Quantitative analysis of Eu2+ and Eu3+ in LiCl–KCl eutectic melt by spectrophotometry and electrochemistry

The redox behavior of Eu2+ and Eu3+ in the LiCl–KCl eutectic at 773 K was investigated. Since the equilibrium potential of the melt is very close to the redox potential of the Eu3+|Eu2+ couple, the Eu2+ and Eu3+ species coexist. Quantitative analysis of Eu2+ and Eu3+ was performed by spectrophotometry and by potentiometry. Under the coexistence of Eu2+ and Eu3+, potentiometric titration of Eu ion using a yttria-stabilized zirconia membrane electrode (YSZME) was performed by changing the concentration of O2−. The formation of the europium oxychloride, EuOCl, was confirmed by X-ray diffraction analysis, whiles no precipitation of the oxides, EuO and Eu2O3, was found. The equivalent point of the EuOCl formation (x = [O2−]added/[Eu]total) was shifted to a smaller value from the theoretical value (x = 1) due to the coexisting Eu2+. The contribution of the coexisting Eu2+ to the formation of EuOCl was estimated by using [Eu2+]/[Eu3+] as determined by potentiometry and this was subtracted from the titration data. The solubility product of EuOCl was determined to be pks(EuOCl) = 7.81 ± 0.10.