Use of Potentiometric Titration for the Study of Thermodynamic Properties of UCl <sub>4</sub> and UO <sub>2</sub> in Fused LiCl-KCl Eutectic

Abstract

The interaction between uranium oxygen-free and oxygen ions in molten LiCl-KCl eutectic (LKE) at 773 K was studied to investigate the chemical and electrochemical properties of U 4+ and O 2- ions by potentiometric titration method. Combining theoretical and experimental data, the E - p O 2- Pourbaix diagram for stable uranium compounds has been constructed. The stable states of U(IV) in molten LiCl-KCl salt at 773 K were UO 2 and UCl 4 during the concentration of oxygen ions reaches equivalent point α =2.39. The calculated values of the activity coefficient of UCl 4 and UCl 3 were 5.05×10 - 5 and 1.01×10 - 3 , respectively. This is indicated that U(III) ions are less active than U(IV) ions probably due to the formation of strong complexes with the chloride ions. Tungsten cathode was used to deposit metal uranium, and the equilibrium potentials vs. the Ag/AgCl reference electrode are -1.5005 V and -2.5305 V separately by using the emf of the galvanic cell. The potential of urnium ions in the melt was changed linearly with the concentration function of oxygen ions ( p O 2- ). According to the titration reaction, solid UO 2 was formed in the range 0 < α < 2.39. The equilibrium between UO 2 and U (IV) ions was described by the equation: E =3.678+0.0383´ p O 2- , by which the stability constant of UO 2 can be calculated. Meanwhile, the equilibrium of UO 2 with U(III) is as shown in the formula: E =2.107+0.154´ p O 2- .