Polythermal investigation of course of molten ionic bromide deoxidization by action of ‘bromine + carbon’ red-ox pair

Abstract

The process of removal of oxide ion admixtures by the action of the ‘C + Br₂’ red-ox pair (usually called carbohalogenation) from bromide melts of KBr–NaBr (0.5 : 0.5), BaBr₂–KBr (0.51 : 0.49) and KBr–LiBr (0.4 : 0.6) compositions at 943, 973, 1000 and 1023 K was studied by a potentiometric method using Pt(O₂)|YSZ oxygen electrode as an indicator. The chemical stage of this process in all the molten mixtures is characterized by the 2^nd kinetic order with respect to oxide ions. The thermal dependences of the conditional rate constants are described by the equations: ln k = 22.06 (±1.5) − 4880 (±1400)T⁻¹ for KBr–NaBr, ln k = 15.5 (±0.8) − 3700 (±800)T⁻¹ for BaBr₂–KBr and ln k = 12.02(±1) − 4011(±900)T⁻¹ for KBr–LiBr, which makes it possible to estimate the activation energies of the carbohalogenation as 42 ± 12 kJ mol⁻¹, 31 ± 7 kJ mol⁻¹ and 33 ± 8 kJ mol⁻¹, respectively. Thermodynamic characteristics of the activation complex formation (ΔH^#, ΔS^#) are estimated. The purification limits (residual concentration of O²⁻) of the said melts in the 943–1023 K temperature range are (1 ÷ 2) × 10⁻⁹ mol kg⁻¹ for KBr–NaBr, (2 ÷ 3) × 10⁻⁷ mol kg⁻¹ for BaBr₂–KBr, and (1.4 ÷ 1.8) × 10⁻⁵ mol kg⁻¹ for KBr–LiBr. There exists a good correlation between the purification limits and oxoacidic properties (the oxobasicity indices) of the studied melts.