Temperature dependence of the Tafel slope. The reduction of bromate in alkaline media

The temperature dependence of the transfer coefficient of bromate reduction at pH > 12 is studied in several supporting electrolytes (0.1-1 M LiCl, NaCl, KCl, CsCl) in the temperature range 0-90° at the dropping mercury electrode. The transfer coefficient is found to be independent of temperature. The treatment of the raw current-potential data includes correction for the ohmic drop in solution, calculation of the activation controlled current density from the ratio I/I_d through Koutecky's tabulated χ values and diffuse-double-layer correction according to Frumkin's theory. The Tafel lines are linear over four to six decades of activation current density. Strong emphasis is placed on accuracy. A small but definite dependence of the transfer coefficient on the cation of the supporting electrolyte is found (0.61, 0.64, 0.67 and 0.67 for Li⁺, Na⁺, K⁺ and Cs⁺ respectively). The diffuse double-layer effects are suitably accounted for by Frumkin's theory. The corrected reaction rates for the individual electrolytes, except for CsCl, are independent of ionic strength. A negative apparent enthalpy of activation is observed with all electrolytes along the polarographic wave.