Transient methods: comparison of three methods employing the Cd²⁺|Cd(Hg) couple

A combination of three transient techniques (coulostatic, modified coulostatic and galvanostatic) has been applied for the study of the electrode reaction in aqueous solution, 1 M Na₂SO₄, pH 4. The techniques are applied under optimal conditions as determined by the information content curves. The modified coulostatic method, for which charging is performed by a galvanostatic pulse of a width of several τ_c, has been applied for the first time. The classical coulostatic method has not been applied in its pure form, i.e. t_p ≪ τ_c, but with somewhat longer pulses and smaller current densities, in order to avoid saturation of the measuring device during the charging step, and the evaluation of the results was performed using the equation for the coulostatic method with finite pulse width, derived for the modified coulostatic method. The three methods complement each other: with the classical coulostatic method the optimization of the experimental conditions is simplest; the modified coulostatic method ensures the highest accuracy in the determination of j₀ and the galvanostatic method makes it possible to study the process on a longer time scale. With the three methods combined, the reaction can be studied in a time range of (1-40)τ_c or more, depending on the ratio of τ_c τ_d. A two-indicator electrode cell was used in order to increase the overpotential for which linearization of the overpotential/time plot is valid. The maximum value of the overpotential (not including IR_s) was 6 mV. The exchange current density and the double layer capacitance were determined at different concentrations of Cd²⁺ and Cd(Hg). The stoichiometric number ν was estimated to be 1. Cathodic and anodic transfer coefficients were calculated and corrected for the diffuse-double-layer effect, according to Frumkin's theory. The transfer coefficients were found to be independent of temperature in the range 5-65 °C. α_a = 1.59, α_c = 0.23. The values corrected for diffuse double layer were found to be α^{ø₂ = 0}_a = 1.69, α^{ø2 = 0}_c = 0.12. The values of the standard rate constant at 25 °C are: k_sh = 0.026 cm s^-1; k^{ø2 = 0}_sh = 0.013 cm s^-1. The standard enthalpy and entropy of activation are: ΔH^{≠ 0}_{η = 0uncorr} = 33 kJ mol^-1, ΔH^{≠ 0}_{η = 0corr} = 32 kJ mol^-1; ΔS^{≠ 0}_{η = 0uncorr} = 5 J (mol K)^-1, ΔS^{≠ 0}_{η = 0corr} = -7 J (mol K)^-1.