The oxidation of potassium hexacyanoferrate(II)trihydrate by potassium dibromodicyanoaurate(III) in acidic solution: a kinetic study

The reduction of the dibromodicyanoaurate(III) ion by hexacyanoferrate(II)trihydrate was studied in an acidic medium. The reaction was first order in both Au(CN)₂Br₂⁻ and Fe(CN)₆⁴⁻ and a second order rate constant of k₂ = 255 ± 5 M⁻¹ s⁻¹ at [H⁺] = 2.041 × 10⁻⁴ M, an ionic strength of 0.51 M (NaBr) and 20.0 ± 0.1 °C was found for the reaction. The reaction rate decreases with increasing [H⁺] in the region 0.0004 ≤ [H⁺] ≤ 0.065 M. An equilibrium constant of K_a = (3.00 ± 0.01) × 10⁻³ M (pK_a = 2.52) at 20.0 ± 0.1 °C was found for the deprotonation of H₂Fe(CN)₆²⁻. Activation parameters of ∆H^# = 47.8 ± 0.9 kJ mol⁻¹ and ∆S^# = −37 ± 3 J K⁻¹ mol⁻¹ have been obtained by a least squares fit of temperature data directly to the Eyring equation.

Graphical abstract