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

Abstract

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 Ka = (3.00 ± 0.01) × 10−3 M (pKa = 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