The Kinetics of the ferrous-ferric redox reaction in aqueous perchloric acid

Wadden, John Terrence (1978) The Kinetics of the ferrous-ferric redox reaction in aqueous perchloric acid. Masters thesis, Memorial University of Newfoundland.

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Abstract

The rotating disc electrode (RDE) has been used to investigate the kinetics of the ferrous-ferric redox reaction on platinum, palladium, and gold in aqueous perchloric acid. The electrode shape was optimized, precise speed control was used for the rotating disc and possible noble metal dissolution was minimized during cyclic activation of the test electrode. Each system was studied using counter and reference electrodes constructed from the same metal as the RDE. -- Mass transport effects were eliminated by making a long extrapolation to infinite rotation speed (i₀⁻¹ versus ω⁻¹/²). The data were analyzed in several ways including the slope-intercept method of Jahn and Vielstich and the constant mass transport method of Randles. In this regard, diffusion coefficients of the ferrous and ferric species were determined as well as their temperature dependencies. -- Observed rates were corrected for real surface areas as measured by oxygen desorption of by hydrogen adsorption techniques. Heterogeneous rate constants at 298 K were measured on platinum, gold and palladium and were found to be 5.41 x 10⁻¹³ cm.s⁻¹, 2.11 x 10⁻³ cm.s⁻¹, and 1.77 x 10⁻³ cm.s⁻¹, respectively. A different trend was observed for the pre-exponential factor, A⁺, decreasing from gold to platinum to palladium. Experimental activation enthalpies for all three metals were essentially constant within experimental error having a median value of 39 ± 3.5 kJ.mol⁻¹. An attempt was made to correlate kinetic parameters with the thermionic work function of each metal. The possible effect upon the kinetics of chloride impurity, arising from reduction of the perchlorate ion was also considered. Utilizing radiochemical chloride adsorption data, the rate constants of the reaction on a chloride-free and fully covered platinum electrode were estimated to be 2.62 x 10⁻⁵ and 6.76 x 10⁻³ cm.s⁻¹ respectively.

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