Tracer diffusion coefficients of I–, I –3, Fe2+ and Fe3+ at low temperatures

Abstract

Tracer diffusion coefficients (D_i) have been determined by measuring the limiting oxidation or reduction currents at a large platinum rotating-disc electrode. The systems studied at 5 °C were I^–, Fe²⁺ and Fe³⁺ in an aqueous supporting medium of 0.05 mol dm^–3 HClO₄ made up to an ionic strength of 0.1 mol dm^–3 with NaClO₄, and I^–₃ in 0.05 mol dm^–3 HClO₄–0.05 mol dm^–3 Nal. The last three ions were also studied at 10, 15 and 20 °C. The resulting activation energies were similar to the value expected from the Stokes–Einstein equation. The values of D_iη/T, where η is the viscosity of the solution and T the absolute temperature, have been compared with appropriate literature data at 25 °C. The effective hydrodynamic radii of the four ions have been calculated and interpreted in structural terms.