Determination of the Diffusion Coefficient of Hydrogen in Aqueous Solution Using Single and Double Potential Step Chronoamperometry at a Disk Ultramicroelectrode

An assessment is made of single and double potential step chronoamperometry (SPSC and DPSC, respectively) at Pt disk ultramicroelectrodes (UMEs) as methods for determining the value of the diffusion coefficient of hydrogen in aqueous solutions. In SPSC, measured currents for the oxidation of dissolved hydrogen (at concentrations close to saturated solution values) comprise a significant contribution, at short to moderate times, from the oxidative desorption of adsorbed hydrogen as well as the diffusion-controlled oxidation of the solution species. Provided that the electrode is preconditioned using a well-defined potential cycling procedure, the behavior for the oxidative desorption step alone can be established in an Ar-saturated solution. The chronoamperometric characteristics for the solution diffusion-controlled process may then be determined, from which the diffusion coefficient of hydrogen can be measured. In DPSC, a locally supersaturated solution of hydrogen is created transiently through the diffusion-controlled reduction of a known concentration of protons in an initial potential step. Hydrogen is subsequently collected back through oxidation to protons; the current flowing depends on the diffusion coefficients of the two species and the duration of the forward step. Under these conditions, the contribution from surface electrochemical processes to the forward and reverse chronoamperommograms is shown to be negligible. By solving the mass transport problem for DPSC with arbitrary diffusion coefficients of the redox species, the diffusion coefficient of hydrogen is readily determined. Both methods yield a consistent value for the diffusion coefficient of hydrogen, D H 2 , in 0.1 mol dm-3 KNO3 of 5.0 × 10-5 cm2 s-1.