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 chronoampero metry (SPSC and DPSC, respectively) at Pt disk ultramicroelectrodes (U MEs) 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 mode rate times, from the oxidative desorption of adsorbed hydrogen as well as the diffusion-controlled oxidation of the solution species, Provid ed that the electrode is preconditioned using a well-defined potential cycling procedure, the behavior for the oxidative desorption step alo ne can be established in an Ar-saturated solution. The chronoamperomet ric characteristics for the solution diffusion-controlled process may then be determined, from which the diffusion coefficient of hydrogen c an 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, Hydroge n is subsequently collected back through oxidation to protons; the cur rent flowing depends on the diffusion coefficients oft he two species and the duration of the forward step, Under these conditions, the cont ribution from surface electrochemical processes to the forward and rev erse chronoamperommograms is shown to be negligible, By solving the ma ss transport problem for DPSC with arbitrary diffusion coefficients of the redox species, the diffusion coefficient of hydrogen is readily d etermined. Both methods yield a consistent value for the diffusion coe fficient of hydrogen, D-H2, in 0.1 mol dm(-3) KNO3 of 5.0 x 10(-5) cm( 2) s(-1).