Time scanning pulse polarography, a new approach to chronoamperometry

Abstract

A new approach has been developed for discriminating at the dropping mercury electrode (DME) between currents controlled by various types of rate processes, including diffusion and chemical kinetics of reactions coupled with electron transfer. The crux of time scanning pulse polarography (t.s.p.p.) is a combination of chronoamperometry with pulse polarography. Typically, pulses of 200 ms were used and applied instantaneously (i.e. with the aid of a potentiostat whose rise-time was 2 ms) at a time, t 0 (on the order of 1 s) which corresponded to an appreciable fraction of the total drop time of the DME. Limiting currents, ( i) were subsequently “sampled” on successive drops as function of a time-shift, 1<θ<200 ms. These current measurements were taken at times ( t 0+θ) using sampling “time-windows” of 5 ms or less. Diagnostic plots of log i versus long θ were obtained whose slopes were characteristic of the controlling rate process, viz, ∂ log i/∂ log θ=−0.47, for a diffusion controlled electrode reaction; ∂ log i/∂ log θ=zero, for an electrode process whose kinetics are controlled by a preceding first order chemical reaction. Similar diagnostic plots were found to characterize “catalytic current” sequences. The significance of t.s.p.p. is inherent in the feature that it is applicable to trace concentration levels (10μ M or less) which are intractable by classical polarography.