Hydrodynamics and mass transport in wall-tube and microjet electrodes: An experimental evaluation of current theory

The application of steady-state and fast-scan linear sweep voltammetry to a high-speed wall-tube electrode (HWTE) is reported in different solvents to investigate the response of the HWTE over a wide range of Reynolds' numbers (Re). Experiments are reported for the oxidation of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in propylene carbonate (PC), water, butyronitrile (BN), acetonitrile (AN), and acetonitrile-water mixture solutions containing 0.10 M supporting electrolyte for a 24 μm radius platinum microdisk electrode housed within the HWTE using a range of average flow jet velocities from 0.03 to 19.8 m s (corresponding to volume flow rates of 0.003-0.25 cm s and center-line jet velocities from 0.05 to 39.5 m s). Fast scan linear sweep voltammetry is presented for the oxidation of TMPD in PC and of 9,10-diphenylanthracene (DPA) in AN. Theoretical results are derived using finite element methods for both one- and two-dimensional mass transport models. It is found that, for solvents with a kinematic viscosity above ca. 7.5 × 10 cm s, the hydrodynamic behavior for Re <2000 is as expected with current responses in accordance with those predicted for a laminar, parabolic inlet flow profile. In low viscosity solvents, where Re <2000, currents are lower than expected, indicating a departure from laminar flow in practical cells even at low Re. The HWTE is compared to the channel electrode in the light of the experimental results, theoretical limits of electron-transfer rate detectable, and conclusions drawn that the channel electrode is more reliable for kinetic measurements.