Probe effects on concentration profiles in the diffusion layer: computational modeling and near-surface pH measurements using microelectrodes

A finite-element method was applied to the problem of measuring near-surface concentration values with a probe placed in front of the electrochemical interface. Those probes, like pH-sensing microelectrodes, interfere with mass transport and unreliable results may be obtained due to the local distortions. A pure diffusion problem was solved for a one-component two-dimensional axisymmetric model, under Dirichlet and Neumann-type boundary conditions at the substrate surface. The generalized results may be used to estimate the effects of the sensor tip-substrate distance on local current density and surface concentrations, and on measurement accuracy in absence of homogeneous reactions. To account for the presence of homogeneous reactions, simulations were performed for specific system compositions, using the near-surface pH measurement in solutions containing a buffering agent as an example. An IrOx microelectrode was used for experime ntal measurements of near-surface pH carried out in solutions displaying different buffering capacities. The results were compared to simulations performed for a one-dimensional model with simplified electrode kinetics, but fully describing the kinetics of homogeneous reactions. It is shown that valuable information on the electrochemical processes may be extracted by using this experimental approach, even when the measured pH does not strictly correspond to the pH effectively experienced by the substrate surface.