hydrodynamic methods.ppt

Methods involving forced convection—hydrodynamic methods
Methods involving convective mass transport of reactants and products are sometimes called hydrodynamic methods.
The advantage of hydrodynamic methods is that a steady state is attained rather quickly and measurements can be made with high precision, often without the need for recorders or oscilloscopes. In addition, at steady state, double-layer charging does not enter the measurement.
Rotating disk electrode
Construction of hydrodynamic electrodes that provide known and reproducible mass transfer conditions is more difficult than for stationary electrodes.
The most convenient and widely used system involves the rotating disk electrode.
This electrode is amenable to rigorous theoretical treatment and is easy to construct with a variety of electrode materials.
Rotating disk electrode
Schematic resultant streamlines
Rotating disk electrode
RDE is rather simple to construct and consists of a disk of the electrode material imbedded in a rod of an insulating material.
RDE is rotated at a certain frequency, f(revolutions per second), but the parameter of interest is the angular velocity, ω(sec-1), where ω=2πf.
Theoretical treatment of convective systems
Within the layer, 0≤x≤δ, no solution movement occurs and mass transfer takes place by diffusion. Thus the convection problem is converted to a diffusional one in which the adjustable parameter δis introduced.
In this model, it is assumed that convection maintains the concentrations of all species uniform and equal to the bulk values up to a certain distance from the electrode, δ
The convective-diffusion equation
Drive of fluid flow: diffusion, migration and convection. For solutions containing an excess of supporting electrolyte, the ionic migration term can be neglected.
When fluid flow is smooth and steady and occurs at the separate layers, the flow velocity is small right at the walls because of friction between the fluid and the wall.
Levich equati