10-48
10.7
Hydrodynamic Techniques (RDE,
HDM)
The current response to an applied potential may be determined by a number of
parameters. Two of the most important are the rate of electron transfer and the rate of
mass transport from the bulk solution to the surface of the working electrode. The
rate of electron transfer can be controlled by the applied potential, and is not
discussed further. It is the rate of mass transport that is of interest in this section.
There are three ways by which mass transport can occur:
a) Diffusion - motion down a concentration gradient.
b) Migration - motion down an electric gradient
c) Convection - molecular motion imposed by external influences, e.g., stirring
In order to obtain quantitative data from voltammetry experiments, it is important that
the mode of mass transport be mathematically well defined. Migration is eliminated
from all voltammetry experiments by the addition of a fully dissociated electrolyte.
Therefore, diffusion and convection only are used.
In many voltammetry experiments, convection is eliminated by using an unstirred
solution and protecting the solution from external vibrations (although such
conditions can only maintained for relatively short time periods). Voltammetry
techniques that use stationary solution include Cyclic Voltammetry,
ChronoCoulometry, pulse and square wave techniques. In addition to the
experimental difficulties of maintaining convection-free conditions, diffusion
controlled techniques are limited in that there is no way to vary the rate of mass
transport.
In hydrodynamic techniques, the molecules are brought to the electrode surface is
some well defined manner. This can be done by stirring the solution or pumping the
solution through a flow cell (as in Liquid Chromatography/Electrochemistry). One
widely used method is to rotate the electrode using a Rotating Disk Electrode.
Hydrodynamic techniques have a number of advantages over stationary solution
techniques, due to the increase in the rate of mass transport to and from the electrode
surface. The faster rate allows steady state conditions to be rapidly achieved (i.e., the
rates of mass transport and electron transfer are balanced), and steady state conditions
can be maintained during a potential scan provided the linear
Scan Rate
is
sufficiently slow (typically, about 20 mV/sec). One advantage of steady state
voltammetry is that the current at a given potential is independent of both the scan
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