126
10
Theory
10.1
Forced Convection
The current signal recorded during an electrochemical experiment is easily
influenced or disturbed by the convection of various molecules and ions due to
bulk movement of the solution. Proper interpretation of the current signal must
accurately account for any contributions (desired or undesired) from solution
convection. Thus, the control of solution movement is a critical part of any
electrochemical experiment design, and the issue of convection cannot be
ignored. Two opposing approaches are typically used to address the convection
issue. At one extreme, an experiment can be conducted in a quiescent solution,
so that convection makes little or no contribution to the observed current. The
opposite extreme involves forced convection, where the solution is actively stirred
or pumped in a controlled manner.
At first glance, it may seem that the simplest and most obvious way to account
for convection is to try to eliminate it entirely by using a quiescent (non-moving)
solution. This is the approach used in many popular electroanalytical
techniques
[1]
(including cyclic voltammetry, chronoamperometry, square wave
voltammetry, and differential pulse voltammetry). The timescale for these
methods is generally less than 30 seconds, and on such short timescales, the
influence of convection in an unstirred solution is generally negligible. On longer
timescales, however, even unstirred solutions are prone to convective
interference from thermal gradients and subtle environmental vibrations.
For long duration (steady-state) experiments, convection is unavoidable, so
actively forcing
[2]
the solution to move in a well-defined and controlled manner is
the preferred approach. An entire family of electroanalytical methods (broadly
categorized as hydrodynamic voltammetry) couples precise control of solution
flow with rigorous mathematical models defining the flow. Some of the many
examples of hydrodynamic voltammetry include placing an electrode in a flow
cell,
[3]
firing a jet of solution at an electrode target,
[4-5]
embedding an electrode
in a microfluidic channel,
[6]
vibrating a wire-shaped electrode,
[7]
subjecting the
solution to ultrasonication,
[8]
and rotating the electrode.
[2,9-14]
By far the most popular and widely used hydrodynamic methods are those that
involve a rotating electrode. The rotating electrode geometries most amenable
to mathematical modeling are the rotating disk electrode (RDE),
[9-14]
the rotating
ring-disk electrode (RRDE),
[15-26]
and the rotating cylinder electrode (RCE).
[27-32]
Researchers take advantage of the stable, steady-state laminar flow conditions
adjacent to an RDE or RRDE to carefully gather information about electrode
reaction kinetics.
[13,14,21,26,33-43]
In contrast, the relatively chaotic and turbulent
conditions adjacent to an RCE are exploited by corrosion scientists
[44-69]
wishing to
Summary of Contents for MSR 636A
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