|
Voltammetry is one of the techniques which electrochemists employ to investigate electrolysis
mechanisms. There are numerous forms of voltammetry
* Potential Step
* Linear sweep
* Cyclic Voltammetry
For each of these cases a voltage or series of voltages are applied to the electrode and the
corresponding current that flows monitored. In this section we will examine potential step
voltammetry, the other forms are described on separate pages
Experimental cell
For the moment we will focus on voltammetry in stagnant solution. The figure below shows a
schematic of an electrolysis cell. There is a working electrode which is hooked up to an external
electrical circult. For our purposes at the moment we will not worry about the remainder of the
circuit, obviously there must be more than one electrode for current to flow. But as we shall see
later it is only the so called working electrode that controls the flow of current flow in the
electrochemical measurement.
The essential elements needed for an electolysis measurement are as follows:
* The electrode: This is usually made of an inert metal
(such as Gold or Platinum)
* The solvent: This usually has a high dielectric constant
(eg water or acetonitrile) to enable the electrolyte to dissolve and help aid the passage of
current.
*A background electrolyte: This is an electrochemically
inert salt (eg NaCl or Tetra butylammonium perchlorate, TBAP) and is usually added in high
concentration (0.1M) to allow the current to pass.
*The reactant: Typically in low concentration
10-3 M
Potential Step Voltammetry
In the potential step measurement the applied voltage is instantaneously jumped from one value
V1 to another V2
The resulting current is then measured as a function time. If we consider the reaction
Usually the voltage range is set such that at V1 the reduction of (Fe3+ ) is thermodynamically unfavourable. The
second value of voltage (V2) is selected so
that any (Fe3+) close to the electrode
surface is converted to product (Fe2+ ).
Under these conditions the current response is shown below
The current rises instantaneously after the change in voltage and then begins to drop as a function
of time. This occurs since the instant before the voltage step the surface of the electrode is
completely covered in the reactant and the solution has a constant composition below
Once the step occurs reactant (Fe3+) is
converted to product (Fe2+) and a large
current begins to flow. However now for the reaction to continue we need a supply of fresh
reactant to approach the electrode surface. This happens in stagnant solution via diffusion. As
we noted in a previous section the rate of diffusion is controlled by the concentration gradient.
So the supply of fresh (Fe3+) to the surface
(and therefore the current flowing) depends upon the diffusional flux. At short times the
diffusional flux of (Fe3+) is high, as the
change in concentration between the bulk value and that at the surface occurs over a short
distance
Concentration verses distance above the electrode before voltage
step
Concentration verses distance above the electrode a short
time after a voltage
step
As the electrolysis continues material can diffuse further from the electrode and therefore the
concentration gradient drops. As the concentration gradient drops (see concentration profiles
below) so does the supply of fresh reactant to the surface and therefore the current also decreases.
For a large electrode the appropriate mass transport equation for the reaction is
where the distance x is normal to the electrode
surface. If we recall the expression for the electrolysis current for a reduction reaction:
Clearly this is not particularly helpful for experimental purposes since we require the surface
concentration as a function of time. However Cottrell showed it is possible to rearrange and solve
the mass transport equation in terms of the flux. This gives
Now the current is related to the bulk reactant concentration and we can see by inspection that
if the current is measured as a function of time we would expect a plot of
to be a straight line if the reaction is occuring by diffusion control. Such plots allow the
estimation of the diffusion coefficients of the species to be obtained.
| 
