Some Electrochemical Methods/Experiments Using a Potentiostat
Cyclic Voltammetry (CV)
The trace opposite shows a cyclic voltammogram for K3Fe(CN)6.
Here, the initial potential is set to 0.80V (point A). The scan first goes in the negative direction and at point B, Fe(III) begins to be reduced to Fe(II), with the current peaking at point D and falling away as the concentration of Fe(III) near the electrode becomes depleted. Meanwhile the concentration of Fe(II) near the electrode is increasing.
The scan direction reverses at -0.15V. In the region F-H Fe(III) is still undergoing conversion to Fe(II). At point I oxidation has commenced, resulting in anodic current in the region I-K. At the end of the scan we have predominantly Fe(III) near the electrode, as we had at the start of the cycle.
In an analysis of a CV trace for a diffusion-controlled, reversible electrode reaction one can make the following observations:
• The anodic and cathodic peak currents are approximately the same.
• The difference in peak potentials is approximately 0.059/n (V), where n is the number of electrons involved in the half reaction.
• The potential midway between the two peak potentials is the formal electrode potential (E0’) (corrected for the reference electrode being used) for the redox couple.
• The peak currents for the forward and reverse sweeps are proportional to the square root of the scan rate.
Anodic Stripping Voltammetry (ASV)
In ASV there is a “deposition period”, during which the working electrode is held at a negative potential compared to the reference electrode and metal ions deposit at the WE.
Thereafter, the potential difference between the WE and the RE is ramped during a computer generated potential "sweep" as shown in the diagram at left.
During application of the potential sweep the current is measured at the WE, so the experimental data appears in the form of a current vs. voltage plot.
Square Wave Voltammetry (SWV)
SWV is a further improvement of staircase voltammetry (itself a derivative of linear sweep voltammetry e.g. ASV).
In staircase voltammetry the potential sweep is actual a series of staircase steps. In square wave voltammetry, a square wave is superimposed on the potential staircase sweep.
The current is measured at the end of each half-wave, just prior to the potential change. The differential current is then plotted as a function of potential, and the reduction or oxidation of species is measured as a peak or trough.
Pulsed Amperometric Detection (PAD)
This technique employs a tailored, stair-step waveform in situations where the working electrode needs to be regenerated prior to each detection event.
In the waveform shown opposite, an electro-active species is detected by monitoring the WE current in a time window τ near the end of the E1 step. This step typically lasts ~ several hundred milliseconds.
Immediately thereafter a positive step to E2 is made, which results in oxidative cleaning of the WE. This again is for several hundred milliseconds.
The potential is then taken negative to reactivate the WE, and the potential is once again held for several hundred milliseconds.
The whole sequence can then be repeated - this being done at a repetition rate of 1-2 Hz.
PAD is a useful electrochemical method for detecting sugars in chromatography, as these molecules do not have strong UV absorptions at the wavelength(s) of typical HPLC detectors.