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Electrochemical Voltammetry
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BAS (Bioanalytical Systems) CV-27
Voltammograph, BAS C-1B Cell Stand, coupled with a BAS XYT Analog
Recorder. The Cell Stand features magnetic stirring and selectable
gas purge rate for the sample. The gas control provides for blanketing
the sample when active purging is not occurring.
The magnetic stirrer controls mass transport to the electrode
surface and mixing for titration's. The lift arm with detachable
cell top allows removal and replacement of the cell vial. The
cell is enclosed in a Faraday Cage to minimize electrical interference.
The BAS Voltammetric Analyzer is an electrochemical instrument
capable of performing a variety standard controlled-potential
techniques. It is ideal for demonstrating basic electrochemistry
and doing routine electroanalysis. It is capable of: digital smoothing,
peak or wave height determination transformation (the Anson and
Cottrell transforms, convolution, deconvolution. etc.),
peak potential and current measurement, half-wave and limiting
current measurement, and cyclic voltammetry.
BAS CV27 allows for cyclic voltammetric and bulk electrolysis
experiments. Cyclic voltammetry is an electrolytic method
that uses microelectrodes and an unstirred solution so that the
measured current is limited by analyte diffusion at the electrode
surface. The electrode potential is ramped linearly to a more
negative potential, and then ramped in reverse back to the starting
voltage. The forward scan produces a current peak for any analytes
that can be reduced through the range of the potential scan. The
current will increase as the potential reaches the reduction potential
of the analyte, but then falls off as the concentration of the
analyte is depleted close to the electrode surface. As the applied
potential is reversed, it will reach a potential that will reoxidize
the product formed in the first reduction reaction, and produce
a current of reverse polarity from the forward scan. This oxidation
peak will usually have a similar shape to the reduction peak.
Cyclic Voltammetry (CV) is a versatile electroanalytical technique for the
study of electroactive species. CV consists of linearly cycling
the potential of an electrode immersed in an unstirred solution
while measuring the resulting current. Thus, a voltammogram is
a display of current versus potential. The most useful
aspect of this technique is its application to the qualitative
diagnosis of electrode reactions. Such as voltammetry of a redox
couple (e.g. ferro-/ferricyanide or Fe2+/Fe3+ in aqueous solution ) using a rotating disc electrode (Pt or Au)
or cyclic voltammetry of a redox system. A simple potential wave
form that is often used in electrochemical experiments is the
linear wave form i.e., the potential is continuously changed
as a linear function of time. The rate of change of potential
with time is referred to as the scan rate. Cyclic voltammetry
can be used as a technique to:
- Reveal surface contamination.
- Estimate relative surface area and roughness.
- Evaluate electrolyte leakage at electrode-insulator interfaces.
- "Fingerprint" electrochemical reactions for benchmarking
and quality control
- Estimate potentials at which reduction-oxidation reactions
occur.
- Determine charge storage capacity
The simplest technique that uses this wave form is linear sweep
voltammetry. The potential range is scanned in one direction,
starting at the initial potential and finishing at the final potential.
A more commonly used variation of the technique is cyclic voltammetry,
in which the direction of the potential is reversed at the end
of the first scan. Thus, the waveform is usually of the form of
an isosceles triangle. This has the advantage that the product
of the electron transfer reaction that occurred in the forward
scan can be probed again in the reverse scan. In addition, it
is a powerful tool for the determination of formal redox potentials,
detection of chemical reactions that precede or follow the electrochemical
reaction and evaluation of electron transfer kinetics.
An example wave form that can be used in cyclic voltammetry
is shown below (Reload to view the cycle). In this example
it is assumed that only the reduced form of the species is initially
present. Thus, a positive potential scan is chosen for the first
half cycle during which an anodic current is observed. Because
the solution is quiescent, the product generated during the forward
scan is available at the surface of the electrode for the reverse
scan resulting in a cathodic current.