Description
Square-wave Voltammetry (SWV) is used for both quantitative chemical analysis and study of the mechanism, kinetics, and thermodynamics of chemical reactions.
SWV used as an analytical tool offers three major advantages when compared to other electrochemical techniques:
•SWV is extremely sensitive, often allowing direct analyses at the ppb (parts per billion) level and even the low ppt (parts per trillion) level when used in a stripping mode.
•SWV requires less time per sweep than older techniques such as Differential Pulse Polarography. A SWV sweep can often be recorded in less than ten seconds, in contrast with a Differential Pulse Polarogram that typically requires more than two minutes for data acquisition.
•The square-wave frequency can be used to differentiate between processes with fast and slow kinetics. In some cases, kinetically fast processes can be measured without interference from slower processes that occur in the same potential range.
Other techniques, such as Cyclic Voltammetry, are generally preferred over SWV for mechanistic and kinetic studies. However, square-wave voltammetry’s sensitivity allows mechanistic and kinetic measurements in solutions that are too dilute for more conventional study.
SWV is generally performed on a stationary solid electrode or a hanging mercury drop electrode. The SWV script in the Pulse Voltammetry software provides for mercury-drop generation, solution de-aeration, and experiment sequencing suitable for the most common applications for square-wave voltammetry.
Choose the type of electrode in the Electrode Setup Panel.
Additional sequencing steps suitable for square-wave anodic (or cathodic) stripping are implemented in the Pulse Voltammetry’s SWS (Square Wave Stripping) technique.
Applied Waveform and data acquisition
The Pulse Voltammetry software implements a popular form of square-wave voltammetry generally called “Osteryoung” SWV. Another variant, known as “Barker” SWV involving a linear voltage-ramp, is less common, and not implemented in the software.
In a square-wave voltammetric experiment, a complex voltage sweep is applied to the working electrode in an electrochemical cell. This waveform can be constructed as a sequence of fixed height pulses (Curve A, below) added to a voltage staircase (Curve B, below). The resulting waveform is seen in the figure below.
Step signal for Square Wave Voltammetry.
The pulse height is generally 20 to 50 mV. The staircase has 2 to 10 mV steps. In the diagram above, notice that the square-wave and staircase waveforms are synchronized and that the square-wave has a 50% duty cycle.
The staircase waveform in Pulse Voltammetry is defined to begin at Initial E and end at Final E. The duration of the sweep is defined by the staircase step height (Step Size), the square-wave frequency (Frequency), and the initial and final potentials.