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As we discussed in a previous post, cyclic voltammetry is normally the go-to technique to start characterizing electrochemical systems. But it has two main drawbacks: speed and background. So, what do scientist do when they want to overcome these drawbacks? They use square wave voltammetry.
Square wave voltammetry is a powerful electrochemical technique used by scientists to investigate redox reactions. From all the pulsed techniques, it is the one which output resembles cyclic voltammetry the most and can be processed to reduce the capacitive background signal.
From its fundamentals to its practical applications, in this blog post we will unravel everything there is to know about this powerful electrochemical method.
What is square wave voltammetry?
Square wave voltammetry is a specific type of pulse voltammetry, together with normal pulse voltammetry and differential pulse voltammetry, that minimizes the capacitive background current of the working electrode, whilst maximising the signal-to-noise ratio of the Faradic currents of the oxidation and reduction waves.
Square wave voltammetry excitation waveform
The excitation waveform of square wave voltammetry is a variation of linear sweep voltammetry that combines a square wave with a staircase potential sweep. As a result, the working electrode experiences two pulses, one with a forward potential and one with a reverse potential, for each step of the staircase.
This unique waveform allows for two current sampling locations that can be processed to minimize capacitive current contributions to the signal.
How does square wave voltammetry work?
When performing square wave voltammetry, potentiostats sample the current on the working electrode at two specific times:
- Once at the end of the forward potential pulse,
- Once at the end of the reverse potential pulse
In both cases, this sampling occurs immediately before the potential direction is reversed.
These two sampling times give rise to the two datasets generated by this technique: the forward and reverse i-v curves.
Superimposing these two curves results in a plot that resembles a cyclic voltammogram. It must be noted that forward and reverse currents consist of the sum of Faradic and non-Faradic contributions.
To obtain the best signal-to-noise ratio, the forward and reverse current curves should be subtracted to cancel out the capacitive contribution of the current. The resulting plot from this operation resembles that of differential pulse voltammetry.
What is the difference between cyclic voltammetry and square wave voltammetry?
While the results between cyclic voltammetry and square wave voltammetry may be similar to each other (specially when plotting both forward and reverse currents), there are a number of differences between the techniques.
3 differences between cyclic voltammetry and square wave voltammetry
1. Potential waveform
The first difference between the two techniques is the potential excitation waveform. While cyclic voltammetry uses a staircased triangular potential waveform, square wave voltammetry uses a combination of a staircase and a square potential waveform.
2. Scan rates
The second difference between cyclic voltammetry and square wave voltammetry is the speed of the techniques or scan rates used. Thanks to the rapid switching between potentials in square wave voltammetry, data can be acquired faster and faster scanrates are typically used. For cyclic voltammetry, however, slower scan rates are normally used to obtain voltammograms with enough detail.
Lastly, the sensitivity. Both techniques are very sensitive, but the capacitive background of cyclic voltammetry often masks redox reactions at low concentrations. In these cases, the capacitive background elimination of square wave voltammetry allows for better detection limits.
Square wave voltammetry plots
Square wave voltammetry data is typically plotted in two ways:
- using the Forward and Reverse currents
- using the difference between the Forward and Reverse currents
Forward and Reverse current plots
Whilst not used as often these days, the traditional plot for square wave voltammetry used the currents from both the Forward and Reverse currents.
When plotted together in a graph, the forward and reverse currents resemble a cyclic voltammogram.
Once upon a time, when mercury drop electrodes were used in the study of electrochemistry, acquiring full cyclic voltammograms within a single mercury drop with sufficient quality was extremely challenging. But the invention of square wave voltammetry enabled scientists to obtain similar datasets in a much shorter timeframe
Besides displaying cyclic voltammetry like data, square wave voltammetry can also be used to minimize the background capacitive current. Thus resembling the output of differential pulse voltammetry.
This is accomplished by subtracting the Reverse current from the Forward current, yielding a single peak for each oxidation/reduction pair on a baseline. Both the baseline height and drift, as well as the signal-to-noise ratio and width of the peaks will heavily depend on the parameters chosen during the acquisition. Thus, proper optimization studies for each application are required for best results.
Differential plots are particularly useful when working with complex samples that contain multiple redox peaks that may be overlapping, like heavy metal detection in waste water.
Parameters of square wave voltammetry
Modern potentiostats have a variety of parameters that can be tuned for square wave voltammetry measurements. All these parameters have a profound effect on the resulting curves and it is important to understand their function.
As with other electrochemistry techniques, the parameters can be divided into 2 types of settings: pretreatment and square wave voltammetry settings
Stage 1: Pretreatment Settings
The pretreatment settings are designed to prepare the electrochemical cell to improve the reproducibility of the results.
In most modern potentiostats, these parameters are normally part of the square wave voltammetry technique. But in some models, these may need to be set up separately.
In general, the pretreatments available are:
- Conditioning: this pretreatment equalizes the electrode potential at the begining of the experiment. It does so by holding the working electrode at a specific potential for a set amount of time. It is set by providing the following parameters:
- Conditioning potential (V)
- Conditioning time (s)
- Deposition: This pretreatment is performed after conditioning. It is mainly used for stripping voltammetry or applications where preconcentration of the analyte is required. This pretreatment helps improve both reproducibility and sensitivity. Similarly to conditioning, it is set up by providing 2 simple parameters:
- Deposition potential (V)
- Deposition time (s)
Stage 2 square wave voltammetry settings
- Equilibration time (s): this parameters determines the amount of time that the equipment will wait before starting the measurement. Depending on the potentiostat model, this might be a wait time with the cell off, or at the starting potential of the scan. The purpose of this parameter is to ensure that the initial currents of the scan are stable and no unexpected jumps are observed.
- Starting potential (V): this parameter specifies the first potential that will be applied to the electrochemical cell during the measurement.
- Ending potential (V): this parameter specifies the last potential that will be applied to the electrochemical cell during the measurement.
- Potential step (V): this parameter specifies the step of the staircase of the linear sweep component of the excitation wave of the experiment.
- Amplitude (V): this parameter specifies the amplitude of the square component of the excitation wave of the experiment.
- Frequency (Hz): this parameter specifies the speed at which the square wave will be applied during the experiment.
- Measure forward/reverse currents (boolean): this parameter may not be available in all potentiostats. It normally consists in a checkbox that determines whether the individual forward and reverse currents will be measured or not. If unchecked, only the differential current will be recorded.
How do square wave voltammetry parameters affect the resulting voltammogram?
When starting to use square wave voltammetry for characterisation of electrochemical systems it can be difficult to understand how tweaking all the different parameters affect the final result. So often, rookie scientist would end up testing out random parameters to try and elucidate how the results change.
A more effective approach that we would recommend is to perform a matrix optimization experiment to ensure that the best results are obtained each time, for each application.
However, if you would like to know roughly how to tweak the outcome of square wave voltammetry, here we explain to you how the effect of the potential step, amplitude and frequency affect the resulting voltammograms.
Effect of potential step
The potential step is, perhaps, the least flexible of the 3 crucial parameters of square wave voltammetry.
The reason is simple: it affects directly the resolution of the voltammograms.
Therefore, you will likely want to keep it below 10 mV, since any larger will start to make it difficult to discern any peaks clearly. For challenging applications, 1 mV will normally be preferred.
But, there’s a catch. The potential step also affects the acquisition time. We’ll explain more in detail once we explain the effect of frequency, but since the acquisition time changes, increasing the potential step not only makes peaks less defined, but also broader.
Effect of amplitude
The amplitude of the square wave is one of the most tuneable parameters of square wave voltammetry.
It affects two main characteristics of the resulting voltammograms: background signal and signal-to-noise ratio of the resulting peaks.
As we discussed above, the forward and reverse currents show a combination faradic and capacitive currents. To maximize the capacitive background correction, minimizing the amplitude helps, as it reduces the difference between the capacitances of the forward and background currents. This comes, however, at the expense of lower peak intensities.
Large amplitudes do offer better peak intensities, but as amplitudes become larger, the background current increases, the peaks become broader and shift from the location they should be. Thus, if inapropriate amplitudes are used, it can lead to inaccurate results.
Effect of Frequency
Frequency is a parameter that strongly affects the speed at which data is recorded together with the potential step. For comparison with cyclic voltammetry, square wave voltammetry scan rates can be calculated as follows:
Increasing the frequency not only reduces the acquisition time, but in general also improves peak intensity. However, for diffusion-limited experiments, utilizing frequencies that are too high can lead to peak broadening and distortion. So for most applications, the frequency tends to range 25 to 100 Hz.
Who invented square wave voltammetry?
The iinvention of square wave voltammetry is attributed to G. C. Barker and the first appearance of the technique in scientific literature was in 1960 with a paper entitled “Pulse polarography”.
50 years ago, the working electrode for electrochemistry was often a dropping mercury electrode (DME). This type of electrode consisted, literally, in a liquid mercury column that would eject droplets of mercury that would be utilized as the working electrode. Since the mercury drop surface area was constantly changing throughout the experiment, reliable data was difficult to obtain. In fact, complex maths were required to analyze the collected data. Square wave voltammetry changed this. Thanks to the fast acquisition speeds, voltammograms could be obtained within a single mercury drop. So, complex maths for analysis were no longer required.
Luckily, nowadays DMEs are no longer used. We use safer and more reliable electrode systems. But square wave voltammetry remains as a useful technique for acquiring reliable voltammograms at a fast speed.
Advantages and disadvantages of Square Wave Voltammetry
- Enhanced sensitivity compared to other voltammetric techniques like cyclic voltammetry.
- Reduced capacitive background
- Faster scanning rates that enable quicker analysis.
- Better peak resolution, ideal for complex samples with overlapping peaks
- Specialized equipment capable of generating and detecting square waves is necessary. This is not necessarily an issue for today’s potentiostats, but may be a hurdle for point of care dignostics due to increased reader cost.
- The diffusion layer is not regenerated after the first scan, which can lead to deteriorated performance if unmanaged.
We hope that our guide on square wave voltammetry has helped you get a deep understanding of this versatile technique. With the rise of low cost potentiostats with pulse capabilities, square wave voltammetry is sure to become an even more widely used technique. So it is important to understand its principles and how it can be used to develop assays.