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From all the pulsed techniques available in electrochemistry, Normal Pulse Voltammetry (NPV) is, perhaps, the least used nowadays. Likely because the shape of the resulting voltammogram, a sigmoid, is less attractive from a presentation point of view. However, that does not mean that NPV is an obsolete technique. In fact, there are a number of situations where characterising electrochemical systems with NPV is the better choice.
So, if you would like to know more about Normal Pulse Voltammetry keep reading, as we will cover its principles, parameters and applications in this guide.
1.1 What is Normal Pulse Voltammetry?
Normal Pulse Voltammetry is an electroanalytical technique that involves the application of a series of increasing potential steps onto a constant DC potential to study redox processes and characterize electrochemical systems.
The data arising from a typical Normal Pulse Voltammetry experiment typically displays a sigmoidal shape centered at the redox potential of the species being oxidized/reduced.
1.2 Importance and Applications
Normal Pulse Voltammetry is one of those techniques that you rarely see in scientific papers. This is mainly due to the fact that scientists prefer other techniques, like cyclic voltammetry or square wave voltammetry, over normal pulse voltammetry. The reason is simple. They offer more information, are quicker and are more straightforward to analyze.
However, Normal Pulse Voltammetry offers a unique approach to characterize diffusible redox molecules, like ferricyanide, and determine the optimal applied potential in DC experiments to make electrooxidation/reduction as efficient as possible. One application of this type of characterisation is in second generation glucose biosensors, where an artificial mediator is continuously oxidized at a DC potential. In this case, NPV can be used to determine the optimal DC potential to apply by determining at what potential the reaction starts to be diffusion limited and not potential limited.
2. Understanding the Waveform
In Normal Pulse Voltammetry (NPV), the waveform used for excitation plays a crucial role in obtaining meaningful results. Let’s explore the characteristics of the excitation waveform and its significance in the NPV technique.
2.1 Excitation Waveform in Normal Pulse Voltammetry
Unlike the aforementioned pulsed techniques, where pulses are superimposed on a staircase linear sweep, in NPV the pulses always go back to the initial potential value, and instead, the pulse height increases pulse after pulse.
3. Working Principle of Normal Pulse Voltammetry
To gain a comprehensive understanding of Normal Pulse Voltammetry, it’s crucial to explore its working principle and how it distinguishes itself from other pulse voltammetry techniques. Let’s delve into the fundamental concepts behind NPV and its unique characteristics.
3.1 Electrochemical Reactions and Current Measurements
In NPV, electrochemical reactions occur at the working electrode surface when potential steps are applied.
As we discussed, the waveform of NPV consists in a series of increasing potential pulses superimposed on a DC potential. Thanks to this waveform, when the parameters are properly optimized, the initial status of the redox species before the pulse will always be the same.
Let’s further clarify this concept with an example.
Imagine you have a solution of 100% ferrocyanide and you start scanning from -0.2 V vs Ag/AgCl up to 0.5 V vs Ag/AgCl. Let’s see what happens when you scan these potentials using differential pulse voltammetry and normal pulse voltammetry.
In differential pulse voltammetry, the resulting data would display a peak around 0.2 V vs Ag/AgCl. This can be explained by mathematical equations of Butler-Volmer and Cottrell. As we reach 0.2 V the oxidation of ferrocyanide becomes more efficient and the current increases exponentially (Butler-Volmer), but beyond 0.2 V there is less ferrocyanide available due to mass transport limitations and decays rapidly (Cottrell). It is the combination of these 2 processes that result in the peak like shape.
In normal pulse voltammetry, however, the data would display a sigmoidal curve centred at 0.2 V vs Ag/AgCl.
Because after each pulse, the potential applied goes back to the initial DC potential, which, in this case, would be -0.2 V vs Ag/AgCl. At this potential, the ferrocyanide that has been oxidised to ferricyanide during the pulses as they reach the oxidation threshold, get reduced back to ferrocyanide. Therefore, since the amount of ferrocyanide ions available is regenerated at the end of each pulse, the current continues to increase until it flattens out. Thus the Cottrell behaviour is eliminated. However, this plateau is caused by mass transport limitations and displays the total amount of ferrocyanide ions that can be oxidized in these conditions.
3.2 Comparison with Other Pulse Voltammetry Techniques
To understand the distinct advantages of Normal Pulse Voltammetry (NPV), it’s helpful to compare it with other commonly used pulse voltammetry techniques.
Let’s examine the key differences between Square Wave Voltammetry (SWV), Differential Pulse Voltammetry (DPV), and Normal Pulse Voltammetry (NPV) in the following table:
|Square Wave Voltammetry||Square potential waveform||Precise determination of reversible and quasi-reversible reactions.|
Improved sensitivity compared to cyclic voltammetry
|Limited sensitivity for low concentration species|
|Differential Pulse Voltammetry||Differential potential pulses||Improved sensitivity for low concentration species||Limited ability to investigate irreversible and complex systems|
|Normal Pulse Voltammetry||Increasing potential steps||Removes Cottrell behavior|
Improved sensitivity compared to linear sweep voltammetry
|Requires careful selection and optimization of parameters|
By utilizing potential steps instead of square waveforms or differential potential pulses, NPV offers the unique advantage of removing the Cottrell behaviour. However, it requires careful parameter selection and optimization.
4. Interpreting Normal Pulse Voltammograms
Normal Pulse Voltammograms serve as essential graphical representations of the electrochemical processes occurring during the application of potential steps. These plots provide valuable insights into the redox behavior and characteristics of the electrochemical system under study. In this section, we will explore how to analyze and interpret normal pulse voltammograms.
4.1 Analysis of a Typical Normal Pulse Voltammogram
A typical normal pulse voltammogram consists of a distinctive sigmoid curve that represents the transition between oxidation states of a reversible redox molecule.
By carefully examining the voltammogram, it is possible to determine the reversibility of the redox reactions and quantify the concentration of analyte species.
4.2 Characteristics and Significance
The sigmoid curve observed in normal pulse voltammograms provides valuable information about the electrochemical system:
- The sigmoid centre position (potential) relates to the standard redox potential of the analyte species.
- The step height (current) is directly proportional to the concentration of the redox species.
- The plateau position (potential) relates to the point where the electrochemical reaction is primarily dominated by mass transport instead of potential.
Now that we have gained insights into the interpretation of normal pulse voltammograms, let’s proceed to the next section, where we will discuss the importance of pretreatment techniques and experimental parameter settings.
5. Pretreatment and Experimental Parameter Settings
To obtain accurate and reliable results in Normal Pulse Voltammetry (NPV), proper pretreatment techniques and careful selection of experimental parameters are crucial. In this section, we will explore the importance of pretreatment and discuss the key experimental parameters that significantly influence the voltammogram.
5.1 Importance of Pretreatment Techniques
Pretreatment techniques are employed to prepare the working electrode surface, ensuring a clean and well-defined electrochemical status before the measurement starts. Common pretreatment methods include cleaning as well as applying conditioning and deposition potentials to enhance reproducibility and/or preconcentrate analytes on the surface of the electrode.
Proper pretreatment enhances the accuracy and precision of the NPV measurements, leading to reliable and consistent voltammograms. It also minimizes background noise, and interfering species, while facilitating the detection of analyte signals.
5.2 Influence of E step (Potential Step)
The choice of the potential step, or E step, plays a crucial role in the resolution of NPV experiments. Larger potential steps result in less datapoints. Therefore, this parameter has a direct impact on the final file size arising from the measurement.
5.3 Influence of t pulse (Pulse Duration)
The pulse duration, or t pulse, determines the duration for which the potential step is maintained during NPV experiments. It affects the duration of the forward and reverse electrochemical reactions and, thus, the resulting current response.
For most potentiostats, the max pulse time allowed is limited to 50% of the period time. This changes with scan rate and can be calculated as follows:
Longer t pulse values allow for more extended reactions and improved signal-to-noise ratios and lower capacitive current contributions, while shorter t pulse values enable the examination of faster processes.
Proper adjustment of t pulse is crucial to capturing the desired electrochemical behaviour and obtaining reliable voltammograms.
5.4 Influence of Scan Rate
The scan rate in NPV refers to the rate at which the potential steps are applied. It determines the speed at which the potential changes and affects the kinetics of the electrochemical processes.
In general, higher scan rates allow for faster measurements, while slower scan rates offer improved resolution and detailed examination of the redox processes.
Choosing an appropriate scan rate is essential to balance measurement speed and the ability to capture the desired electrochemical information accurately. In most cases, slow scan rates are preferred as they allow for larger pulse times and better resolution of the expected sigmoidal curve with minimal capacitive contributions.
6. Performing Normal Pulse Voltammetry Experiments: A Step-by-Step Guide
Performing Normal Pulse Voltammetry (NPV) experiments requires careful planning and execution to obtain accurate and reliable results. In this section, we will provide a step-by-step guide to performing NPV experiments, highlighting best practices and considerations along the way.
6.1 Step 1: Sample Preparation
Begin by preparing the electrochemical sample for analysis.
This involves selecting the appropriate working electrode material (in most cases gold will be the best choice thanks to its inert nature) and preparing the supporting electrolyte solution with the analyte of interest.
If you are working on a quantitative assay to determine the concentration of the target analyte, try to eliminate, where possible, contaminants that may interfere with the Normal Pulse Voltammetry measurement.
6.2 Step 2: Electrode Pretreatment
Thoroughly clean and pretreat the working electrode to ensure a reliable and reproducible result.
In most cases, electrode cleaning requires an electrochemical polishing step.
In terms of pretreatments, normal pulse voltammetry normally requires applying conditioning and deposition potentials to ensure that the electrochemical state and anlyte concentration near the surface are consistent across experiments.
6.3 Step 3: Experimental Setup
Immerse the electrodes of a 3 electrode electrochemical cell, screen printed electrode or PCB electrode in the electrolyte containing the analyte of interest and connect it to a potentiostat with normal pulse voltammetry capability.
6.4 Step 4: Parameter Selection
Carefully select the experimental parameters, including the potential step (E step), pulse duration (t pulse), and scan rate.
You will need to optimize these parameters of normal pulse voltammetry to obtain the best results. So conducting preliminary experiments to determine the best parameters is recommended.
But as a rule of thumb, using scan rates up to 25 mV/s, pulse times above 10 ms and potential steps of at least 5 mV tend to give good initial results.
6.5 Step 5: Measurement and Data Acquisition
Start the NPV experiment. For most modern potentiostats this will just require pressing ‘Run’ on the control software.
Verify that the applied potential ranges capture the full extent of the spectrum where your analyte is electrochemically active and save the data.
To ensure the results are reproducible it is recommended to repeat the measurement multiple times.
6.6 Step 6: Data Analysis and Interpretation
To analyze the acquired normal pulse voltammetry data it is best to use specialized software suite. Doing so it is possible to extract the key information more reliably, and even automate it.
Ideally fitting a sigmoidal curve is best to extract the potentials and currents of interest from the voltammogram. Alternatively it can also be done manually, but it is more time consuming and prone to human bias.
Normal Pulse Voltammetry (NPV) is a powerful electrochemical technique that enables the investigation of electrochemical reactions without experiencing the Cottrell behaviour. In this comprehensive guide, we have explored the fundamentals of NPV, its waveform, working principles, its applications and the key experimental parameters that influence the voltammogram.
By following the step-by-step guide provided and using the information about how each parameter affects the results, you should be able to perform NPV experiments effectively, optimize your results, and obtain valuable insights from electrochemical systems.
We hope this guide has provided a comprehensive understanding of Normal Pulse Voltammetry and its applications. Whether you are new to the field of electrochemistry or seeking a refresher, we encourage you to explore further and apply this powerful technique to your research endeavours.
If you are interested in learning more about electrochemistry techniques and applications check out our blog, where you will find lots of useful information on electrochemistry and biosensing in general.
For further academic reading on Normal Pulse Voltammetry, we recommend visiting this link from Science Direct. Science Direct is a scientific literature portal from Elsevier that has up to date curated topics of scientific interest.