Electrochemistry is the study of techniques that use electrical stimulation to analyze the chemical reactivity of a system. More specifically, it analyzes the loss and gain of electrons i.e. the oxidation and reduction mechanisms in a reaction. Oxidation and reduction reactions are called redox reactions- these provide vital information related to the kinetics, concentration, mechanism of reaction, and chemical status of the reactants in solution.
Electrochemical analysis is very helpful in many applications including the study of neurotransmitter behavior and polymerizations reactions. Electrochemistry is different from spectroscopy as electrochemical techniques analyze a different set of parameters.
Electrodes in electrochemical analysis
Electrochemical methods make use of electrically conductive probes or electrodes which are usually linked to electronic devices that measure the electrical parameters of the reactants in solution. Most electroanalytical techniques use three electrodes, namely; the working electrode, the reference electrode and the counter (auxiliary) electrode. These electrodes are connected to a potentiostat that controls the working electrode potential and determines the resulting current.
The working electrode is a critical component of an electrochemical experiment as this is where the key reactions take place. This electrode is usually made of an inert material. The first step in a typical electrochemical analysis is the application of a potential to the working electrode. Next, the resulting current is measured and plotted against time. Alternatively, the potential can be varied and the resulting currents can be plotted against the applied potential.
Several types of electrode substrates are used in electrochemical experiments. Some of these are highlighted below:
- Carbon as an electrode substrate is beneficial as it can be easily renewed for electron exchange.
- Mercury electrodes are very popular as they are renewable and reproducible.
- Nanomaterial-based electrodes have a high surface area for an increased immobilization of functional groups. Certain semiconductor nanomaterials promote the rate of electron transfer between proteins and electrodes. Metal nanomaterials such as gold nanoparticles, carbon nanotubes (CNTs), graphene, and metallic oxide/sulfide nanomaterials are commonly used as electrode substrates. Some biocompatible nanomaterials can help proteins or cells maintain their activities on the electrode for a long period for the analysis of proteins and cells.
- Chemically modified electrodes seek to enhance specific properties of the ordinary electrode such as compatibility with reactants such as proteins.
- Noble metals such as gold, silver, and platinum are also widely used as electrode substrates. Silver is usually used for the preparation of chemically modified electrodes (CMEs) whilst pure gold and platinum electrodes are both very chemically stable and conveniently manufactured.
Four key parameters are generally measured in an electrochemical experiment:
- Potential (E) is defined as the quantity of energy or electrical force in a system. Its base unit is the volt (V). An increase in E indicates the availability of more energy for the reaction.
- Current (I) is the measure of electron flow in a reaction. The base unit of current is amperes or amps (A). Current is usually measured in the microamp or nanoamp scale in electrochemical experiments.
- Charge (Q) denotes the number of electrons used per equivalent and its base unit is coulomb (C).
- Time (t) denotes duration of the experiment. It is expressed in second (s).
Due to the many different combinations of working electrode types and parameters possible in electrochemical experiments, a number of techniques are possible using electrochemical principles. Some are as follows:
A commonly used electroanalytical technique that can characterize an electrochemical system. Multiple CV experiments help in the determination of Nernstian or non-Nernstian behavior, rate constants, formation constants, formal potentials, and diffusion coefficients. Unfortunately, it is not a technique which is good for quantitative analysis.
- Linear Sweep Voltammetry (LSV)
LVS can be used for quantitative electrochemical analysis. In LSV, the potential of the electrode is varied at a constant rate during the reaction and the resulting current is measured.
Can be used to generate three current-potential plots, namely; reverse current versus potential, forward current versus potential, or difference current versus potential.
Used to determine diffusion coefficients and investigate reaction kinetics and mechanisms.
Used to determine higher concentrations. In this technique, a constant current is applied to the electrode and the resulting potential change is plotted against time.
Is another version of chronoamperometry which can give a relatively more accurate measurement of a kinetic rate constant and also aids easy detection of reactant adsorption on an electrode surface.