VOLTAMMETRY: WHAT IT CONSISTS OF, TYPES AND APPLICATIONS - CHEMISTRY

The voltammetry is an electroanalytical technique that determines information of a chemical or analyte from the electrical currents generated by the variation of an applied potential species. That is, the applied potential E (V), and the time (t), are the independent variables; while current (A), the dependent variable.

The chemical species must ordinarily be electroactive. What does it mean? It means that it must lose (oxidize) or gain (reduce) electrons. For the reaction to start, the working electrode must supply the necessary potential theoretically determined by the Nernst equation.

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An example of voltammetry can be seen in the image above. The electrode in the image is made of carbon fibers, which is immersed in the dissolution medium. Dopamine does not oxidize, forming two carbonyl groups C = O (right side of the chemical equation) unless the proper potential is applied.

This is accomplished by scanning E with different values, limited by many factors such as the solution, the ions present, the same electrode, and dopamine.

By varying E over time, two graphs are obtained: the first E vt (the blue triangle), and the second, the response C vs t (yellow). Their forms are characteristic for determining dopamine under the conditions of the experiment.

What is voltammetry?

Voltammetry was developed thanks to the invention of the polarography technique by the 1922 Nobel Prize winner in chemistry, Jaroslav Heyrovsky. In it, the electrode of the drop of mercury (EGM) is constantly renewed and polarized.

The analytical deficiencies of this method at that time were solved with the use and design of other microelectrodes. These vary enormously in material, from carbon, noble metals, diamond and polymers, to in their design, discs, cylinders, sheets; and also, in the way in which they interact with the solution: stationary or rotating.

All these details are intended to favor the polarization of the electrode, which causes a decay of the registered current known as limit current (i ₁). This is proportional to the concentration of the analyte, and half the power E (E _1/2) to achieve half of said current (i _1/2) is characteristic of the species.

Then, by determining the values of E _1/2 in the curve where the current obtained with the variation of E is plotted, called a voltamperogram, the presence of an analyte can be identified. That is, each analyte, given the conditions of the experiment, will have its own value of E _1/2.

Voltammetric wave

In voltammetry you work with many graphs. The first of these is the E vs t curve, which allows a follow-up of the applied potential differences as a function of time.

But at the same time, the electrical circuit records the C values produced by the analyte by losing or gaining electrons in the vicinity of the electrode.

Because the electrode is polarized, less analyte can diffuse from the solution into it. For example, if the electrode is positively charged, the species X ^- will be attracted to it and will be directed towards it by mere electrostatic attraction.

But X ^- you are not alone: there are other ions present in your environment. Some M ⁺ cations can get in the way of the electrode by enclosing it in "clusters" of positive charges; and likewise, N ^- anions can trap around the electrode and prevent X ^{- from} reaching it.

The sum of these physical phenomena causes current to be lost, and this is observed in the C vs E curve and its shape similar to that of an S, called the sigmoid shape. This curve is known as a voltammetric wave.

Instrumentation

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The instrumentation of voltammetry varies according to the analyte, solvent, type of electrode and application. But, the vast majority of them are based on a system made up of three electrodes: a working one (1), an auxiliary one (2) and a reference one (3).

The main reference electrode used is the calomel electrode (ECS). This, together with the working electrode, makes it possible to establish a potential difference ΔE, since the potential of the reference electrode remains constant during measurements.

On the other hand, the auxiliary electrode is in charge of controlling the charge that passes to the working electrode, in order to keep it within the acceptable E values. The independent variable, the applied potential difference, is the one obtained by adding the potentials of the working and reference electrodes.

Types

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The image above shows an E vs t plot, also called a potential waveform for linear sweep voltammetry.

It can be seen that as time passes, the potential increases. In turn, this sweep generates a response curve or voltamperogram C vs E whose shape will be sigmoid. There will come a point where no matter how much E increases, there will be no increase in current.

Other types of voltammetry can be inferred from this graph. How? Modifying the potential wave E vs t by means of sudden potential pulses following certain patterns. Each pattern is associated with a type of voltammetry, and encompasses its own theory and experimental conditions.

Pulse voltammetry

In this type of voltammetry, mixtures of two or more analytes whose E _1/2 values are very close to each other can be analyzed. Thus, an analyte with an E _1/2 of 0.04V can be identified in the company of another with an E _1/2 of 0.05V. Whereas in linear sweep voltammetry, the difference must be greater than 0.2V.

Therefore, there are higher sensitivity and lower detection limits; that is, analytes can be determined at very low concentrations.

Waves of potentials can have stair-like patterns, inclined stairs, and triangles. The latter corresponds to cyclic voltammetry (CV for its acronym in English, first image).

In CV, a potential E is applied in one direction, positive or negative, and then, at a certain value of E in time t, the same potential is applied again but in the opposite direction. When studying the voltamperograms generated, the maxima reveal the presence of intermediaries in a chemical reaction.

Redissolution voltammetry

This can be of the anodic or cathodic type. It consists of the electrodeposition of the analyte on a mercury electrode. If the analyte is a metal ion (such as Cd ²⁺), an amalgam will form; and if it is an anion, (such as MoO ₄ ^2–) an insoluble mercury salt.

Then, pulses of potentials are applied to determine the concentration and identity of the electrodeposited species. Thus, the amalgam is redissolved, just like the mercury salts.

Applications

-Anodic redissolution voltammetry is used to determine the concentration of dissolved metals in fluid.

-Allows to study the kinetics of redox or adsorption processes, especially when the electrodes are modified to detect a specific analyte.

-Its theoretical base has been used for the manufacture of biosensors. With these, the presence and concentration of biological molecules, proteins, fats, sugars, etc. can be determined.

-Finally, it detects the participation of intermediaries in the reaction mechanisms.

References

González M. (November 22, 2010). Voltammetry. Recovered from: quimica.laguia2000.com
Gómez-Biedma, S., Soria, E., & Vivó, M.. (2002). Electrochemical analysis. Journal of Biological Diagnosis, 51 (1), 18-27. Recovered from scielo.isciii.es
Chemistry and Science. (July 18, 2011). Voltammetry. Recovered from: laquimicaylaciencia.blogspot.com
Quiroga A. (February 16, 2017). Cyclic Voltammetry. Recovered from: chem.libretexts.org
Samuel P. Kounaves. (sf). Voltammetric Techniques.. Tufts University. Recovered from: brown.edu
Day R. & Underwood A. Quantitative Analytical Chemistry (5th ed.). PEARSON Prentice Hall.

VOLTAMMETRY: WHAT IT CONSISTS OF, TYPES AND APPLICATIONS - CHEMISTRY - 2025