10.17
Voltammetry: Overview
Voltammetry is an electroanalytical technique in which the current flowing through an electrochemical cell is measured as a function of applied potent…
In voltammetry, the current flowing through an electrochemical cell is measured as a function of applied potential under conditions of concentration polarization.
The voltammetric cell comprises three electrodes immersed in a dilute solution of the analyte along with a supporting electrolyte.
A potentiostat varies the working electrode's potential relative to the reference electrode. The resulting current is recorded against the working electrode potential as a voltammogram.
Working electrodes in voltammetry have minimal surface areas to enhance polarization.
Generally, mercury working electrodes are preferred for their relatively large negative potential range because of the high overvoltage of hydrogen on mercury.
Additional advantages include amalgam formation and the ability to renew the electrode surface by new drop formation.
In dilute solutions, the current that results when an electroactive species is reduced or oxidized reaches a limiting value directly proportional to the analyte's concentration. By measuring voltage and current at the interface between the electrode and an electroactive analyte, voltammetry reveals information about the reduction or oxidation processes of the analyte.
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Q1: What are the three electrodes used in a voltammetric cell?
A voltammetric cell contains a working electrode where redox reactions occur, a reference electrode that maintains constant potential for accurate measurement, and an auxiliary electrode, typically platinum wire, that completes the circuit. These three electrodes are immersed in a dilute solution containing the analyte and supporting electrolyte.
Q2: Why are mercury electrodes preferred as working electrodes in voltammetry?
Mercury electrodes offer a large negative potential range, preventing unwanted hydrogen reduction that would interfere with measurements. They can form amalgams with various metals, enhancing certain redox reactions. Additionally, their surface can be renewed by forming new drops, maintaining measurement accuracy and consistency throughout experiments.
Q3: How does voltammetry determine analyte concentration?
In dilute solutions, the current produced when an electroactive species is reduced or oxidized reaches a limiting value directly proportional to the analyte's concentration. By measuring voltage and current at the electrode-analyte interface, voltammetry reveals reduction or oxidation processes and enables quantitative analysis through this concentration-dependent current response.
Q4: What is a voltammogram and what information does it provide?
A voltammogram is a plot of current versus applied potential at the working electrode. It records the current response as a potentiostat varies the working electrode potential relative to the reference electrode. The voltammogram reveals information about redox-active species, their reduction or oxidation processes, and provides data for identifying and quantifying electroactive analytes in solution.
Q5: What role does concentration polarization play in voltammetry?
Voltammetry operates under conditions of concentration polarization, where current is measured as a function of applied potential. This condition ensures that the current response depends on the analyte concentration at the electrode surface. Working electrodes with minimal surface areas enhance polarization, allowing accurate measurement of electroactive species and reliable quantitative analysis.
Q6: What are the common types of mercury working electrodes used in voltammetry?
The hanging mercury drop electrode (HMDE) and the dropping mercury electrode (DME) are the most common mercury working electrodes. Both electrodes leverage mercury's wide negative potential range and ability to form amalgams. Their surfaces can be renewed by forming new drops, ensuring consistent and reliable results during voltammetric measurements and analysis.
Q7: How does a potentiostat control the working electrode in voltammetry?
A potentiostat varies the working electrode's potential relative to the reference electrode, which maintains a constant reference point. The potentiostat systematically changes the applied potential while measuring the resulting current response. This controlled potential variation allows the instrument to generate a voltammogram that reveals the electrochemical behavior of redox-active species in the solution.