Cyclic voltammetry (CV) is a fundamental electrochemical technique used to study redox reactions. In this demonstration, a ferricyanide solution is analyzed using a potentiostat with a carbon electrode. The potential is scanned from −200 mV to +600 mV and back at 100 mV/s, generating a characteristic voltammogram. Key equations explain the response: the Tafel equation (kinetic control at low potentials), Randles-Sevcik equation (diffusion-limited peak current), and Cottrell equation (current decay due to reactant depletion). The forward scan oxidizes ferrocyanide to ferricyanide; the reverse scan reduces it back. CV reveals reaction reversibility and kinetics, with scan rate influencing peak clarity. This method is vital for electrochemical research and assay development.
Theoretical Introduction to Cyclic Voltammetry
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Setup & Parameters:
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Uses a potentiostat with screen electrodes and a ferricyanide solution (a classic reversible system).
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Software (e.g., “SenseItAll”) controls the waveform:
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Start potential: −200 mV
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Turn potential: +600 mV
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Scan rate: 100 mV/s (total cycle time: 16 seconds for one full scan forward and back).
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Repeats: Typically 1–2 cycles (here, only 1 is used).
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Key Equations in CV:
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Tafel Equation: Describes the “foot of the wave” (low potential), where current depends on electron transfer kinetics (no diffusion effects).
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Randles-Sevcik Equation: Governs the peak current (diffusion-controlled regime). Peak current depends on scan rate and diffusion coefficients.
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Cottrell Equation: Explains the current decay after the peak (depletion of reactants near the electrode).
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Behavior During the Scan:
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Forward Scan (−200 mV → +600 mV): Oxidation of ferrocyanide ([Fe(CN)₆]⁴⁻) to ferricyanide ([Fe(CN)₆]³⁻).
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Reverse Scan (+600 mV → −200 mV): Reduction of ferricyanide back to ferrocyanide.
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Peak Separation: Reversible systems show distinct oxidation/reduction peaks; irreversible systems have wider separation.
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Practical Demonstration
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Electrodes: Carbon electrode (model 501) with ferricyanide solution.
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Data Collection:
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Real-time current vs. time plot shows oxidation/reduction waves.
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After the scan, the voltammogram (current vs. potential) is generated, resembling the simulated “ideal” curve.
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Observations:
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Fast scan rates (e.g., 100 mV/s) may distort peaks but speed up experiments.
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Slower scans yield clearer peaks but take longer.
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Key Takeaways
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CV is a powerful tool to study redox reactions, electron transfer kinetics, and diffusion processes.
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The shape of the voltammogram reveals reversibility (ferricyanide/ferrocyanide is quasi-reversible).
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Equations like Tafel, Randles-Sevcik, and Cottrell help interpret different regions of the curve.