Study Guide · Physical Chemistry

Cyclic Voltammetry: Reading a Voltammogram

What the peaks, currents and potential gaps in a cyclic voltammogram actually tell you about a redox couple.

CSIR-NET · GATE Chemistry · IIT-JAM Chemistry · Electrochemistry · Published 1 October 2026

In short: Cyclic voltammetry sweeps potential forward and back across a redox couple’s range while recording current, producing the characteristic curve seen in GATE and CSIR-NET electrochemistry questions. Reading peak positions, peak currents and the gap between them is what the technique is actually asked about.

What is being plotted

A cyclic voltammogram plots current (y-axis) against applied potential (x-axis) as the potential is swept linearly in one direction and then reversed, producing one anodic (oxidation) peak and one cathodic (reduction) peak for a simple reversible redox couple.

The key quantities examiners ask about

Quantity	What it tells you
E_pa, E_pc (anodic/cathodic peak potentials)	Their average approximates the formal redox potential E°′ for a reversible couple
ΔE_p (peak separation)	Close to 59/n mV at 25°C for an electrochemically reversible couple; a larger gap signals slower electron-transfer kinetics
i_pa / i_pc ratio	Close to 1 for a simple reversible couple with no follow-up chemistry consuming the product

Reversible, quasi-reversible and irreversible behaviour

As electron-transfer kinetics slow down relative to the scan rate, the peak separation grows beyond the ideal 59/n mV and the curve becomes less symmetric — this is the quasi-reversible regime, shading into fully irreversible behaviour where only one clear peak may appear at all. For a diffusion-controlled reversible process, peak current follows the Randles-Sevcik relation: peak current is proportional to the square root of scan rate, with concentration, electrode area and the diffusion coefficient as the other factors.

i_p ∝ n^3/2 A C D^1/2 v^1/2

A common trap: a chemical reaction coupled to the electron transfer — an EC mechanism, where the electrogenerated species reacts further before the reverse scan reaches it — distorts this simple reversible picture, often lowering the reverse peak and shifting the forward peak. Questions testing whether a student can distinguish a simple reversible couple from an EC mechanism are a recurring GATE and CSIR-NET pattern.

Where this fits a physical chemistry prep plan

Cyclic voltammetry sits within the electrochemistry section of GATE Chemistry (CY) and CSIR-NET Part C, usually alongside Nernst-equation numericals and conductance-based questions.

FAQs

Is cyclic voltammetry tested quantitatively or mostly conceptually?

Both appear. Conceptual questions on reading peaks and reversibility are more common than full numerical Randles-Sevcik calculations, but the relation is still worth knowing qualitatively.

What does a large ΔE_p indicate?

Slower electron-transfer kinetics at the electrode surface, moving the system from reversible toward quasi-reversible behaviour.

How does scan rate affect the voltammogram?

Peak current increases with the square root of scan rate for a diffusion-controlled reversible process, and peak separation can increase at higher scan rates for a quasi-reversible system.

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