Voltammetry Explained: Principle, Types and Applications

Voltammetry is a family of electroanalytical techniques in which a voltage is applied to an electrode and the resulting electric current is measured. The voltage forces species in solution to gain or lose electrons (reduction or oxidation), and the current is proportional to how much of the species is present. Sweep the voltage, watch the current, and read both concentration and chemical identity from the response.

Voltammetry matters because it detects trace chemicals with extreme sensitivity. Environmental labs measure lead and cadmium in drinking water at parts-per-billion levels, and researchers study battery reactions with it — all in minutes from a tiny sample.

Key Concepts

• Three-electrode cell: a working electrode (where the reaction happens — glassy carbon, platinum or gold), a reference electrode (usually Ag/AgCl) and a counter electrode (platinum wire).
• Linear sweep voltammetry: voltage ramps one way; current peaks as reactant near the electrode is consumed, then falls.
• Cyclic voltammetry (CV): the voltage is swept forward then reversed, giving anodic and cathodic peaks with potentials Epa, Epc and currents ipa, ipc.
• Reversible diagnostics (25°C): for a fast one-electron reaction: ΔEp = 59.2 mV, ipa/ipc = 1, peak current grows with √(scan rate).
• Polarography: the classic form using a dropping mercury electrode; now largely replaced due to mercury toxicity.
• Stripping voltammetry: metal ions are plated onto the electrode, then stripped off while current is recorded — reaching parts-per-billion detection.

Formulas

• Randles–Sevcik equation (peak current, 25°C, reversible): ip = 2.69 × 10^5 × n^(3/2) × A × C × D^(1/2) × v^(1/2) (ip in amperes; A in cm², C in mol/cm³, D in cm²/s, v in V/s).
• Ilkovic equation (polarographic diffusion current): id = 708 × n × D^(1/2) × m^(2/3) × t^(1/6) × C (id in µA; m in mg/s, t in s, D in cm²/s, C in mmol/L).
• Assumptions and limits: excess supporting electrolyte must suppress migration, dissolved oxygen must be purged with nitrogen (it gives its own reduction current), and Randles–Sevcik applies strictly to reversible, diffusion-controlled reactions at 25°C.

Worked Example

A 1.0 mM one-electron (n = 1) species is scanned: A = 0.020 cm², D = 7.6 × 10^(−6) cm²/s, v = 0.10 V/s. Concentration is 1.0 × 10^(−6) mol/cm³. Then ip = 2.69 × 10^5 × 0.020 × 1.0 × 10^(−6) × (7.6 × 10^(−6))^(1/2) × (0.10)^(1/2). Stepwise: 2.69 × 10^5 × 0.020 = 5380; × 1.0 × 10^(−6) = 5.38 × 10^(−3); × 2.757 × 10^(−3) = 1.483 × 10^(−5); × 0.3162 = 4.69 × 10^(−6) A = 4.69 µA. Measured peaks are about 4.7 µA with ΔEp = 59 mV, confirming a reversible one-electron couple.

Common Misconceptions

• "Bigger peaks always mean more chemical." Peak current also rises with scan rate and electrode area, so concentration needs a calibration curve with all else fixed.
• "Peak positions are equilibrium potentials." They shift with scan rate for slow reactions; only for a reversible couple does (Epa + Epc)/2 give the formal potential.
• "Dissolved oxygen does not matter." Oxygen is reduced at the electrode and can hide trace analytes — always purge with nitrogen first.

Real-World Applications

• Environmental monitoring: stripping voltammetry finds lead, cadmium and mercury in drinking water at parts-per-billion levels.
• Pharmaceuticals: drug purity checks and studies of how drugs oxidise in the body.
• Energy research: characterising battery electrodes, supercapacitors and fuel-cell catalysts.
• Sensors: glucose meters use disposable electrochemical strips.

Voltammetry applies a voltage and reads the answering current: a three-electrode cell, a swept potential, and peaks whose heights give concentration through the Randles–Sevcik equation. From trace lead in water to battery research, it turns a tiny sample into quantitative chemical insight.