6.2 Cyclic Voltammetry and Linear Sweep Voltammetry

Cyclic and linear sweep voltammetry are powerful techniques for studying electrochemical reactions. These methods involve sweeping electrode potentials and measuring resulting currents, providing insights into redox processes, reaction kinetics, and analyte concentrations.

Voltammograms reveal crucial information about electrode reactions, including redox potentials and reversibility. By analyzing peak positions and shapes, researchers can determine formal potentials, electron transfer rates, and quantify electroactive species in various applications.

Cyclic Voltammetry and Linear Sweep Voltammetry

Cyclic vs linear sweep voltammetry

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• Cyclic voltammetry (CV) sweeps potential back and forth between two limits creates a triangular potential waveform allows study of both oxidation and reduction processes (Fe^2+/Fe^3+ redox couple)
• Linear sweep voltammetry (LSV) sweeps potential in one direction only creates a linear potential waveform focuses on either oxidation or reduction process depending on sweep direction (anodic stripping of Pb)

Potential waveforms in voltammetry

• Cyclic voltammetry uses triangular potential waveform
  • Initial potential ($E_i$) swept to switching potential ($E_s$) at constant rate ($v$)
  • At $E_s$, sweep direction reverses potential swept back to $E_i$ at same rate (100 mV/s)
• Linear sweep voltammetry uses linear potential waveform
  • Potential swept from initial value ($E_i$) to final value ($E_f$) at constant rate ($v$)
  • Sweep direction can be anodic (oxidation) or cathodic (reduction) (50 mV/s)

Interpretation of cyclic voltammograms

• Cyclic voltammogram features anodic peak potential ($E_{pa}$) and current ($i_{pa}$) correspond to oxidation cathodic peak potential ($E_{pc}$) and current ($i_{pc}$) correspond to reduction (Fe^2+ oxidation, Fe^3+ reduction)
• Determine redox potentials by estimating formal reduction potential (E^0') as average of $E_{pa}$ and $E_{pc}$: E^0' = (E_{pa} + E_{pc})/2 (0.5 V vs SCE)
• Study electrode reaction mechanisms:
  1. Reversible systems: $\Delta E_p = E_{pa} - E_{pc} \approx 59/n$ mV at 25°C, $n$ = number of electrons transferred (one-electron transfer)
  2. Irreversible systems: $\Delta E_p > 59/n$ mV, peak separation increases with scan rate (slow electron transfer kinetics)
  3. Quasi-reversible systems: peak separation depends on scan rate, lies between reversible and irreversible cases (intermediate kinetics)

Applications of voltammetry techniques

• Qualitative analysis identifies presence of electroactive species based on characteristic peak potentials distinguishes different redox couples in a mixture (ascorbic acid and dopamine)
• Quantitative analysis uses peak current ($i_p$) proportional to concentration of electroactive species
  • Randles-Sevcik equation for reversible systems at 25°C: $i_p = 2.69 \times 10^5 n^{3/2} A D^{1/2} v^{1/2} C$
    • $n$: number of electrons transferred
    • $A$: electrode area (cm^2)
    • $D$: diffusion coefficient (cm^2/s)
    • $v$: scan rate (V/s)
    • $C$: concentration (mol/cm^3)
  • Construct calibration curves by plotting $i_p$ vs $C$ for standard solutions (lead in water samples)