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Electrochemistry
Table of Contents
Unit 11 Overview: Electroanalytical Chemistry and Sensors
11.1 Principles of Electroanalytical Methods
11.2 Potentiometric Sensors and Ion-Selective Electrodes
11.3 Amperometric and Voltammetric Sensors
11.4 Biosensors and Chemical Sensors

Amperometric and voltammetric sensors are powerful tools for measuring chemical concentrations. These techniques rely on electrochemical reactions, measuring current as a function of potential or time to detect and quantify analytes in various applications.

From glucose monitoring to heavy metal detection, these sensors offer versatility and sensitivity. Understanding the principles, techniques, and factors affecting sensor performance is crucial for accurate analysis and interpretation of voltammograms and amperograms in electrochemistry.

Amperometric and Voltammetric Sensors

Principles of amperometric and voltammetric sensors

  • Amperometric sensors measure current resulting from the oxidation or reduction of an analyte at a fixed potential
    • Current is proportional to the concentration of the analyte
    • Applications include glucose monitoring (blood glucose meters), environmental monitoring (dissolved oxygen sensors), and industrial process control (chlorine sensors)
  • Voltammetric sensors measure current while varying the potential applied to the electrode
    • Provide information about the redox processes occurring at the electrode surface
    • Applications include heavy metal detection (lead, cadmium), organic compound analysis (pesticides, pharmaceuticals), and electrochemical reaction studies (electrocatalysis, corrosion)

Techniques in voltammetry and amperometry

  • Cyclic voltammetry (CV) involves sweeping the potential back and forth between two values at a fixed rate
    • Provides information about the reversibility and kinetics of redox reactions
    • Characteristic peaks in the voltammogram correspond to oxidation and reduction processes (anodic and cathodic peaks)
  • Chronoamperometry (CA) involves stepping the potential from a value where no faradaic reaction occurs to a value where the reaction is diffusion-controlled
    • Current-time response is measured and analyzed
    • Useful for studying the kinetics of electrode processes and determining diffusion coefficients (Cottrell equation)
  • Linear sweep voltammetry (LSV) involves sweeping the potential in one direction at a fixed rate
  • Differential pulse voltammetry (DPV) involves superimposing potential pulses on a linear potential ramp to enhance sensitivity
  • Square wave voltammetry (SWV) involves superimposing a square wave on a staircase potential ramp for improved sensitivity and speed

Interpretation of voltammograms and amperograms

  • Voltammograms provide qualitative and quantitative information
    • Peak potential ($E_p$) identifies the analyte (qualitative)
    • Peak current ($i_p$) is proportional to the concentration of the analyte (quantitative)
    • Separation between oxidation and reduction peaks ($\Delta E_p$) indicates the reversibility of the redox reaction (reversible: $\Delta E_p = 59/n$ mV at 25℃)
  • Amperograms provide quantitative information
    • Steady-state current ($i_{ss}$) is proportional to the concentration of the analyte
    • Current-time response can be analyzed using the Cottrell equation: $i(t) = \frac{nFAD^{1/2}C}{\pi^{1/2}t^{1/2}}$
      1. $n$: number of electrons transferred per molecule
      2. $F$: Faraday constant (96,485 C/mol)
      3. $A$: electrode area (cm²)
      4. $D$: diffusion coefficient (cm²/s)
      5. $C$: bulk concentration of the analyte (mol/cm³)
      6. $t$: time (s)

Factors in sensor performance

  • Electrode material affects the potential window, background current, and electrocatalytic properties
    • Common materials include carbon (glassy carbon, carbon paste), noble metals (gold, platinum), and mercury (hanging mercury drop electrode)
  • Surface modification enhances selectivity, sensitivity, and stability of the sensor
    • Methods include chemical modification (self-assembled monolayers), polymer coating (Nafion, polyaniline), and nanomaterial deposition (carbon nanotubes, metal nanoparticles)
  • Electrolyte composition and pH influence the redox behavior of the analyte and potential interfering species
  • Temperature affects the diffusion coefficient and reaction kinetics
  • Interfering species can cause overlapping signals or surface fouling
  • Mass transport conditions (stirring, flow rate) affect the diffusion layer thickness and sensor response time