Electroanalytical methods measure electrical properties to analyze chemical solutions. These techniques use the relationship between electricity and chemical reactions, involving electron transfer at electrode-solution interfaces. They're classified into potentiometric, voltammetric, coulometric, and conductometric methods.
These methods rely on electrical signals from electrode reactions. The signal's magnitude depends on analyte concentration and reaction kinetics. Understanding these principles is crucial for interpreting electroanalytical data and applying these techniques effectively in chemical analysis.
Fundamental Principles and Concepts
Fundamentals of electroanalytical methods
- Involve measuring electrical quantities (current, potential, charge, impedance) to obtain information about analytes in solution
- Based on the relationship between electricity and chemical reactions
- Faradaic processes transfer electrons across the electrode-solution interface
- Non-faradaic processes change the structure of the electrode-solution interface without electron transfer
- Classified into several categories
- Potentiometric methods measure potential difference between electrodes
- Voltammetric methods measure current as a function of applied potential
- Coulometric methods measure charge passed during an electrolysis reaction
- Conductometric methods measure the conductivity of a solution
Electrical signals in chemical processes
- Arise from chemical processes at the electrode-solution interface
- Oxidation reactions involve species in solution losing electrons to the electrode
- Reduction reactions involve species in solution gaining electrons from the electrode
- Magnitude depends on analyte concentration and nature of electrode reaction
- Nernst equation relates electrode potential to analyte concentration $E = E^0 + \frac{RT}{nF} \ln \frac{[Ox]}{[Red]}$
- Faraday's laws relate charge passed to amount of analyte reacted $Q = nFN$
- Kinetics of electrode reactions influence the electrical signal shape and magnitude
- Mass transport of analyte to electrode surface affects current response
- Electron transfer kinetics determine rate of electrode reaction
Types and Applications of Electroanalytical Methods
Types of electroanalytical techniques
- Potentiometric methods
- Measure potential difference between indicator and reference electrodes
- Determine ion concentrations and redox potentials (pH measurement, ion-selective electrodes)
- Voltammetric methods
- Measure current response vs applied potential
- Provide information about analyte concentration, identity, and electrochemical behavior (cyclic voltammetry, square wave voltammetry, stripping voltammetry)
- Coulometric methods
- Measure charge passed during exhaustive electrolysis of analyte
- Quantitatively determine analytes (controlled-potential coulometry, controlled-current coulometry)
- Conductometric methods
- Measure solution conductivity, which depends on ionic species concentration
- Determine total ionic concentration and titration endpoints (conductometric titrations, oscillometric techniques)
Components of electroanalytical measurements
- Electrodes
- Interface between electrical circuit and solution
- Working electrode is site of electrode reaction of interest
- Reference electrode maintains constant potential as reference point for measuring working electrode potential
- Counter electrode completes circuit and balances charge transfer at working electrode
- Electrolytes
- Provide conductive medium for charge transport in solution
- Support electrolytes increase solution conductivity and minimize solution resistance
- Composition influences electrode reaction and electrical double layer at electrode-solution interface
- Redox reactions
- Transfer electrons between solution species and electrode
- Oxidation reactions at anode involve species losing electrons
- Reduction reactions at cathode involve species gaining electrons
- Basis for many electroanalytical methods (voltammetry, coulometry)