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  Section: Practical Skills in Chemistry » Instrumental techniques
 
 
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Electroanalytical techniques

 
     
 
Content
Instrumental techniques
  Basic spectroscopy
    Introduction to spectroscopy
    UV Ivisible spectrophotometry
    Fluorescence
    Fluorescence spectrophotometry
    Phosphorescence and luminescence
    Atomic spectroscopy
  Atomic spectroscopy
    Atomic Absorption Spectroscopy
    Atomic Emission Spectroscopy
    Inductively coupled plasma
    Decomposition techniques for solid inorganic samples
  Infrared spectroscopy
  Nuclear magnetic resonance spectrometry
    1H-NMR spectra
    13C-NMR spectra
  Mass spectrometry
    Interfacing mass spectrometry
  Chromatography ~ introduction
    The chromatogram
    Resolution
    Detectors
  Gas and liquid chromatography
    Gas chromatography
    Liquid chromatography
    High-performance liquid chromatography
    Interpreting chromatograms
    Optimizing chromatographic separations
    Quantitative analysis
  Electrophoresis
    The supporting medium
    Capillary electrophoresis
    Capillary zone electrophoresis (CZE)
    Micellar electrokinetic chromatography (MEKC)
  Electroanalytical techniques
    Potentiometry and ion-selective electrodes
    Voltammetric methods
    Oxygen electrodes
    Coulometric methods
    Cyclic voltammetry
  Radioactive isotopes and their uses
    Radioactive decay
    Measuring radioactivity
    Chemical applications for radioactive isotopes
    Working practices when using radioactive isotopes
  Thermal analysis
    Thermogravimetry
    Applications

Electrochemical methods are used to quantify a broad range of different molecules, including ions, gases, metabolites and drugs.

*Note: The basis of all electrochemical analysis is the transfer of electrons from one atom or molecule to another atom or molecule in an obligately coupled oxidation-reduction reaction la redox reaction).

It is convenient to separate such redox reactions into two half-reactions and, by convention, each is written as:

⇒ Equation [34.1]
oxidized form + electron(s) (ne) reduction reduced form
oxidation

You should note that the half-reaction is reversible: by applying suitable conditions, reduction or oxidation can take place. As an example, a simple redox reaction occurs when metallic zinc (Zn) is placed in a solution containing copper ions (Cu), as follows:

⇒ Equation [34.2] Cu2+ + Zn → Cu + Zn2+

The half-reactions are (i) Cu2+ + 2e → Cu and (ii) Zn2+ + 2e → Zn. The oxidizing power of (i) is greater than that of (ii), so in a coupled system, the latter half-reaction proceeds in the opposite direction to that shown above, i.e. as Zn − 2e → Zn2+. When Zn and Cu electrodes are placed in separate solutions containing their ions, and connected electrically (Fig. 34.1), electrons will flow from the Zn electrode to Cu2+ via the Cu electrode owing to the difference in oxidizing power of the two half-reactions.

By convention, the electrode potential of any half-reaction is expressed relative to that of a standard hydrogen electrode (half-reaction 2H+ + 2e → H2) and is called the standard electrode potential, E0. Table 34.1 shows the values of E0 for selected half-reactions. With any pair of half-reactions from this series, electrons will flow from that having the lowest electrode potential to that of the highest. E0 is determined at pH = 0. It is often more appropriate to express standard electrode potentials at pH 7 for biological systems, and the symbol E0' is used: in all circumstances, it is important that the pH is dearly stated.

 
A simple galvanic electrochemical cell. The KCI salt bridge allows migration of ions between the two compartments but prevents mixing of the two solutions.
Fig. 34.1 A simple galvanic electrochemical cell. The KCI salt bridge allows migration of ions between the two compartments but prevents mixing of the two solutions.
The arrangement shown in Fig. 34.1 represents a simple galvanic cell where two electrodes serve as the interfaces between a chemical system and an electrical system. For analytical purposes, the magnitude of the potential (voltage) or the current produced by an electrochemical cell is related to the concentration of a particular chemical species. Electrochemical methods offer the following advantages:
  • excellent detection limits, and wide operating range (10−1 to 10−8 mol L−1);
  • measurements may be made on very small volumes (µL) allowing small amounts (pmol) of sample to be measured in some cases;
  • miniature electrochemical sensors can be used for certain in vivo measurements, e.g. pH, glucose, oxygen content.

Standard electrode potentials* (E0) for selected half-reactions
Table 34.1 Standard electrode potentials* (E0) for selected half-reactions
 
     
 
 
     



     
 
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