Electrochemical reactions involve exchange of electrons between reactants and products. Such process can be induced by application of electrical energy to electrodes placed in electrically conducting solutions. Broadly speaking electro-analytical techniques involve measurement of potential difference, current or conductance of solutions and are grouped under the following categories :
- Electrogravimetry & Coulometry
Potentiometry involves measurement of potential difference across the indicator and reference electrodes immersed in the analysis solution. In case of potentiometric titration the potential difference is plotted against volume of reagent added and equivalence point read from the plot.
Measurement of pH is also a form of potentiometry where the potential of the glass electrode is measured as a function of hydrogen ion concentration in the solution. pH based titrations are popular in chemical and biochemical processes and control of waste water treatment processes. In the laboratory potentiometric titration is largely in used for determinations of endpoints in redox titrations.
Amperometry involves measurement of current between two electrodes at constant potential difference. The current is plotted against volume of titrant to find the endpoint of the titration.
Amperometric titrations are free of personal errors arising due to estimation of colour changes in visual indicator titrations as end point is arrived at by extrapolation of the graphical segments before and after end point. Further amperometric titrations can be carried out at dilutions at which visual indicator or potentiometric titrations lack the required accuracy.
Voltammetry involves changes in concentration of the electoactive entity through oxidation or reduction at the surface layer at the indicator electrode such as gold, platinum, glassy carbon or mercury. The resulting current is plotted as a function of applied potential.
In case of Polarography a dropping mercury electrode replaces the flat surface electrodes. A continuously varying potential program is applied between the dropping mercury electrode and the reference electrode and the resulting current changes are plotted against the applied voltage. Half wave potential is used for qualitative estimation of analyte and wave height can be used for quantitative estimations.
Voltammetry has been adapted to several analysis applications involving both inorganic and organic analytes in environmental, pharmaceuticals, foods, clinical and metallurgical samples.
Electrogravimetry & Coulometry
In both these techniques electrolysis is carried out till such time that the available analyte is completely oxidised or reduced to the final product. In electrogravimetry the product at end point is weighed as a deposit on one of the electrodes. On the other hand in coulometry the quantity of current is measured.
Conductometry involves measurement of conductance of solution during titrimetric determinations.
Conductometric determinations are useful for studies on dissociation constants of weak acids and bases, ionic product of water and solubility products of sparingly soluble salts.
Analytical chemistry stands on a strong foundation of analytical techniques. Subsequent articles will cover other groupings.
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