Most electroanalytical techniques can be classified into three categories such a

Question

Most electroanalytical techniques can be classified into three categories such as potentiometry, coulometry, and voltammetry in terms of the underlying principles. (a) Explain the underlying principles of these techniques and their advantages and disadvantages (b) For electrochemical analysis, samples should be prepared and analyzed in the form of the electrochemical cell regardless of types of electroanalytical techniques. However, there are big differences between potentiometry and coulometry/voltammetry in constructing an electrochemical cell. Explain the differences between them in terms of the roles and selection of electrodes. Assume that you are going to use only two electrodes for electrochemical cells. (c) In many cases, it is generally more preferred to use a three electrode system than a two electrode system in electrochemical analysis. Explain why it is and describe the role of each electrode in a three electrode system.

Explanation / Answer

a)

Potentiometry
Principle
1. Potentiometry measures the difference in electrode potential between the reference electrode and indicator electrode.
2. Reference electrode has a constant potential and indicator electrode potential changes with the composition of the sample.
3. Difference of potential between the electrodes give the composition of the unknown sample.
Advantages
1. Non destructive measurement.
2. Good selectivity
3. High sensitivity
Dis advantages
1. Establishing equilibrium is very time consuming.

Coulometry
Principle
1. Coulometry measures the current of the cell over time for the conversion of the analyte from one oxidation state to another oxidation state.
2. The measured current helps to determine the number of electrons passed which indicate the concentration of the analyte.

Advantages
1. Unknown concentration of the analyte can be determined.
2. Number of electrons involved in the redox reaction can be estimated.
Dis advantages
1. Only redox substances can be estimated.

Voltammetry
Principle
1. Voltammetry applies a constant and/or varying potential at an electrode's surface and measures the resulting current with a three electrode system.
2. This method can reveal the reduction potential of an analyte and its electrochemical reactivity.
Advantages
1. Nondestructive
2. Very small amount of the analyte is consumed
Dis advantages
1. Separation of the analyte from the bulk solution is difficult for estimation.

b)

Differences between potentiometry and voltammetry/coulometry
Electrochemical cell for potentiometry:
1.   Potentiometry is based on the measurement of the potential of an electrode system (e.g. electrochemical cell).
2.   Potentiometric measurement system consists of two electrodes called reference and indicator electrode, potentiometer and a solution of analyte
3.   Reference electrode is an electrode with potential which is a) independent of concentration of analyte (or other) ions in solution; b) independent of temperature.
4.   Potential of an indicator electrode depends mainly on the concentration of the analyte ions (in this case hydrogen ions).
5.   Potentiometric measurements enable selective detection of ions in presence of multitude of other substances.
6.   The potential of the indicator electrode is sensitive to hydrogen ions. In a system like this, the potential is measured in reference to a calomel electrode, e.g. calomel electrode functions as the reference electrode.

Electrochemical cell for voltametry:
1.   A typical arrangement for a voltammetric electrochemical cell consists of the working electrode, the reference electrode, and the auxiliary electrode
2.   The working electrode, which typically consists of a chemically inert conductive material acts as a donor or acceptor of electrons participating in the general electrode reaction.
3.   The reference electrode keeps the potential between itself and the working electrode constant.
4.   The potential is measured between the reference and working electrodes, and the current is measured between the working and counter electrodes.
5.   A counter electrode is employed to allow for accurate measurements to be made between the working and reference electrodes.
6.   The counter electrode's role is essentially to ensure that current does not run through the reference electrode, since such a flow would change the reference electrodes potential.

c)

•   The working electrode, which typically consists of a chemically inert conductive material acts as a donor or acceptor of electrons participating in the general electrode reaction.
•   The reference electrode keeps the potential between itself and the working electrode constant.
•   The potential is measured between the reference and working electrodes, and the current is measured between the working and counter electrodes.
•   A counter electrode is employed to allow for accurate measurements to be made between the working and reference electrodes.
•   The counter electrode's role is essentially to ensure that current does not run through the reference electrode, since such a flow would change the reference electrodes potential.

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