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Potentiometry- Electrochemical Cell, Construction and Working of Reference and Indicator Electrodes

Two half-cells make up electrochemical cell, each contains a metal electrode that is immersed in ions whose activity determines electrode's potential.

Electrochemical cell

In Figure, you can see a schematic illustration of an electrochemical potentiometric cell. Two half-cells make up an electrochemical cell, each containing a metal electrode that is immersed in ions whose activity determines the electrode's potential. Connecting the two half-cells is an electrolyte salt bridge using KCl, an inert electrolyte. Porous frits are fixed to the ends of the salt bridge, allowing the ions in the electrolyte to freely move between half-cells and the salt bridge. By moving between the half-cells and the salt bridge, the ions complete the electrical circuit.

Electrochemical cell

Conventionally, the electrode on the left is called the anode and it represents the oxidation reaction; thus, we have the anode on the left.

Zn(s )⇋ Zn2 + (aq)+2e

Right above the electrode is the cathode, which performs the reduction reaction.

Ag + (aq)+e⇋ Ag(s)

Figure shows the reaction potential of an electrochemical cell

Zn(s) + 2Ag+(aq) ⇋ 2Ag(s) + Zn2+(aq)

Potentiometric electrochemical cells also include a cathode and anode that serve as an indicator and reference electrodes, respectively.

It measures the potential difference between two electrodes that is produced by the addition of titrants or by the changes in ion concentration in the sample solution or analyte. As a reference electrode and an indicator electrode, these two electrodes are referred to as such. The reference electrode is the electrode that remains stable and maintains its potential in the presence of the sample solution. Indicator electrodes respond to changes in analyte solution potential. To prevent interference between analytes and reference electrodes, salt bridges are used.

A potential difference or electromotive force is calculated using the following formula -

Ecell= Eind – Eref + Ej

  • A cell's electromotive force is equal to its electromotive force Ecell
  • The End of an indicator electrode is its electromotive force
  • Referring to the reference electrode, If equals the electromotive force
  • Across the salt bridge, Ej equals electromotive force

Construction and working reference

  1. Potentiometry method - the potentiometric method analyzes electrochemical cells in the absence of a current to determine the potential
  2. The basic components required are the indicator electrode, reference electrode, salt bridge, and potential measuring device.
For reference electrode, desired characteristics are as follows:
  • Eref is a fixed or known potential
  • Solution composition does not affect the results
  • Cellular response (even when the cell is experiencing a net current)
  • Complies with the Nernst Equation
  • It should be reversible
  • Easily assembled and rugged
  • It is always considered as the electrode on the left

Indicator electrode

Recognizes or responds when an analyte is present. When an analyte ion (or group of ions) concentration changes, the ideal indicator electrode responds rapidly and reproducibly.

The following three types are common:
1. Metallic Indicator Electrodes
  • Electrodes of the First Kind
  • Electrodes of the Second Kind
  • Electrodes of the Third Kind
  • Metallic Redox Indicators
2. Membrane Indicator Electrodes Crystalline Membrane Electrodes Non-crystalline Membrane Electrodes
3. Ion-selective Electrode (field effect transistor) ISFET

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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