Decomposition Potential

Decomposition Potential | Factors Affecting Decomposition Potential | Significance of Decomposition Potential

Decomposition Potential

Decomposition Potential or Decomposition Voltage

The minimum potential which must be applied externally between the two electrodes immersed in an electrolytic solution to continue the decomposition of an electrolyte is called decomposition potential. It is also known as decomposition voltage.
When 1N H2SO4 solution is electrolyzed by using Pt electrodes and applying an external potential of 1 volt, it is found that current flows initially and after some time it stops due to polarization (i.e. back emf becomes greater than applied emf due to H2 and O2 form a layer at the surface of the electrodes) at the respective electrodes.
Now, if the applied potential is gradually increase , a point reached when the electrolysis continues and evolution of H2 and O2 gas takes place at the two electrodes and the electrolysis does not stop even though the applied emf is increased not more. This applied potential necessary forcontinuous electrolytic decomposition is called decomposition potential and for H2SO4 its vale is found to be 1.7 volts at 25° C.

The decomposition potential varies from solution to solution. During electrolysis, the metallic ions travel towards the cathode. If the metal has a higher solution pressure, it will try to send these ions, back into the solution i.e., the solution will have high decomposition potential. Thus, a metal with high solution pressure, e.g., zinc, requires a high emf to be deposited on the cathode than a metal like copper with low solution pressure. This fact is made use of in refining and electrometallurgy of metals. For example, copper and zinc can be separated electrically. The decomposition potentials of copper sulphate and zinc sulphate are 1.5 and 2.55 volts respectively with copper and zinc electrodes. When an emf. of less than 2.5 volt is applied between copper electrodes, only copper will be deposited on the cathode while zinc remains in solution.

Experimental Measurement of Decomposition Potential

The set up for the experimental measurement of decomposition potential or voltage is as shown in the figure given below-
The beaker is filled with the experimental solution. Two electrodes and a stirrer are immersed in the beaker. One electrode is connected through milliammeter to the negative terminal of the battery and act as cathode. The other electrode through the variable resistance is connected to the positive terminal of the battery and act as anode. The voltmeter is connected across the anode and cathode.
Experimental Measurement of Decomposition Potential

To start with , first the sliding contact is moved slowly on the wire and the value of the applied potential and current flowing are noted down from the voltmeter and milli ammeter. This process of recording current and potential is continued till sudden rise in the current is shown by the milli ammeter, then a graph of current and applied potential is plotted. The curve obtained from graph is as shown below-
Decomposition Potential

Factors Affecting Decomposition Potential

The magnitude of decomposition Voltage of the electrolyte depends upon-
1. Nature of electrolytes
2. Nature of electrodes
3. Temperature
4. Concentration of electrolytes
5. Nature of product of electrolysis

Significance of Decomposition Potential

Decomposition potential plays a very important role in the following processes-
1. Electroplating: The minimum potential required for electroplating of metal under a given set of conditions is understood by decomposition potential.
2. Separation of metals: A solution containing copper and zinc ions are often separated by electrolysis. Since the decomposition potential of copper metal is low (1.2V), so at this voltage, only copper metal is deposited. When all copper is deposited, zinc ions get deposited later at a better decomposition potential (2.55V).
3. Refining of metals: A particular metal can be made to get deposited on the cathode by applying a potential equal to its decomposition potential (electrolytic refining).