A simple voltaic cell is made by immersing a zinc plate and a copper plate in a solution of sulfuric acid diluted in water. As shown in the figure, if a copper plate and a zinc plate are connected externally to an electric load, an electric current starts flowing from the copper plate to the zinc plate through the load. This means that some electrical potential difference has developed between the copper plate and the zinc plate. As the current flows from the copper to the zinc, it is obvious that the copper plate becomes positively charged and the zinc plate becomes negatively charged.
Voltaic Cell Working Principle:
The working principle of the voltaic cell is based on the principle that whenever two different metals are immersed in an electrolyte solution, the more reactive metal tends to dissolve in the electrolyte as positive metal ions, making the metal Electrons are left behind on the plate. This phenomenon causes the more reactive metal plate to become negatively charged.
A less reactive metal will attract the positive ions in the electrolyte, and hence these positive ions accumulate on the plate, giving the plate a positive charge. Here in the case of simple voltaic cell, zinc precipitates as positive ion in sulfuric acid solution and then reacts with negative SO4 − − ion of the solution to form zinc sulfate (ZnSO4). Since copper is a less reactive metal, the positive hydrogen ions of the sulfuric acid solution tend to accumulate on the copper plate. More zinc ions in solution mean more electrons are released in the zinc plate. These electrons then pass through an outer conductor sandwiched between zinc and copper plates.
On reaching the copper plate, these electrons then combine with the hydrogen atoms deposited on the plate to form neutral hydrogen atoms. These atoms then combine in pairs to form molecules of hydrogen gas and the gas eventually comes along the copper plate in the form of hydrogen bubbles. The chemical reaction taking place inside a voltaic cell is as follows,
However, this process stops when the contact potential between Zn and dilute sulfuric acid reaches a value of 0.62 volts. During the operation of a voltaic cell, the zinc layer is at a low potential with respect to the adjacent solution film as shown in the figure below.
Similarly, when a Cu plate is placed in contact with an electrolyte, the positive hydrogen ions in the solution tend to accumulate on it until its potential rises above that of the solution by about 0.46 V. Therefore, the electric potential difference across the voltaic cell is 0.62 − (− 0.46) = 1.08 volts.
A simple voltaic cell has two main faults, known as polarization and local action.
Polarization of Voltaic Cell:
It is observed that the current in this cell decreases gradually and after a certain time of its operation the current stops completely. This reduction in current is due to the accumulation of hydrogen on the copper plate. Although the hydrogen escapes from the cell in the form of bubbles, a thin layer of hydrogen forms on the surface of the plate. This layer acts as an electrical insulator, thereby increasing the internal resistance of the cell. Because of this insulating layer, no more hydrogen ions can gain electrons from the copper plate and accumulate as ions. This layer of positive hydrogen ions on the copper plate exerts a radiant force on other hydrogen ions that are approaching the copper plate. Hence the current decreases. This phenomenon is known as polarization.
Local Action of Voltaic Cell:
It is found that even when the voltaic cell is not supplying any current, zinc continues to dissolve in the electrolyte. This is due to the fact that some traces of impurities such as iron and lead in commercial zinc form small local cells that short-circuit through the main body of zinc. The action of these parasitic cells cannot be controlled so that some loss of zinc occurs. This phenomenon is known as local action.