In this topic, you study the definition, working & diagram of Voltaic cell.
It is a primary cell. It consists of a glass vessel containing dilute H2S04, and two dissimilar metals used as the electrodes. The copper and zinc plates are used. Both the electrodes are immersed in dilute H2S04 solution as shown in Fig. 9.3. If both the electrodes are connected through glavanometer or otherwise, the current will flow from copper to zinc outside the cell and from Zn to Cu inside the cell.
Fig. 9.3. Voltaic cell.
The Zn rod reacts with the dil. H2S04 and forms hydrogen.
zn + H2S04 ZnS04 + 2H
The hydrogen being positively charged travels towards the copper plate and after gaining two electrons liberates in the form of atomic hydrogen around the copper plate in the shape of bubbles. Thus the copper plate behaves like the anode and Zn plate as cathode.
Defects. The cell suffers two major defects the local action and polarization.
Local Action. The Zn rod used in this cell is not a pure Zn rod but has some impurities like Sn, Fe, Cu. etc. In that case, even if the load is not connected even than small cells are formed. The hydrogen gives away its charge to the dissimilar metal particles. As a result due to this action there is a wastage of Zn, and life is also decreased. This defect is known as the local action as shown in Fig. 9.4.
This defects can be removed by amalgamating the Zn rod. In this method the mercury layer is deposited over the zinc rod. Thus the mercury will not permit the other impurity to react and will only allow the particles to react with dilute H2S04
Polarization. The nascent hydrogen after giving its charge to copper and finally gaining two electrons, liberates in the form of atomic hydrogen which is neutral and behaves as an insulator for the other charged particles of hydrogen. It does not allow other hydrogen particles to give its charge to copper plate and hence the current is reduced as shown in Fig. 9.5. The rate of deposition will increase with the time and finally the cell will stop its working. This action is known as the polarization.
Fig. 9.5 Polarisation
Fig. 9.4. Local Action
The defect can be removed by two ways:
(a) By mechanical means. In this method the copper plate is taken out, brushed and again used.
(b) By using depolarizer. In this case a chemical agent is used, which converts the hydrogen into other forms say water etc. The chemical is known as the depolarizer as Mn02 etc.