Defects in Simple Cells

Defects in Simple Cells:

There are two major defects in simple cells, namely, polarization and local action.

(1) Polarization- As we have seen that the hydrogen ions H⁺ travel to the positive electrode (Cu) and transfer their positive charge to it, the neutral hydrogen gas bubbles are expected to escape out from the electrolyte. But a part of such hydrogen bubbles gets deposited as a neutral layer on the positive electrode, which tends to stop the fresh inflow of positive charge to this positive electrode. Thus the potential difference (emf) starts falling down, and the cell ultimately stops functioning. This process is called polarization of the cell. Fresh hydrogen ions traveling towards the positive electrode, now form a positively charged layer on the neutral layer of hydrogen. In due course of time, the positive charge of the copper rod gets transferred to the zinc rod through an external circuit, and its positive potential falls down. Thus the copper rod becomes less positive than the positive hydrogen layer around itself, which sets up an opposite emf. This reverse emf further ensures the blockage of cell functioning. Thus if a bulb is connected to the external circuit, the intensity of its glow gradually reduces due to polarization.

The problem of polarization may be reduced by any of the following methods.

• Mechanical Method- The layers of hydrogen may be brushed away from time to time, but this method may be adopted in open-cell construction. It is not at all a convenient method, particularly when the cell is closed from all sides.

• Chemical Method- Oxidizing agents like nitric acid, manganese dioxide, potassium dichromate, etc. are used to convert hydrogen into water. Thus, the hydrogen layers may be removed.

(2) Local Action- The negative electrode used in simple cells is commercial zinc which contains impurities like carbon, iron, etc. These impurities lying on the surface of the zinc rod come in contact with the acid and thus minute cells are formed on the zin rod itself. Due to the circulation of these local currents on the zin rod, zin is wasted. This defect is called local action.

In order to remove this defect, amalgamated zinc rods are used. The zinc rod is initially washed with hydrochloric or sulfuric acid and then mercury is rubbed on the surface of the rod. Sometimes 4 to 5% of mercury is initially mixed with molten zinc before casting the rod. Zinc dissolves in mercury and a coating of pure zinc ion is formed on the surface of mercury while the impurities remain inside the coating. Thus only the zinc ions come in contact with the acid.