Defects of a simple Electric Cell

4.2.1.1 Defects of a simple Electric Cell

The practical value and performance of simple cells is limited by the following defects:

(a)          Polarization

In the reaction in the simple cell, hydrogen gas is evolved. It collects in bubbles around the positive electrode and-eventually insulate the positive electrode from the solution.  This stops the reaction. This process is known as polarization.

It is minimized by use of a depolarizer.  This is a chemical which reacts with the hydrogen to produce water e.g. manganese dioxide.

(b)          Local Action

Impurities such as iron and lead embedded in the zinc electrode form small local cells. The impurity acts as the positive electrode and zinc as the negative. The formation of these local cells between the impurities and the zinc electrode is referred to as local action. It tends to wear the zinc electrode and the electrolyte; this happens even when the cell is not in use.

Local action may be minimized by using pure zinc, but zinc in its pure state is very expensive. Instead, a cheaper option is used alloying the zinc electrode with mercury. This process is referred to as amalgamation and resultant alloy is called zinc amalgam.