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.
