Standard, Secondary Cell
Standard cell is that cell which can provide nearly a constant over a very long period of time. The Daniel cell and leclanche cell as discussed above do not have constant with time due to change in the ionic concentration of their electrolytes. The commonly used standard cell is the Weston cadmium cell it consists of H-shaped glass vessel whose two vertical limbs A and B are 3 long and diameter. The positive electrode or cathode of the cell is pure mercury at the bottom of limb A. above which there is a paste of miraculous sulphate (Hg2.SO4) which acts as a depolarizer. Over this paste cadmium sulphate (CdSQ) crystals are placed.
The negative electrode or anode of the cell is cadmium –mercury (Cd – Hg) amalgams placed at the bottom of the other tube B. over this amalgam cadmium sulphate crystal are placed.
The electrolyte is a saturated solution of cadmium sulphate which is filled in both the limbs of the tube platinum wires are sealed at the bottom of each limb of the tube which are connected to the terminals on the insulated box in which H-shaped vessel is kept. The cell can be connected to the external circuit through the terminals on the box.
When the cell is in use the chemical reactions involved in this cell are given below
(i) At the negative electrode
Cd = Cd ++ + 2e –
Thus the cadmium atoms give the negative charges.
(ii) At the positive electrode
Cd ++ + CdSO4 + Hg2SO4 = 2 CdSO4 + 2 Hg+
The mercury ions acquire negative charge from the positive electrode, get neutralized and deposit there 2E- + 2 Hg + = 2 Hg
Due to these reactions a constant potential difference is developed between two electrodes of the cell and a constant electrons current is maintained in the external circuit, The independent of temperature over a considerable range.
A secondary cell is that cell in which the electrical energy is first stored up as a chemical energy and when the outside circuit is closed to draw the current from the cell the chemical energy is reconverted into electrical energy. The chemical reactions are reversible in this cell.
The secondary cells are also called stronger cells or accurnulators because they act in such a way as if they were reservoir of electricity the current can be drawn from them whenever required and when they are discharged they can be recharged. The commonly used secondary cells are lead-acid accumulator and Edison cell.
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