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9.11 Electrochemical Cells
An electrochemical cell is a single arrangement of two electrodes and an electrolyte for producing an electric current due to chemical action within the cell, or for producing chemical action due to passage of electricity. Thus electrochemical cells may be used for two purposes namely.
(i) to convert chemical energy into electrical energy.
(ii) to convert electrical energy into chemical energy.
The common galvanic (or voltaic) cell, dry cell and lead storage battery are devices for converting chemical energy into electrical energy, while the electrolytic cell used for the purification of metals is the device in which electrical energy is used in causing a chemical reaction (electrolysis).
9.11.1 Salt bridgeIt is U-shaped glass tube filled with a gelly like substance, agar-agar, mixed with an electrolyte like KCl, KNO3, NH4NO3, etc. It performs following function.
(i) It prevents voltage drops, i.e. prevents junction potential.
(ii) It allows flow of current by completing the circuit, i.e. migration of anion from anode to cathode half cell.
(iii) It prevents accumulation of charges and maintains the electrical neutrality of solutions by intermigration of ions into two half cell reactions.
Characteristics of electrolyte used as salt bridge
(i) The electrolyte should be inert and should not react chemically with the electrolyte of either of the two half cells.
(ii) The cation as well as anion of the electrolyte should have same ionic mobility and almost same transport number, viz. KCl, KNO3, NH4NO3 etc.
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