Does a salt bridge have to consist enough electrolyte to keep in supplying anion and cation to balance the electric potential build up in two half cells? If yes, then how can a string of cotton with limited electrolyte work as a salt bridge?
Salt bridge is a link between the electrolytes in the two half cells to transfer potential from one cell to another. It consists of strong inert electrolytes, which will not react with any of the electrolytes in either half cell. We assume the salt bridge is an infinite source for supplying both positive and negative ions to the electrolytes. A salt bridge can be of two types: inverted U-shaped glass tube filled with a gel mixed with up to ten percent strong electrolytes, or filter paper or cotton dipped in strong inert electrolytes.
Answer and Explanation:
The function of the salt bridge is to supply positive and negative ions to each half cell when there is an accumulation of either positive or negative ions to neutralize them. This makes the cell always neutral, and the cell supplies continuous current through the external conductor.
A string of cotton dipped in a strong inert electrolyte cannot be an infinite source of both positive and negative ions, but it can act as a source of ions for a considerable time, because the cotton string dipped in the electrolyte contains billions of positive and negative ions.
A salt bridge needs to contain enough electrolytes to supply sufficient positive and negative ions to supply each half cell when they build up charge. If a porous partition is used it permits the migration of ions from one cell to the other, but the function of a salt bridge is not the migration of ions from one half cell to another but rather the transfer of potential to complete the circuit. This happens when ions from the salt bridge move to each half cell.
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from AP Chemistry: Help and ReviewChapter 1 / Lesson 9
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