Why do we need a Salt Bridge?
Reviewing, the last thing we studied Mechanism of Electrochemical cell, we made it work without a salt bridge, and in fact it will, But not for long, the reason is better explained if we analyse what is happening next in the solution.
Here we can see that the process we studied, if it continues on happening, the concentration of Positive ion (here Zn+2) will increase and continue on increasing till it will hinder the next coming Positive ion so much that it is not able to come out in the solution.
Something similar happen to the other beaker but with negative charges.
What happened here? In simpler terms, the driving force was repelled by the repulsion created due to its happening, it’s just like the principle in physics that induced field tries to kill the source of its origin.Here let’s solve this problem in the engineering way to do things,
Problem – Increasing number of positive ions around the metal.
A simple solution – Decrease the number around the metal.
Applying it – we need to remove positive ion from metal, put some negative ions it will pull them towards it.
That is exactly what we do, when we put a salt bridge into the circuit. It consist of a gel agar-agar which in simple language is consisting of positive and negative ions, so when positive ion concentration increase in one solution it pulls the negative ion from the bridge and the positive ions surround the negative ion and hence, we get our main reaction going on again.
Now since we know what gel we used, can you think of some features that we might have considered while choosing the gel? As we can see a salt bridge is nothing, but a upside down U-tube filled with a gel.
The gel consist mainly of KCl, it is prepared as gel by adding it in boiling water. Why KCl? This is logically assumed, that production of both positive and negative ions are at the same rate so we need a combination of positive and negative ion (K+ and Cl–) such that they have almost same conductivity
So what is the role of a salt bridge? A traditional question
- It completes the circuit (which is actually, it’s keep the circuit working)
- To maintain neutrality
- To minimize Liquid junction potential and polarization(discussed later)
Abhishek kumar jha
(Chemistry at Utkarshini)
Notes for the lectures1-102-electrochemistry-why-a-salt-bridge
video link- https://www.youtube.com/watch?v=JgRC0n4yM38