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I searched for the answer for this question on Wikipedia and Stackexchange a lot but couldn't find same question or a required answer in some other similar question.

I understand that the function of the salt bridge is to complete the circuit and maintain electrical neutrality in both the half cells of a galvanic cell. This is achieved by movement of cations and anion in the salt bridge. the anions move towards anodic half cell and cations move towards cathodic half cell.

My question is- Are these cations and anions are provided by salt bridge itself and it depletes over time as the electrochemical reaction progresses?

OR

Is salt bridge is just a medium and passing the anions from solution of cathodic cell to anodic cell and passing the cations from solution of anodic cell to cathodic cell?

OR

Both of this happens?

I will be grateful for insight on this topic.

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Consider a salt bridge as a part of the cell electrolyte, that is not allowed to mix with the rest of electrolyte.

For a particular ion, there are 2 cases:

  1. The ion is shared with the electrolyte part, from which the ion migrates toward the bridge. Then the ion gets depleted from electrolyte and not from the bridge.
  2. The ion is not shared with the electrolyte part, from which the ion migrates within the bridge toward the other electrolyte part. Then the ion gets depleted from the bridge.

But there is needed to add the bridge maintains ion neutrality, so ions that leave the bridge are replaced at the other bridge end by the same or other ions of the same total charge.

Bridges are not usually used in power cells and cells where significant portion of chemicals gets depleted or react. If they are used, they usually use at least one of ions shared.

Power cells often use a single electrolyte and a diaphragm, soaked with this electrolytes, so ion depletion does not apply.

Bridges are used mostly in cells where accurate potentials matter, to minimize phase potentials, where the current is minimal. Like calomel ( $\ce{Hg2Cl2}$ ) or $\ce{AgCl}$ reference cells, or in laboratory test cells.

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  • $\begingroup$ @Poutnic: Do the components of the salt bridge change over time on cell operation? How? $\endgroup$ – Adnan AL-Amleh Nov 27 '19 at 22:56
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    $\begingroup$ The ions migrate in and out, that is the change. Plus, in a particular case, there may be chemical changes, not directly related to being a cell bridge. $\endgroup$ – Poutnik Nov 28 '19 at 4:21
  • $\begingroup$ I am replying late but thank you @Poutnic for the answer. $\endgroup$ – Peeyush Sharma Dec 4 '19 at 17:51
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Salt Bridge is like a source which provides cations to the the half cell where the negative ions are more than the positive ones in the electrolyte .

Similarly it gives anions to the half cell where positive ions are more than the negative ones in the electrolyte.

It basically ensures continuity of the flow of electrons from anode to the cathode by neutralising the ions present in the respective electrolytes by providing an electric contact between the two solutions without allowing them to mix with each other.

One thing which should be taken care of while choosing appropriate salt bridge is that the substance/solution present in it should have equal migratory aptitude of both the cations and the anions so as to ensure that the neutralisation occurs almost simultaneously in both the half cells.eg Potassium chloride, ammonium nitrate.

Usually an electrochemical cell stops functioning when no more electrons are released at the anode. There can be multiple reasons for this and malfunction of the salt bridge might be one of the reasons.

Eg.the salt bridge getting depleted i.e. it is not releasing enough counter ions in the electrolyte. Therefore the solution in the anode compartment would become positively charged and the solution in the cathode compartment would become negatively charged, because of the charge imbalance, the electrode reaction would quickly come to a halt

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