Also, the purpose of a salt bridge is to optimize certain electrodes' properties using two, different (independent) electrolytes; so, each one of the two (half reaction) cells is, usually, free to use it's own electrolyte, independently, without disturbing the behavior of the other (half cell).

In short, a salt bridge bridges the internal ionic current, only.

In more details, a salt bridge is an analog of a series connection in terms of the circuit engineering schematic electronic diagrams, e.g. the same, nominal, currents' value; however, the internal nature of this current is, chemically, very special. Even though, this current is the,

1) same, ionic nature (of the current), in both half cells,

stock-still, it's own internal (ionic) nature is composed from very

2) different (chemically) ions, in each one of the half cell,

accompanied, also, by a very:

3)different (standard, and RE[1]) potential values, and 4)different (ionic) window[2] of the potential values,

5)different mobility[3,4],

6)different spin, ..., etc.

1. a) Reference Electrode Salt Bridge Kit https://www.pineresearch.com/shop/wp-content/uploads/sites/2/2017/06/DRP10038-Reference-Electrode-Salt-Bridge-Kit-Product-Guide-REV001.pdf b) Electrochemical Cell Kit http://www.acminstruments.com/media/ACM/Datasheets/ds-ElectrochemicalCellKit-v1.0.pdf c) https://www.waters.com/waters/partDetail.htm?partNumber=700001958&locale=en_GR

2. https://chemistry.stackexchange.com/questions/12427/why-does-increasing-number-of-salt-bridges-increase-voltage-of-electrochemical-b https://chemistry.stackexchange.com/questions/5477/why-is-it-important-to-use-a-salt-bridge-in-a-voltaic-cell-can-a-wire-be-used

3. SEI[3] in batteries' electrodes might be view

inside the wider context of a salt-bridge (in a voltaic-cell) presenting a much lower mobility than the high mobility values inside the common liquid electrolyte (salt-bridge).

4. The SEI Layer Inside Lithium Batteries Explained https://www.upsbatterycenter.com/blog/sei-layer-inside-lithium-batteries/

Dear Ioannis Samaras and Habib M. N. Ahmad

Thanks so much for your explanations about salt bridge but honestly I'm not looking for the salt bridge definition. I asked crystal clear questions and I really really appreciate it if I can have to the point answers for them.

Thanks again.

Dear Sina Hamedi ,

the cell is not (fully) specified in your question[1].

1. Quotation (of your question): The function of salt bridge in electrochemistry?

Hi

We use a salt bridge in a Galvanic cell to maintain electron neutrality by transferring anions to the anode side and cations to the cathode side. The question is that why the existing SO4(2-) (for example) in the anode half cell electrolyte cannot neutralize the produced Zn2+? If true, can we omit the salt bridge and increase the concentration of the electrolyte in two half cells?The second question is that how salt bridge really works? how is it possible for it to pass just anions to the anode and cations to the cathode? i.e. why doesn't it pass Cu2+ from the cathode electrolyte to the anode electrolyte? Why doesn't it pass the existing KCl in the salt bridge to either sides?Thanks!

Sina Hamedi , the Equations you're looking at go in both directions, but the directionality is dependent on the charge, so the electrons being gained on one side are lost on the other side. Thus the lead acid (and SO4(2-)) releases electrons while the zinc ions needs them, or vice versa.

"why doesn't it pass Cu2+ from the cathode electrolyte to the anode electrolyte? Why doesn't it pass the existing KCl in the salt bridge to either sides?"

Because the copper ions are electromagnetically attracted to receive the electrons from the cathode.

It's all about the electrochemical gradients.

Ioannis Samaras

I just took a simple cell for example. Zn|ZnSO4||CuSO4|Cu and the salt bridge contains KCl.

Jack Don McLovin

Thanks for the answer. I think you just answered the second question but I still don't get why we need something like KCl for ions transfer to both sides and why doesn't KCl go either directions and neutralize excess of ions?

About the first question I don't know what equations you talk about and I just took a simple cell (Zn|ZnSO4||CuSO4|Cu ) for example not lead acid battery.

Thank you.

Dear Sina Hamedi ,

the ionic (electrolyte's) intermixing depends, mainly, on the current (actually the charge). There is, practically, no (ions' intermixing) issue, upon zero current tests, either at rest, say for OCV measurements, or for an educative (demonstration) day time. Some minor, aging issues (including ions' intermixing) depend on the kind of the salt bridge (huge number of) specs. The salt bridge electrolyte is, always, strong, having high specific conductivity[1] (like diluted KCl[2,3]).

Also, the electrolyte's mass ratio ("bridge electrolyte" over "(any) half cell electrolyte") is "near zero" (infinitesimal).

1. https://www.traskinstrumentation.com/pdf/app_notes/Conductivity_Chart_of_Liquids.pdf

2. Why is potassium(and not lithium or sodium) always used in salt bridges as KCl or KNO3? https://answers.yahoo.com/question/index?qid=20100720091314AAfdshu

3.

Article Electrical Conductivity of Sodium Chloride and Potassium Chl...