# Why is it important to use a salt bridge in a voltaic cell? Can a wire be used?

I was learning about voltaic cells and came across salt bridges. If the purpose of the salt bridge is only to move electrons from an electrolyte solution to the other, then why can I not use a wire?

Also, will using $\ce{NaCl}$ instead of $\ce{KNO3}$ in making the salt bridge have any effects on voltage/current output of the cell? why?

Plus if it matters, I'm using a Zinc-Copper voltaic cell with a tissue paper soaked in $\ce{KNO3}$ as salt bridge

There's another question related to salt bridges on this site.

The purpose of a salt bridge is not to move electrons from the electrolyte, rather it's to maintain charge balance because the electrons are moving from one-half cell to the other.

The electrons flow from the anode to the cathode. The oxidation reaction that occurs at the anode generates electrons and positively charged ions. The electrons move through the wire (and your device, which I haven't included in the diagram), leaving the unbalanced positive charge in this vessel. In order to maintain neutrality, the negatively charged ions in the salt bridge will migrate into the anodic half cell. A similar (but reversed) situation is found in the cathodic cell, where $\ce{Cu^{2+}}$ ions are being consumed, and therefore electroneutrality is maintained by the migration of $\ce{K+}$ ions from the salt bridge into this half cell.

Regarding the second part of your question, it is important to use a salt with inert ions in your salt bridge. In your case, you probably won't notice a difference between $\ce{NaCl}$ and $\ce{KNO3}$ since the $\ce{Cu^{2+}}$ and $\ce{Zn^{2+}}$ salts of $\ce{Cl-}$ and $\ce{NO3-}$ are soluble. There will be a difference in the liquid junction potential, but that topic is a bit advanced for someone just starting out with voltaic/galvanic cells.

• But if you consider the salt bridge as a solution that is part of the two half cells (which connect the two half cells so they are one), it doesn't really matter which side is positive or negative since the whole thing (two half cells connected by a salt bridge) Is neutral. – most venerable sir Apr 19 '15 at 16:03

Without the salt bridge, 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.

It helps to maintain the flow of electrons from the oxidation half-cell to a reduction half cell, this completes the circuit.

• Why it would come to a halt? Is this because the accumulation of positive charge will attract the negative electrons? – Anton Mar 10 at 22:20

The purpose of the salt bridge is to move ions.

If you use enough electrolyte solution on both sides, though, it doesn't matter; in that case, the salt bridge can be neglected.

I was learning about voltaic cells and came across salt bridges. If the purpose of the salt bridge is only to move electrons from an electrolyte solution to the other, then why can I not use a wire?

If you connect the two electrodes with a wire, you will short-circuit anything else connected to the electrodes. If you connect the two electrolyte solutions with a wire, there are two things that could happen:

1. nothing (if there is no redox reaction happening at the wire/electrolyte interface, there will be no charge transfer. The wire can't transport ions, and the electrolyte can't transport electrons).
2. two more half-reactions (if there are half-reaction that the respective combinations of electrolyte and wire material can support)

In either case, this will not have the desired effect of a salt bridge, which is to balance the charge buildup that occurs when electrons travel along the wire from anode to cathode.

Also, will using $$\ce{NaCl}$$ instead of $$\ce{KNO3}$$ in making the salt bridge have any effects on voltage/current output of the cell? why?

The voltage and the current depend on what load you attach to the voltaic cell, and properties of the cell itself.

If ion transport along the salt bridge is the rate-determining step, it will affect the current (current is movement of charge per time). If you measure the voltage under load, it will also be affect by the nature of the salt bridge.