Salt Bridge Questions: Will Zinc Affect Cell Potential?

[AnathemaXI]

I've just about had it with these salt bridges. It's such an easy concept but there are too many questions about it.

I understand that the salt bridge enables the flow of ions for the cathodes and anodes by neutralizing charges, but if you use a different salt bridge (Say zinc nitrate instead of potassium nitrate) would it effect the actual cell potential at all? Although zinc has a lower reducing strength and higher oxidizing strength than potassium, it's still going to neutralize the charges in the solution.

Can anyone explain to me if it would or wouldn't effect the cell potential?

 

[Topher925]

The answer is, it depends. If you are talking about open circuit voltage, then no, theoretically the salt bridge material will not affect the cell potential. However, if the cell is not at equilibrium, that is there is some kind of net transfer of charge, then yes it will. This change in potential is due to the ionic conductivity of the bridge which generates losses in the cell that manifest as a lower cell potential. A bridge with a higher ionic conductivity will yield a higher cell potential when current is being drawn and vice-versa.

 

[AnathemaXI]

In the example I'm thinking about it refers to an electrochemical cell, a closed one at that. The molarities of the of the solutions are not at standard conditions so it is not at equilibrium. I'll give you the whole picture b/c maybe that would help more.

I was given the problem that a student is making a Fe|Fe2+ || Ni2+|Ni cell. He had .25M Nickel nitrate solution and .75 Iron II chloride solution. I discovered the cell potential under those conditions would be .18 V using the Nernst equation.

Now the question I'm trying to solve after that is if replacing the original potassium nitrate solution with a zinc nitrate solution would effect the cell potential. I see that zinc is still capable of reducing Fe and Ni albeit not as much as K. It is also a strong electrolyte so it would dissociate completely and neutralize the cathode and anode.

I'm not sure exactly how to judge the ionic conductivity of the salt bridge in relation to what you said. Zinc 2+ isn't as strong of a reducing agent as potassium, so does that mean it yields a lower ionic conductivity as a salt bridge which would lead to a decrease in cell potential?

 

[Borek]

You mean zinc nitrate replaces potassium nitrate in salt bridge?

As Topher wrote, as long as there is no current flowing through the circuit and ions in the salt bridge don't mix with the electrode solutions, potential of the cell shouldn't change.

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