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I recently set up an experiment with 1M half cells made up of zinc and copper nitrate. They were connected with a sodium nitrate salt bridge of varying concentrations. As the concentration doubled 5 times from 0.125M to 2M the voltage also increased. I couldn't find much explaining why this is however there is another question on this site which is quite similar and I imagine the answer may explain my problem as well. Link is attached underneath. Could increasing the concentration allow for a higher flow of electrons through the salt bridge due to lowering internal resistance and therefore increasing voltage?

(Why does increasing number of salt bridges increase voltage of electrochemical battery?)

Any help would be greatly appreciated. Thanks

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  • $\begingroup$ How do you measure the voltage ? If it is with an analogical apparatus, the cell will produce some current, and the voltage will decrease from the theoretical value. And then if you improve the connection between the two compartments, the internal resistance decreases and this will produce an increase in the voltage. If you use a digital voltmeter, this effect sill not be observed, as practically no currant is consumed. $\endgroup$ – Maurice Mar 23 '20 at 12:49
  • $\begingroup$ Electrons do not flow through a salt bridge, only ions do so. Increasing the ion concentrations in the salt bridge lowers its resistance, which matters if substantial current flows. As @Maurice noted, a digital voltmeter results in very low (negligible in your case) current draw because the input resistance of the digital voltmeter is typically 10 M ohms. $\endgroup$ – Ed V Mar 23 '20 at 13:12
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Based largely on a prior answer to the question: 'Why is it important to use a salt bridge in a voltaic cell? Can a wire be used?', the explanation centers on maintaining the charge balance (electrical neutrality) of the battery cell.

The electrons are efficiently transported via the electrodes connected by wires as sourced from the half-cell:

$\ce{Zn -> Zn(II) + 2 e-}$

However, to balance the charge in this half-cell, NO3- must enter from the salt bridge (and conversely, K+ must enter the other half-cell to balance the charge in both that cell and the salt bridge).

Not surprisingly, the less efficient part of this cell is the salt bridge itself creating resistance. Raising the concentration in the salt bridge lowers relative water content (a poor conductor and source of resistance) and increases the potential rate of diffusion of the respective NO3- and K+ ions, resulting in a corresponding increase of the cell efficiency (as witnessed by an increase in voltage). Some supporting comments per Wikipedia:

If no salt bridge were present, the solution in one half cell would accumulate negative charge and the solution in the other half cell would accumulate positive charge as the reaction proceeded, quickly preventing further reaction, and hence production of electricity.[1]

Also:

The conductivity of a glass tube bridge depends mostly on the concentration of the electrolyte solution. At concentrations below saturation, an increase in concentration increases conductivity. Beyond-saturation electrolyte content and narrow tube diameter may both lower conductivity.

And with respect to filter paper bridges, more details:

Conductivity of this kind of salt bridge depends on a number of factors: the concentration of the electrolyte solution, the texture of the filter paper and the absorbing ability of the filter paper. Generally, smoother texture and higher absorbency equates to higher conductivity.

Another interesting cited desirable property of the salt bridge is that the mass ratio of the bridge electrolyte relative to half-cell electrolyte should be small.

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