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Let's say we need to measure the standard electrode potential of a zinc electrode dipped in an aqueous solution of $\ce{ZnSO4}$. To do this, we use the standard hydrogen electrode(SHE)-the reference electrode-as the anode and the zinc electrode as the cathode, and measure the emf of the cell.

The electrolyte corresponding to the SHE is a solution whose hydronium ion concentration is 1M. To construct the zinc half cell, we dip a strip of zinc in a 1M solution of $\ce{ZnSO4}$(for the STANDARD value). We connect the two electrolytes(of the zinc electrode and the SHE) with a salt bridge. Now, we measure the emf of this cell using a high resistance voltmeter, to find the potential difference between the electrodes without allowing any significant current through the cell.

When the zinc strip is dipped in the solution, it initially has no excess electrons on it. It is just a neutral piece of zinc metal. Now an equilibrium will be established between the solid zinc and zinc ions in the solution, finally leading to an excess of electrons on the electrode and a specific concentration of zinc ions in the solution. This, and a similar process at the SHE would cause an electrostatic potential difference between the two electrodes, since the relative positions of the two equilibria would be different. The voltmeter measures this potential difference to be the emf of the cell.

Here's my question: In setting up the two equilibria, won't the concentrations of the electrolytes change? Initially the zinc electrode had zero charge. But later it acquires a negative charge. This means that a certain number of zinc ions had to join the solution, leaving behind the electrons on the metal. So the concentration of zinc ions in the solution would now have to be more than 1M. So how would the STANDARD electrode potential be measured? How would the electrode potential be measured at ANY arbitrary concentration of zinc ions? It seems to me that the electrode potential can only be measured when the resulting concentration of zinc ions is equal to the equilibrium concentration. Does such a concentration even exist? Similarly, how is the hydronium concentration of the electrolyte corresponding to the SHE maintained at 1M?

According to me, the concentrations have to change for any net charge to develop on either electrode, hence establishing a potential difference. How does a net charge develop on each electrode without the ion concentration in the solution changing?

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    $\begingroup$ Your are really asking the wrong question. The real point is that with a modern high impedance voltmeter virtually no current has to flow to measure the voltage. So there is virtually no change in concentration at the cell's electrodes compared to the bulk solution. If you do draw a high current through the cell then the ions have a lower concentration at the electrode surface because diffusion limits how fast the ions can travel from the bulk of the solution to the electrode surface. So for high currents the EMF of a galvanic cell will drop. $\endgroup$
    – MaxW
    Commented Jan 27, 2017 at 10:38

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A net charge does not need to develop for you to measure a potential difference. A potential difference by definition is the energy it takes for you to take a unit charge from point A to point B.

By the very act of putting a zinc strip into a solution of $\ce{ZnSO_4}$ you established an electrochemical system in which there is an energy difference associated with moving two electrons out of $\ce{Zn}$ to make $\ce{Zn^2+}$.

Think of how thermocouples work. Two dissimilar metals are put into contact and there is a measureable potential difference that is a function of temperature. You don't need to flow any current to establish a potential difference, nor do you need any charge imbalance.

You may be confusing a zero potential with the potential of zero charge. The potential of zero charge $E_{pzc}$ is the electrochemical potential where there is no net charge on the electrode surface. This potential has no connection to the equilibrium potential of a given reaction.

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A voltimeter has very high resistance and consumes a very small amount of current, so if you make the measurement quickly, the concentrations are essentially constant for the duration of the measurement.

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  • $\begingroup$ But there is no net charge on either electrode initially $\endgroup$
    – Newton
    Commented Jan 8, 2017 at 15:33
  • $\begingroup$ So there won't be any potential difference. For charges to develop, the concentrations have to change. $\endgroup$
    – Newton
    Commented Jan 8, 2017 at 15:42
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    $\begingroup$ If you're in a large enough system, the concentration change is so small, it would nearly impossible to measure it. $\endgroup$
    – Zhe
    Commented Jan 8, 2017 at 20:51

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