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I am reading about electrochemical cells from the chapter: Electrochemical cells, Batteries and Fuel cells in the book 'Physical Chemistry' by Thomas Engel and Philip Reid, 3rd edition.

I came across the following paragraph and I want to know if my understanding of this paragraph is correct or not?

The paragraph is about $\ce{Zn}$ elctrode partially immersed in an aqueous solution of $\ce{ZnSO4}$


Paragraph: Can we measure $\Phi$( half-cell potential) directly? Let us assume we can carry out the measurement using two chemically inert Pt wires as probes. One Pt wire is placed on Zn electrode and the second Pt wire is placed in the $\ce{ZnSO4}$ solution. However, the measured voltage is the difference in electrical potential between a Pt wire connected to a zinc electrode in $\ce{ZnSO4}$ solution and a Pt electrode in a $\ce{ZnSO4}$ solution, which is not what we want.


My question is whether the author wants to say that the second platinum wire will act as an electrode in zinc sulphate solution and hence the potential difference measured will be between zinc electrode and platinum wire which won't be the potential difference between zinc electrode and zinc sulphate solution?

Any help is appreciated. Thanks!

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  • $\begingroup$ See also the topic Absolute electrode potential $\endgroup$
    – Poutnik
    Jun 27, 2023 at 10:18
  • $\begingroup$ @Poutnik. Thanks for pointing about absolute electrode potential. To be sure, absolute electrode potential refers to the potential difference between a point inside the metal and a point outside the electrolyte solution? $\endgroup$
    – Natasha J
    Jun 27, 2023 at 13:02
  • $\begingroup$ @Poutnik, did you read the comment that I wrote below the answer? $\endgroup$
    – Natasha J
    Jun 27, 2023 at 13:04
  • $\begingroup$ You should use $\ce{ZnSO4}$ for $\ce{ZnSO4}$ and other formulas, as element symbol should not be in italics. $\endgroup$
    – Poutnik
    Jun 27, 2023 at 14:10
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    $\begingroup$ Yes, the author wanted to say what you have assumed. $\endgroup$
    – Poutnik
    Jun 27, 2023 at 15:28

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Absolute measurements of half-potentials have no sense. They do not exist. They can be measured with respect to something, like for example the hydrogen electrode. It is similar to measuring the absolute altitude. The absolute altitude does not exist. The altitude is measured with respect to the sea level. The altitude cannot have an absolute value.

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  • $\begingroup$ So half cell potentials do not exist independently? But in the book it is written that as zinc atoms leave the electrode leaving behind electrons, the zinc electrode acquires a negative charge and a corresponding positive charge builds up in the surrounding solution. This charging leads to a difference in electric potential $\phi$ between the electrode and the solution, which we call the half-cell potential. $\endgroup$
    – Natasha J
    Jun 27, 2023 at 10:13
  • $\begingroup$ @NatashaJ Direct measurement of the potential difference between the metal and the electrolyte is like an attempt to separate 2 quarks of a pion. If you succeeded, you would not have 2 quarks, but 2 pairs of quarks, providing during the separation energy for 2 other quarks. this potential difference has to be determined indirectly, or estimated theoretically. $\endgroup$
    – Poutnik
    Jun 27, 2023 at 14:46
  • $\begingroup$ @Poutnik But we can determine the potential difference between zinc electrode and zinc sulphate solution indirectly if not directly? $\endgroup$
    – Natasha J
    Jun 27, 2023 at 16:13
  • $\begingroup$ @NatashaJ Possibly yes, but I am not aware of the way. Things get very complicated by extreme reactivity of free electrons in water. We cannot measure their work function as there are none. All are chemically bound. $\endgroup$
    – Poutnik
    Jun 27, 2023 at 16:24
  • $\begingroup$ Okay, thank you for being patient with me $\endgroup$
    – Natasha J
    Jun 27, 2023 at 16:29

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