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Voltmeter is an instrument which measures electric potential difference between two points.

When measuring electrode potential of some redox system (vs SHE for example), it is said that voltmeter reading contains sum of all potential differences present in a cell. This includes all electrode/electrolyte potential differences, contact potential differences between electrodes and probes of voltemter and possibly liquid junction potential. Probes of voltmeter are connected with two electrodes in a cell.

However, since voltmeter measures potential difference between two points, how can it sense sum of all potential differences in a cell if it is connected only between two electrodes and not all potential differences are created at these two points?

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    $\begingroup$ In addition to the answer by @Poutnik, you might also look into three and four wire potentiostats. They can be a bit hard to fathom, but they are part of electrochemistry’s common instrumentation. $\endgroup$
    – Ed V
    Jul 19 at 14:10
  • $\begingroup$ @EdV These devices are indeed interesting, but they are still kind of like separation of two quarks ( leading to 2 quark doubles ). Advantage is some of the potential differences can be considered in this case constant and some variable. $\endgroup$
    – Poutnik
    Jul 19 at 14:21
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    $\begingroup$ @Poutnik No problem: I already upvoted your answer. $\endgroup$
    – Ed V
    Jul 19 at 14:24
  • $\begingroup$ Yes, I think I need to check how voltage devices work as I thought they can measure only potential differences between two points on which they are in direct electronic contact. I will check potenciostats as well. $\endgroup$ Jul 19 at 14:49
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    $\begingroup$ Maybe I misunderstand, but are you familiar with Kirchhoff’s voltage law? There is an article in wikipedia on Kirchhoff’s circuit laws. $\endgroup$
    – Ed V
    Jul 19 at 15:35
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It is the topic for electronics, as measuring of electrochemistry related potential differences by a voltmeter is not different to measuring other potential differences. In both cases they are potentials of different pieces of conductor.

Potential of the wire connected to SHE is conventionally taken as the zero potential.

Even in pure electronics, all what a voltmeter measures is the sum of potential differences across all the circuit path from one measuring point to the other one. That includes all kinds of intended or parasitic components of the circuit. It is the same as for circuits involving electrochemical cell.

There is a close parallel, as you e.g. cannot in general directly measure potential differences inside enclosed integrated circuits.

Partial potential differences within the cell are not measurable this way, must be measured indirectly ( e.g.by analysis the cell behaviour under the load ) , or differently, or estimated theoretically.

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