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My textbook says that in a galvanic cell, it's not possible to measure single electrode potential independently. Instead, a Standard Hydrogen Electrode is used as the system is under equilibrium. Can someone explain this concept more clearly?

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    $\begingroup$ What is unclear there? Closed circuit is needed, you can't have that with only one electrode in solution. $\endgroup$ – Mithoron Jul 29 '17 at 14:40
  • $\begingroup$ But what does the system being in equilibrium have to do with it? Sorry if the question is trivial, just learning these stuff! $\endgroup$ – Lordinkavu Jul 29 '17 at 14:43
  • $\begingroup$ Probably that nothing happens so it's sort of "equilibrium". $\endgroup$ – Mithoron Jul 29 '17 at 14:45
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    $\begingroup$ Equilibrium means that the chemical potential has to be the same for the two side of the cell. One thing cannot be in equilibrium, as it makes no sense, it needs another system to be equilibrium with - which can act as a reference electrode $\endgroup$ – Greg Jul 29 '17 at 15:26
  • $\begingroup$ @Greg Can't the escaping of metal ions from electrode into electrolyte and the deposition of metal on the electrode be considered for equilibrium? $\endgroup$ – Lordinkavu Jul 29 '17 at 15:32
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Electrodes transport electrons produced from one half-cell to another, thus producing an electric charge. You need to have a reference value to compare it to in order to obtain the relative potential. Relative to the Standard Hydrogen Electrode (SHE), that is.

The SHE has a potential that is set arbitrarily to zero giving you a basis with which to compare the potentials from the electrode you're testing.

For further reading, I feel this article on the Standard Hydrogen electrode is pretty descriptive.

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Potential is a relative measurement. You will need to set a reference for measuring any potential.

As in this case, Hydrogen electrode is set as the reference.

All other potentials are measured against Standard Hydrogen electrode to get their potentials under standard conditions.

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