Standard Hydrogen Electrode Assignment

Question

How did scientists decide that 0V is the Standard Hydrogen Electrode? Was 0V measured through experimentation or was it decided in a meeting/convention? I am a little confused by the cited texts below. Was the principle behind assigning 0V the same as when we would tare a scale?

Unfortunately, we have no way to measure the absolute standard reduction potential of an isolated half-cell. All that can be measured are potential differences when two half-cells are connected. Therefore, to assign values to the various standard reduction potentials, a reference electrode has been arbitrarily chosen and its standard reduction potential has been assigned a value of exactly 0 V. This reference electrode is called the standard hydrogen electrode, SHE (Blackman, 05/2015).

_{Blackman, A., Bottle, S. E., Schmid, S., Mocerino, M., Wille, U., Brady, J. E., Sen, F. (2015). Chemistry, 3rd Edition. Retrieved from vbk://9780730324928}

To simplify the collection and sharing of potential data for half-reactions, the scientific community has designated one particular half-cell to serve as a universal reference for cell potential measurements, assigning it a potential of exactly 0 V. This half-cell is the standard hydrogen electrode (SHE) and it is based on half-reaction below:

$\ce{2H+(aq) + 2e− ⟶ H2(g)}$ (Flowers, 2019)

_{Neth, E. J., Flowers, P., Robinson, W. R., Teopold, K., & Langley, R. (2019). Chemistry: Atoms First 2e. Openstax. https://openstax.org/details/books/chemistry-atoms-first-2e}

Answer 1

The hydrogen electrode potential was set to zero more than hundred years ago by the German Society (Bunsen-Gesellschaft) in 1911 after collecting all the data from hundreds of sources for most of the elements. The original referene is Abhandlungen der Deutschen Bunsen -Gesellschaft für angewandte physikalische Chemie. This work was edited by none other than Nernst, who was a Nobel laureate (its the season!). In 1911, the Society decided

"Wir setzen also willkürlich die Potentialdifferenz zwischen einer von Wasserstoff von Atmosphärendruck umspülten Platinelektrode und einer Lösung, die 1 Mol Wasserstoffion in 1 Liter enthält, = 0. Die Beziehungen zwischen dieser „Normal- Wasserstoff-Elektrode" und den am meisten gebrauchten Bezugselektroden, nämlich Wasserstoffelektroden in Lösungen anderer H--Konzentration, Kalomel- Elektroden, Mercurosulfat- Elektroden usw. sind mit einer für die Berechnung von Einzelpotentialen hinreichenden Genauigkeit bekannt."

"We set arbitrarily the potential difference between a platinum electrode surrounded by hydrogen at atmospheric pressure and a solution containing 1 mol hydrogen ion in 1 liter = 0. The relations between this "normal hydrogen electrode" and the most used reference most commonly used reference electrodes, namely hydrogen electrodes in solutions of other H- concentrations, calomel electrodes, mercurosulfate electrodes, etc., are available with an accuracy sufficient for the calculation of sufficient accuracy for the calculation of single potentials."

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Textbooks do a poor job of explaining electrochemistry concepts. For any experimental measurement you a reference. When you quote height of a moutain, the sea level is set as zero height. For pressure, you define vacuum to be reference, zero. When you measure velocity of a car, we set Earth's velocity to be zero or ignore it. Electrical engineers consider the Earth at 0 V potential. In short electrochemists also needed a reference, and the normal hydrogen electrode was a reasonable choice.

Well there is a long story. A very influential physical chemist named G. N. Lewis, had issues with their definitions and the sign conventions of the electrodes. This war between the European conventions vs. American conventions lasted for decades and confused thousands and thousands of students.