# When we are looking for volts vs SHE, is it that the potential of the element is compared to hydrogen potential?

I'm new to understanding this aspect of electrochemistry and only just now refreshing my memory of high school chemistry for a specific purpose. If I've worded the question or anything else incorrectly, don't hesitate to help me adjust.

This one link I'm reading on Corrosion has a standard EMF Series table with a column for reaction and a column for volts vs SHE. Under reaction we have for hydrogen $$\ce{2H+ + 2e- -> H2}$$ and then under volts vs SHE 0.0000.

I understand the reaction from high school chem 32 yrs ago (wow). Then, I look down to one of the two elements I'm interested in, Mg... $$\ce{Mg^2+ + 2e- -> Mg}$$ and its redox potential w.r.t SHE is -2.372 V.

The other element which I can't seem to find hard data on is carbon in the form of 4k carbon filament. I have a table that puts it in the range of +0 to +0.2. So, if Mg and carbon filament are used as two electrodes and connected into a circuit, theoretically with the right electrolyte and surface area I could get a potential of 2.5 volts?

The current (μAs) is what I'm trying to understand next and I'm sure I'll have more questions about that, but I just want to make sure I have this first part right first.

Thank you

• Your title has got a mistake. You should remove the word "atom", because there is no monoatomic hydrogen in a hydrogen half-cell. There are only hydrogen molecules $\ce{H2}$ (or ions). So your last words might be : ...is compared to hydrogen potential ! Dec 30, 2021 at 21:41
• I'm quite over my head in the electrochem section... quite obviously. But definitely needing help. Jump to the comments below and see [from @RobMcN] comment from @ToddMineHardt. I've got a chat room open and a little direction in conduction experiements and formulating.... formulas would be fantastic. Dec 30, 2021 at 23:43

The answer is yes, you will get a potential difference of 0.2-(-2.37) = 2.57 V, if Mg and carbon filament are used as the electrode materials and connected into one circuit. The third digit after the decimal is removed, because different resources give slightly different values. From our own experiments using glassy carbon, its open circuit potential is -0.07 V w.r.t. Ag/AgCl (sat) reference, which in turn is 0.197 V w.r.t. SHE. Therefore, the potential of glassy carbon is 0.197-0.07=0.127 V w.r.t. SHE, close to your quoted value of 0 to 0.2 V for 4k carbon filament. Note that SHE implies pH=0. In practice, people often use RHE as the reference point, which has a pH-dependent potential. This is convenient because experiments done at any pH values can be converted to a difference w.r.t. RHE reference.

Noticed that your question title is also a question. "When we are looking for volts vs SHE, is it that the potential of the element is compared to hydrogen atom's potential?" Strictly speaking, the potential is the redox potential of a half electrochemical reaction. By comparing two potentials, you are comparing the degrees of ease or difficulty to lose or gain electrons between two half reactions. In a sense, this is related to work functions or Fermi levels of different species. The half reactions with negative potentials can lose electrons more easily than the ones with positive potentials. Or conversely, the half reactions with positive potentials can gain electrons more easily than the ones with negative potentials. Therefore, when you are looking at volts vs. SHE, you can comparing the redox of the element vs. the $$\ce{H2 / H+}$$ (or sometimes written as $$\ce{H+ / H2}$$) redox pair.

• Please use MathJax and mhchem for the whole expression: \ce{...}. Other uses will break in different browsers and for different users and while it might look fine for you, it looks wrong for many others. Dec 30, 2021 at 21:20