Talk:Electrochemical potential

Comments
Can one of you gentlemen explain to me how a Daniel Cell works? As a young lad, age 7, (it is now 60 years later!) I built ALL the cells but NEVER understood why the Daniel cell was so LONG_LASTING. HOW does the porous pot STOP the zinc supphate mixing with the copper sulphate?

Is the volts of the daniels cell that BETWEEN zn and Cu suplhate "electrodes" or what?

Many thanks. REMEMBER they taught ME that in the electrolysisis of dilute H2So4 the H went one way and the so4 the other. When I complained that it was NONSENSE, they said "Well we know that, but THAT is what you MUST say to get thru the exam!"

So I took up PHYSICS!

Many thanks. Sorry if I am doing all this wrong - NO INSTRUCTIONS! John

Couple points:

Electrochemical, or chemiosmotic potentials, are use for far more in biology than the production of ATP. Limiting the biological usage of transmembrane potentials to just ATP production ignores their creation in other organelles, or for example their existence across nerve membranes. David Myers

This is for the person who thinks "chemiosmotic potential" isn't a term. If you edit out my use of the term, I will post, in the talk area of this encyclopedia reference, all 63 hits that Google gets to the term chemiosmotic potential, and I'll make sure every link to the term works. David Myers

168...
 * I'm the person who complained about "chemiosmotic potential." Hi. Before I made my comments I looked around the Web for usage. I didn't find anything that struck me as authoritative. Xrefer and other online scientific dictionaries I searched didn't have it. Plus, I gave some weight to my simple gut feeling, because if it were the popular term for the potential, I suspect I'd have heard it when I learned about this subject in a college course for physical science majors. But it struck me as a word I'd never heard. To me, the number of Google hits is less important than what they hit. If you can show me the Web site of a professor or a text book publisher or something else authored by someone who ought to know, you'll persuade me.


 * O.K. I'll back off. I see that this guy, who really ought to know, uses it here [shiva.msu.montana.edu/courses/mb480_580_2002_fall/ docs/firstlivingsystems.pdf]. 168... 04:07 Feb 9, 2003 (UTC)
 * I'd be appreciative if you would realize that it is a term used among scientists involved in studies of bioenergetics. I use it because I recall its use among class notes at Rice University, notes penned by Professor John Olson (Rice is a small school. Professors teach the classes and write their own class notes). My major professor when I was at Rice, Professor Graham Palmer, is an expert on ESR of electron transport proteins and genuinely an expert on cytochrome c oxidase. It is from these men that I recall the term. That it is a term among specialists I will not deny.The important issue is the breakdown of the protonmotive force into three component terms, relative concentration of protons, which of course determines pH, the voltage difference across the membrane, and the remaining non-proton concentration differences across the membrane(e.g. Ca2+, Na+, etc). By the way the last rewrite of the chemiosmotic potential portion is totally confusing. It makes it seem as if the membrane has some magic permeability characteristics that makes it transport protons via a magic wand, instead of using active transport mechanisms to maintain the chemical potential across the membrane.David Myers Dwmyers 14:00 Feb 9, 2003 (UTC)


 * How much crow do you want me to eat? So what if I challenged you initially? I didn't do it without some basis. Plus the evidence you mustered in reply was mostly of the "listen, I know what I'm talking about" sort, which just doesn't cut the mustard here where no one knows you're a dog. In the end, it was me who found my own compelling evidence for your claim before you ever cited a text book or an unchallengeable source. You might thank me for my conscientiousness and good-spirited concession. Anyway, what I wrote above is _exactly_ the concession you seem to resent not having been offered. The professor whose use I cited is evidently an old-school bioenergeticist (he's emeritus, recalled). If he uses the term, it follows without saying that it's "a term used among scientists involved in studies of bioenergetics." But there anyway, I've said it.


 * Meanwhile, whether it's used by the majority of such people has not been established. Although this question is not the one I raised when I challenged you (I hypothesized the term was an imaginary conflation of two others), I'm curious about it, because if few experts use the term, that would make me even less of a dweeb for challenging it. But I'd say the jury is still out on this question. While complimenting me for my conscientiousness and good spiritedness, you might throw in a concession for that.

168... 18:08 Feb 9, 2003 (UTC)
 * I don't know what the answer is. It wouldn't surprise me to find that it isn't common, because Google doesn't pull up a lot of meeting abstracts; but also I could believe it is common. Since you don't claim yourself to be a bioenegeticist or somebody who hears lots of presentations from bioenergeticists at meetings, I'm supposing that, if you do have an opinion on this question, you won't be asking me to take your word for it.

Is the electrochemical potential expressed in volt? In the article this is kept very vague, as if it is something qualitative, not quantatitive - Patrick 21:21 Feb 7, 2003 (UTC)

In the context of an electrochemical half cell, electrochemical potentials can be expressed in units of voltages or in units of energy. In the mitochondrion, because the components of the potential have other physiological effects, the tendency is to break chemiosmotic potentials into three parts, a voltage difference, a concentration difference, and a pH difference across the membrane. David Myers208.17.215.247 21:32 Feb 7, 2003 (UTC)

168...
 * My hunch is you would use volts in a context where you are thinking about the movement of a charged particle. Volts are units of energy per unit charge. But in complicated membrane situation involving many solutes and coupled transport (where a pump used the transmembrane concentration gradient of an uncharged solute to drive the transport of a charged solute against its gradient, say), I can imagine you might have to consider uncharged molecules too, and for them the leap across the memembrane would be a potential energy change, though the voltage exerts no force on them. You'd probably want to talk about them in units of pure energy. Probably somebody should look in a book at this point.


 * Thanks. - Patrick 20:55 Feb 8, 2003 (UTC)

This statement is confusing:

Therefore a transmembrane difference in pH, in addition to the factors above, represents a distinct contributing component of the electrochemical potential in these contexts. This more complex potential might sometimes be called a chemiosmotic potential, after the chemiosmotic hypothesis of Peter Mitchell.

pH is a concentration term, and therefore is included in the concentration term of the general definition of electrochemical potential. It is "broken out" of the general definition in the biological case because pH affects acidity, and because biological systems usually have tight controls on acidity. Other concentration gradients, such as the sodium and potassium gradients, aren't immediately as complicating as the pH gradient.

To restate, the general definition of an electrochemical potential is complete in the sense that it has all the terms necessary to account for a chemiosmotic potential. But when people speak of chemiosmotic potentials across a membrane, for historical reasons they treat the concentration terms as split into two, one part being purely the proton concentration gradient, which affects pH, and the other being the remaining concentration terms. It's a complex potential to be sure, but not somehow a "more complex" electrochemical potential. Dwmyers 14:16 Feb 9, 2003 (UTC)

I reread the latest edit again and rewrote the section on the chemiosmotic potential. The old version confused chemical reaction energies of protons with a chemiosmotic potential. Nope, reaction energies have nothing to do with a chemiosmotic potential. It's simply that because the membranes are proton translocators, scientists in this area think of the potential in terms of proton concentrations and also in terms of the relative concentrations of other chemical species. This energy is a pure potential and does not include terms involving proton chemical reactions. If you need a reference to see what I'm talking about, try Tzagaloff's book on mitochondria, page 152. (the paperback 1982 edition from Plenum press) 24.98.55.136 15:10 Feb 9, 2003 (UTC)

168... 17:36 Feb 9, 2003 (UTC)
 * O.K. I can believe that was a goof on my part--letting reaction energies seep in. Good that you fixed that.


 * I just simplified the sometimes-protons-are-special section.

168... 18:52 Feb 9, 2003 (UTC)
 * My hunch is you were thinking of a context where one considers only what crossing the membrane would mean for a proton, but the text read as if one might seperate out the protons for any energy calculation for such membranes. In your scenario, I'm guessing you're thinking of the proton energy as a voltage term plus a proton concentration or pH term. But hiding in that voltage component, which I think would have to look like the GHK equation, I think you'd have the proton concentration lurking again, alongside separate "terms" (albeit all within the same numerator and denominator) for the concentrations of the other permeant ions. If that's right, the suggestion that proton concentration can be separately dealt with, while the other ions can be lumped, I find a little misleading. (The suggestion is just by way the ambiguity in talking about "the concentration component" of the potential without having an equation and specific terms to point to. I don't think you deliberately suggested it.) Besides that, I just think simpler would be better for this article.

Another point: this article has been pushed so far to the biological side, and it's done it so poorly, that the link from Lithium to here is more an act of hubris than an attempt to educate anyone. No one can find out why lithium is used in anodes of a battery from this article, because any discussions of freshman level analytical electrochemistry don't appear in this article. This really does need to be more than a bad explanation of some third year university biology, which seems to be the direction this article is headed. Dwmyers 15:38 Feb 9, 2003 (UTC)

168... 17:36 Feb 9, 2003 (UTC)
 * Pushed so far? It's just that it started from a biophysical perspective. There was no electrochemical potential article before I made one. I just happen to be much more familiar with it's use in membrane biophysics and I didn't want to stray far from what I knew about (see what happened with the H-bond reaction energy confusion). Granted, membrane biophysicists are a small minority of users of the term, but indeed it's near and dear to them and represents a use. So I think it's worth the paragraph, which is really all it gets here, devoted specifically to the general case of a biological membrane.
 * Looked at the article again. I don't know who did the rewrite, but the first two paragraphs read more cleanly and more openly point to electrochemical cell. It seems far more suitable than it did a few days ago. Dwmyers 14:29 Feb 13, 2003 (UTC)

But why?
A question:

Why is the electrochemical potential where it is? I understand that it a form of potential energy, but can someone break this down further? For example, why is water at 0.4V vs SHE? [Reaction: O2 + 2H2O + 4e ---> 4OH-]. I understand the driving forces involved between differences in electrochemical potential, but I'm having problems wrapping my head around the concept of why an electrochemical potential is where it is.

Any help would be appreciated.

---

I don't know what you mean by "SHE" and so I'm not positive I understand your question, but I guess the answer to it is that it's at least partly about how tightly each element clings to its most loosely held electron a.k.a. the ionization potential for that electron. 168... 00:56, 11 Aug 2003 (UTC)

--- Standard Hydrogen Electrode

Ok, that makes sense. My next question is a similar idea for a cell, notably a battery cell. Why is the point where the voltage of the battery (or cell) is zero, where it is?

To expand... when a battery is charged it starts with a 0V differential between the two electrodes, this differential then increases very rapidly (usually) and then is approximately flat. If we insert a reference electrode we can see that the 'zero point' is some value on the electrochemical potential scale. Why that value?

Two Questions
In the article it says "In a battery, an electrochemical potential arising from the movement of ions balances the reaction energy of the electrodes. The maximum voltage that a battery reaction can produce is sometimes called the standard electrochemical potential ..."

First, is standard electrode potential meant by standard electrochemical potential?

Second, is this article stating that the maximum voltage a wet cell can produce the difference between the two standard reduction potentials? If so, it is wrong. One can vary the voltage of a wet cell by changing the ratio of concentrations of the two solutions involved. Loggie 02:52, 29 Apr 2005 (UTC)

Conflicting terminologies
I have added "electronic and electrical engineering" to electrochemistry, as a domain in which the term "electrochemical potential" means "total thermodynamic chemical potential". I have also restricted the domain where the other convention applies to "metal physics", because (in my experience) the term "electrochemical potential" is not commonly used in semiconductor physics. (RGForbes (talk) 11:43, 10 April 2009 (UTC)) (Richard)


 * Sorry but you need sources for the changes. Also note that sources were given for the text as is. Using only one's own personal experience is a violation of WP:NOR. --Bob K31416 (talk) 12:44, 10 April 2009 (UTC)


 * Ashcroft and Mermin use the "other convention" in their discussion of the p-n junction. (page 593). They're not primarily semiconductor physicists, but still... --Steve (talk) 04:53, 12 April 2009 (UTC)