Talk:Nernst equation

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This is not a particularly elegant layout for the equation .... can anyone improve on it? David Martland 09:47 Dec 5, 2002 (UTC)

My chemistry textbook, Zuhmdal's Chemistry (third edition) seems to disagree with this equation, listing instead of "[red]/[ox]" the reaction quotient Q, which is equal to [red]^x/[[ox]^y, where x and y are the coefficients of the reduced species and the oxidized species, respectively, in the balanced redox reaction.

Am I simply misanalyzing something?

<<Next comment>> I've been seing variations in the Nernst equation. Some using log functions and some using ln functions. Also I've seen one where everything gets multiplied by 2.303 and I don't know which if any of them is correct!!! <</End comment>>

2.303 ~ 1/log_10(e) . This comes from the change of base of the log from e to 10. Ash Lightfoot 04:23, 14 March 2006 (UTC)

I agree with the comment that the equation should have Q above instead of concentrations. The concentrations imply that there are no protons in the redox equation, and that only one species of each half reaction is an ion. This does not have to be true. The use of Q is much more general. Olin 13:49, 17 March 2006 (UTC)

I think the page is fine.

This is a complex derivation for even PhD's in Material Science, so I don't think it CAN be improved upon. Let it be. 137.99.76.228 (talk) 15:40, 15 February 2008 (UTC)


The equation for biological membranes is misleading in two ways: - We normally express the Nernst equation at 37 degrees C rather than room temperature - It's ONLY [out]/[in] when you are considering CATIONS. If you're considering the effect of anions like chloride, you use [in]/[out].

Will update when I have time to check out my sources

Confuseddave 09:27, 8 February 2006 (UTC).

I don't agree with this comment--Nerst equations are used in many more fields than biology, and 25 degrees C is generally the standard. This stays true for galvanic cell testing, and any general corrosion or oxidation/reduction computation. So yes, if the equation is being used for a biological study, 25 degrees C may not be the correct temperature--but that's why there's a term for temperature (T). 137.99.76.228 (talk) 15:37, 15 February 2008 (UTC)

Contents

[edit] Activities not Concentrations

The Nernst Eq'n is first given with activities and then concentrations. The two are not equivalent, except at very high dilution. I believe the formulation with activities is correct, although the Nernst equation is sometimes given with a formal electrode potential and concentrations rather than a standard electrode potential and activities. However, if concentrations are used the conditions under which the formal potential was measured must be stated (supporting electrolyte, concentration of ions, etc.) Can someone sort this out (with appropriate references)? Ahw001 13:27, 23 March 2006 (UTC)

For more details see external link Ahw001 07:55, 24 March 2006 (UTC)

[edit] Formal Electrode Potential?

Anyone have any idea what the formal electrode potential is? I can't find any mention of it anywhere else, and its not present in any of my text books. Olix 20:20, 13 September 2006 (UTC)

Try "Electrochemical Methods" by A.J. Bard and L.R. Faulkner or "A First Course in Electrode Processes" by D. Pletcher Ahw001 06:48, 28 September 2006 (UTC)

[edit] Too complex

"For simplicity, we will consider a solution of redox-active molecules that undergo a one electron reaction"

This is not simple unless you have just taken a chemistry course. Links, etc needed.

[edit] More on Q versus [red]/[ox]

This was alluded to by Olin and Confuseddave above, but I think there remains unresolved issues with the first equation in the article, which would be fixed by replacing \frac{a_{\mbox{Red}}}{a_{\mbox{Ox}}} by Q, the reaction quotient.

According to the (intro) chem that I know, this equation is a valid way to write the Nernst equation:

E = E^0 - \frac{RT}{zF} \ln Q

But I'm less sure about

E = E^0 - \frac{RT}{zF} \ln\frac{a_{\mbox{Red}}}{a_{\mbox{Ox}}}

For example, take the reaction

CATHODE: solid-A + electrons --> anion-A
ANODE: anion-B --> solid-B + electrons

Then the reduced species is [red]=[anion-A], and the oxidized species is [ox]=[anion-B], and clearly Q=[red]/[ox]. So no problem here; this case works.

But now suppose instead we have:

CATHODE: Cation-A + electrons --> solid-A
ANODE: solid-B --> Cation-B + electrons

then the oxidized species is [ox]=[cation-B] and the reduced species is [red]=[cation-A]. Then Q = [cation-B]/[cation-A], but [red]/[ox]=(1/Q)!

Therefore, I propose switching the equation in the intro of the article to the version with Q, and taking out the version with [red]/[ox] altogether. Unless I'm confused. Thanks! --Steve (talk) 23:56, 4 May 2008 (UTC)

UPDATE: Oh I see, this is the Nernst equation for how a reduction potential depends on concentration. In that case, I'd say the first sentence is wrong when it says "cell (or half-cell)", rather than just half-cell, and ditto for the definition of z. It would be an improvement, I think, to start with the Q version, and have the reduction-potential version as a special case. But at the very least, it could be made much clearer that this is the equation for reduction potential, not for the E of a whole cell or anything else. Anyway, would anyone object to a Q-centric opening? --Steve (talk) 01:27, 5 May 2008 (UTC)

UPDATE 2: Having heard no objections in the last week, I've edited the intro accordingly. Comments? --Steve (talk) 04:09, 13 May 2008 (UTC)