[JackBlack]

Q. Calculate the value of E^o' for the reaction NAD⁺ + H⁺ +2e^- NADH  at PH7 ,given that the E^o= -0.320V at 298K

I am not too sure how i would answer this question. Do i need to use the value of E^o' for the O₂/H₂ half cell reaction: O₂ + 4H⁺ +4e^- 2H₂O which is + 0.816V ?And would i need to use the E^o _cell= E^o_right- E^o_left equation ?


Help please

Thanks!

[Borek]

No, you are just asked about the formal potential of the NAD⁺/NADH half cell. Simple plug and chug with the Nernst equation.

[JackBlack]

No, you are just asked about the formal potential of the NAD⁺/NADH half cell. Simple plug and chug with the Nernst equation.

The nernst equation requires some concentrations, but i am not given that in the question  
Do i assume that the NAD+ and NADH are 1 moldm-3?

[Borek]

Yes, that's what I would do in this case.

[JackBlack]

Yes, that's what I would do in this case.

thank u !!

[Babcock_Hall]

The use of E°' means that we are dealing with the biochemical standard state.  The only difference between this standard state and the ordinary standard state is the assumption of pH 7, where [H+] = 1 x 10^-7 M.  A commonly used Biochemstry textbook (Nelson and Cox, page 515) gives E°' for NAD being reduced to NADH as -0.320 volts, and that is the value I have always seen.  E° for NAD (the value at pH 0) would be a very different number, because the half-cell reaction involves a proton.

To be honest, the question does not make sense to me.  I only see a half cell, not a full cell.  If I were to assume another half-cell to make a full cell, I might be tempted to choose the hydrogen ion:
2H^+ + 2e^-  ===> H2.  The E°' value for this is -0.414 volts (not 0.00 volts).  The only other thing I can think of is that the question is asking the student to convert E° to E°'.  However, the value given in the problem is not E°, if I understand things correctly.

[Borek]

To be honest, the question does not make sense to me.  I only see a half cell, not a full cell.

That's not a problem - tables of standard potentials contain only half cell potentials, measured against 0 - and 0 is by definition standard hydrogen electrode (not that anyone uses it in lab practice).

The only other thing I can think of is that the question is asking the student to convert E° to E°'.

That was my understanding.

However, the value given in the problem is not E°, if I understand things correctly.

And that can be a problem. Perhaps whoever designed the question was not aware of the fact something like "biochemical standard state" exists. I wasn't, for me the standard state means pH=0, period. I am not stating biochemical approach is wrong, I can understand why it is convenient to use pH 7 as a reference, it is just confusing when someone uses term will well know definition, but in different meaning.

[Babcock_Hall]

I had a look in the second edition of Metzler's book, Biochemistry, starting around page 293, and it seems as if the situation is in a bit of flux in the biochemical literature regarding the best choice for the standard state.  The biochemical standard state was something I learned about in graduate school (which was a long time ago).  It is my understanding that it is identical to the regular standard state, except for the hydronium ion concentration.

Metzler discusses electrochemistry starting on page 300.  If I take E°' to be -.0320 volts for NAD/NADH, I can calculate that E° is -0.113 volts for NAD/NADH (the difference being related to the difference in hydronium ion concentration.  Both of these values appear in his table on page 301.

Putting in another way, I would have formulated the question as follows:  Given a value of E° of -0.113 volts, calculate E°' for the NAD/NADH half cell.  I know just enough about this subject to be dangerous, so please let me know if anything was unclear.