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Homework Help: Redox potential (Using the Nernst Equation)

  1. Nov 5, 2008 #1
    The redox midpoint potential (Eo’) of NAD+/NADH at pH 7.0 is -320 mV. If the total concentration of {[NAD+] + [NADH]} is 1.5 mM in cells, and the cellular redox potential is about -200 mV, what are the concentrations of [NAD+] and [NADH], respectively? Under certain oxidative stress conditions, the cellular redox potential is increased to -100 mV. What will be the concentrations of [NAD+] and [NADH] in this cell under the oxidative stress conditions? (Assume pH 7.0 is not changed under oxidative stress conditions).

    Eh = Eo' - (RT/nF)ln(reduced/oxidized) but what does he mean by "oxidized," the molecule being oxidized (NADH) or what has been oxidized (NAD+)?

    When I do this problem I keep getting an extremely small numbers that do me no good in the end; what am I doing wrong here? I'll just show the first part of the question (Eh = -200mV).

    Eh = Eo' - (RT/nF)ln(reduced/oxidized)
    Eh = -200E-3V, Eo' = -320E-3V
    -200E-3V = -320E-3V - [(1.987E-3)(298)/(2)(23.1)]ln([red]/[ox])
    120E-3V = (-0.01174)ln([red]/[ox])
    -10.2 = ln([red]/[ox])
    e^(-10.2) = [red]/[ox]
    6.30957344E-11[ox] = [red]

    [ox] + [red] = 1.5E-3M (total)
    6.30957344E-11[ox] +[ox] = [red]
    From here, because the coefficient for [ox] is so small I get [ox] = [red].

    Any help is greatly appreciated!
  2. jcsd
  3. Nov 5, 2008 #2


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    "reduced/oxidized" means the concentration of the reduced form of the ion or atom or specie divided by the oxidized form of the ion or atom or specie.
  4. Nov 6, 2008 #3


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    What units are you using for R and F?
  5. Nov 6, 2008 #4
    R is 1.987E -3 kcal/mol/degree; F is 23.1 kcal/V/mol.
  6. Nov 7, 2008 #5


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    Staff: Mentor

    RT/nF should give about 60 mV/n, it doesn't.
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