Things You'll Need:
- Scientific calculator
- Pencil and paper
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Step 1
Define the components of the general Nernst equation. E is the half-cell reduction potential, Eo is the standard half-cell reduction potential, z is the number of electrons transferred, aRed is the reduced chemical activity for the chemical in the cell and aOx is the oxidized chemical activity. Furthermore, we have R as the universal gas constant of 8.314 Joules/Kelvin moles, T as the temperature in Kelvin and F as the Faraday constant of 96,485 coulombs/mole.
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Step 2
Calculate the general form of the Nernst equation. The form E = Eo - (RT/zF) Ln (aRed/aOx) provides the half-cell reduction potential.
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Step 3
Simplify the Nernst equation for standard laboratory conditions. For E = Eo - (RT/zF) Ln (aRed/aOx), we can treat RT/F as a constant where F = 298 degrees Kelvin (25 degrees Celsius). RT/F = (8.314 x 298) / 96,485 = 0.0256 Volts (V). Thus, E = Eo - (0.0256 V/z) Ln (aRed/aOx) at 25 degrees C.
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Step 4
Convert the Nernst equation to use a base 10 logarithm instead of the natural logarithm for greater convenience. From the law of logarithms, we have E = Eo - (0.025693 V/z) Ln (aRed/aOx) = Eo - (0.025693 V/z) (Ln 10) log10 (aRed/aOx) = Eo - (0.05916 V/z) log10 (aRed/aOx).
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Step 5
Use the Nernst equation E = RT/zF ln (Co/Ci) in physiological applications where Co is the concentration of an ion outside a cell and Ci is the concentration of the ion inside the cell. This equation provides the voltage of an ion with charge z across a cell membrane.
