well for standard potentials of reactions the equation is
E = E* - (R*T/nF)ln(Q) where Q is the reaction constant of the concentrations of your products and reactants... so Q = (Concentration Products)/(concentration reactants) remember to raise the concentrations to any powers like,
3Fe(aq) would be (concentration of Fe)^3 because of the number moles.
*** Q doesnt include solids.
T is your temp in kelvin
n is number of electrons that are used in the oxidation/reduction half reactions
F is a constant.. (Faraday's i believe) = 96,500
that should about do it