Given standard electrode potentials :

${\mathrm{Fe}}^{3+}+3{\mathrm{e}}^{-}\stackrel{ }{\to }\mathrm{Fe};\mathrm{E}º=-0.036 \mathrm{volt}$

${\mathrm{Fe}}^{2+}+2{\mathrm{e}}^{-}\stackrel{ }{\to }\mathrm{Fe};\mathrm{E}º=-0.440 \mathrm{volt}$

The standard electrode potential ${\mathrm{E}}^0$ for ${\mathrm{Fe}}^{3+}+{\mathrm{e}}^{-}\stackrel{ }{\to }{\mathrm{Fe}}^{2+}$