To find the standard potential of ${\mathrm{M}}^{3+}/\mathrm{M}$ electrode, the following cell is constituted: $\mathrm{Pt}/\mathrm{M}/{\mathrm{M}}^{3+}\mathrm{ }\left(0.001\mathrm{ }\mathrm{mol}\mathrm{ }{\mathrm{L}}^{-1}\right)/{\mathrm{Ag}}^{+}\mathrm{ }\left(0.01\mathrm{ }\mathrm{mol}\mathrm{ }{\mathrm{L}}^{-1}\right)/\mathrm{Ag}$

The emf of the cell is found to be $0.421 \mathrm{volt}$ at $298 \mathrm{K}$. The standard potential of half-reaction ${\mathrm{M}}^{3+}+3{\mathrm{e}}^{-}\to \mathrm{M}$ at $298 \mathrm{K}$ will be:

(Given: ${\mathrm{E}}_{\raisebox{1ex}{${\mathrm{Ag}}^{+}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{Ag}$}\right.}^{⊝}$ at $298 \mathrm{K}=0.80 \mathrm{volt}$)