The magnitude of the change in oxidising power of the $\mathrm{MnO}_4^{-} / \mathrm{Mn}^{2+}$ couple is $\mathrm{x} \times 10^{-4} \mathrm{~V}$, if the $\mathrm{H}^{+}$ concentration is decreased from 1 M to $10^{-4} \mathrm{M}$ at $25^{\circ} \mathrm{C}$. (Assume concentration of $\mathrm{MnO}_4^{-}$and $\mathrm{Mn}^{2+}$ to be same on change in $\mathrm{H}^{+}$concentration). The value of $x$ is $\_\_\_\_$ .