$\mathrm{KCl}$ is used in a salt bridge because:

${\mathrm{Fe}}^{2+}+2\mathrm{e}\to \mathrm{Fe}...\left(\mathrm{i}\right)$

${\mathrm{Fe}}^{3+}+\mathrm{e}\to {\mathrm{Fe}}^{2+}...\left(\mathrm{ii}\right)$

The standard potentials (in volt) corresponding to the reactions $\left(\mathrm{i}\right)$ and $\left(\mathrm{ii}\right)$ are ${\mathrm{E}}_1$ and ${\mathrm{E}}_2$ respectively. The value (in volt) of the standard potential corresponding to the reaction ${\mathrm{Fe}}^{3+}+3\mathrm{e}\to \mathrm{Fe}$ is: