Electrode potential for Mg electrode varies according to the equation
 ${\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}={\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}^{\mathrm{\theta }}-\frac{0.059}{2}\log \frac{1}{\left[{\mathrm{Mg}}^{2+}\right]}$.The graph of ${\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}{\mathrm{v}}_{\mathrm{S}}\log \left[{\mathrm{Mg}}^{2+}\right]$ is

Electrode potential for Mg electrode varies according to the equation,
$\begin{array}{r}{\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}={\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}^{\circ }-\frac{0.059}{2}\log \frac{1}{\left[{\mathrm{Mg}}^{2+}\right]}\\ {\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}={\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}^{\circ }+\frac{0.059}{2}\log \left[{\mathrm{Mg}}^{2+}\right]\\ {\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}=\frac{0.059}{2}\log \left[{\mathrm{Mg}}^{2+}\right]+{\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}^{\circ }\end{array}$
This equation represents equation of straight line. It can be correlated as
$\begin{array}{cccc}{\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}=& \left(\frac{0.059}{2}\right)& \log \left[{\mathrm{Mg}}^{2+}\right]+{\mathrm{E}}_{{\mathrm{Mg}}^{2+}/\mathrm{Mg}}^{\circ }& \\ \uparrow & \uparrow & \uparrow & \uparrow \\ \mathrm{Y}& \mathrm{m}& \mathrm{x}& +\mathrm{C}\end{array}$
So, intercept $\left(\mathrm{C}\right)={{\mathrm{E}}_{{\mathrm{Mg}}^{2+}/}}_{\mathrm{Mg}}$