How would you determine the standard electrode potential

The standard electrode potential of Mg2+ | Mg can be measured with respect to the standard hydrogen electrode, represented by Pt(s), H2(g) (1 atm) | H+(aq)(1 M).

A cell, consisting of Mg | MgSO4 (aq 1 M) as the anode and the standard hydrogen electrode as the cathode, is set up.

$\mathrm{Mg}\left|\mathrm{Mg}^{2+}(\mathrm{aq}, 1 \mathrm{M}) \| \mathrm{H}^{+}(\mathrm{aq}, 1 \mathrm{M})\right| \mathrm{H}_{2}(\mathrm{~g}, 1$ bar $), \mathrm{Pt}_{(s)}$

Then, the emf of the cell is measured and this measured emf is the standard electrode potential of the magnesium electrode.

$E^{\ominus}=E_{R}^{\ominus}-E_{L}^{\ominus}$

Here, $E_{R}^{\ominus}$ for the standard hydrogen electrode is zero.

$\therefore E^{\ominus}=0-E_{L}^{\ominus}$

$=-E_{L}^{\ominus}$