electrolysis of brine

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in the electrolysis of brine, why is not sodium ion discharge but hydrogen ion
discharge?chlorine ion dischrged perferentially than that of hydroxide ion. but they
are same order diffierence in the E.C.S? (Cl & OH vs H & Na)

To determine the product produced, there are three factors.
1. the nature of electrode
2. the position in the electrochemical series.
3. the concentration of solution.

You should correct the idea that they are not at the same position in the E.C.S
If only consider the factor 2.
H+ ion is preferentially discharged than Na+ ion
and OH- ion is preferentialy discharged than cl- ion.

Actually, in most of the cases, H+ ions should be preferentially discharged at the cathode, except the cathode is made by Mercury.

At the anode, because in brine, the concentration of Cl- ions is high, so it preferentially discharged. But, after a while, the concentration decrease, then OH- ion is preferentially discharged.