For the couple MnO4- + 8H+ + 5e- -> Mn^2+ + 4H2O at 298 K, with [MnO4-] and [Mn^2+] kept constant, the reduction potential changes per unit increase in pH by (take 2.303RT/F = 0.0591 V)

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For the couple $MnO X_{4} X^{-} + 8 H X^{+} + 5 e X^{-} Mn X^{2 +} + 4 H X_{2} O$ at 298 K, with $[MnO X_{4} X^{-}]$ and $[Mn X^{2 +}]$ kept constant, the reduction potential changes per unit increase in pH by (take $2.303 RT / F = 0.0591 V$ )

E = E^{\circ} + \frac{0.0591}{5} lo g \frac{[ MnO X _{4} X ^{-} ] [ H X ^{+} ] ^{8}}{[ Mn X ^{2 +} ]} = const - \frac{8 \times 0.0591}{5} pH

. The slope is

\frac{8}{5} \times 0.0591 = 0.0946 V

per pH unit. The ratio of protons to electrons (8/5) sets the pH dependence.