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NEET levelSingle correct optionDifficulty 1 of 3ec110

The reduction potential of which of the following couples is independent of pH?

A $Cr X_{2} O X_{7} X^{2 -} / Cr X^{3 +}$
B $MnO X_{4} X^{-} / Mn X^{2 +}$
C $Fe X^{3 +} / Fe X^{2 +}$
D $O X_{2} / H X_{2} O$

Answer: C

Only couples whose half reaction involves

H X^{+}

or

OH X^{-}

have pH-dependent potentials.

Fe X^{3 +} + e X^{-} Fe X^{2 +}

involves no protons, so its Nernst equation contains no

[H X^{+}]

term. The other three half reactions all consume

H X^{+}

and their potentials fall as pH rises.

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