Studying liquid water in a frame of band theory shows that varying a reduction-oxidation (RedOx) potential of aqueous solution can be identified as shifting Fermi level in its band gap. This medium becomes the reductive one when Fermi level is shifting to the conduction band due to populating hydroxonium level ( ) + 3 3 H O H O by electrons and transforming water in a hypo-stoichiometric state, 2 1 H O − x . Opposite in the hyper-stoichiometric one ( ) 1 2 + -OH OH by holes and the aqueous

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... ion becomes the oxidative one. The energy difference between these electronic levels is estimated of 1.75 eV. It is shown that the standard half-reactions and the typical aqueous electrodes fix their RedOx potential only by the electrons and holes populations [ ] [ ] ( ) 3 H O , OH of these local electronic levels in the band gap of non-stoichiometric water in the corresponding solutions.