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For convenience of calculations, biochemists define a standard state different from that used in chemistry and physics: in the biochemical standard state, [H + ] is 10ˉ 7 M (pH 7) and [H 2 O] is ____ M. For reactions that involve Mg 2+ (which include most of those with ATP as a reactant), [Mg 2+ ] in solution is commonly taken to be constant at 1 mM.