Enthalpy of formation of OH- in aqueous solution is
∆Hf0 of H+aq is arbitrarily fixed as zero, Eq 1 +Eq 2 H2g+12O2g→H+aq+OH-aq, ∆H=-228.5 kJ ; Since ∆f H of H+aq=0, ∆f H of OH-aq=-228.5 kJ