Activity can not be approximated with concentration in concentrated solutions, because an activity coefficient is very different from unity. Activities of OH-and H2O were calculated in NaOH solutions of 1M to 15M and at the temperature of 298.15K to 343.15K. The activity coefficients obtained experimentally by G. Åkerlöf and G. Kegeles were employed for the calculation. The potential-molarity equilibrium diagrams for Fe-NaOH solution system were established. These diagrams are more useful than the potential-pH ones for the purpose of thermodynamic consideration where the theory of dilute solutions can not be applied.
