In the present section, we will first study the equimolar case without taking H 3 O + and OH -into consideration in the relationship for β, and find the same result as Rilbe. Then, we study the mixtures of two weak acids of different concentrations and establish for which value of ∆pK a = pK a2 -pK a1 the minimum exists or not and, finally, we study the effect of the contribution of H 3 O + and OH -and show how to determine the optimal values of concentrations and pK a 's in order to obtain a flat buffer index curve. All this will be done using numerical methods, as they can be extended to more complicated mixtures and to the inclusion of H 3 O + and OH -, which is already difficult analytically for a mixture of two weak monovalent species.