This section deals with solutions of two weak acids with their acidity constant noted pK a1 and pK a2 and concentrations C 1 and C 2 . We will note the average pK a , pK a = (pK a1 + pK a2 )/2. Such mixtures have already been studied at great length. To quote two particularly relevant contributions, one should note that Ricci has studied the presence of a minimum of β in the pH interval between pK a1 and pK a2 , and provided conditions for its existence when neglecting the contribution of H 3 O + and OH -(see pp. 185-191 in ). ) has shown that this minimum disappears when pK a2 -pK a1 ≤ 1.14 for equimolar mixtures, also when neglecting the contributions of H 3 O + and OH -.