Next, we have shown that the ionization of a weak diprotic acid is significantly enhanced in the PEC phase, as compared to the supernatant. Therefore, the partition-ing of such a weak acid between the two phases can be tuned by varying the pH. At low pH, the weak diprotic acid is uncharged, therefore, steric repulsion dominates its interactions, causing that it prefers the supernatant phase. As the pH is increased, the preference for the PEC phase increases concomitantly. Divalent form of the diprotic acid starts to dominate in the PEC phase already at pH values when only its neutral and signle-ionized forms would be found in the supernatant. Consequently, the diprotic acid significantly accumulates in the PEC phase already at much lower pH values than could be expected from its ionization in the aqueous solution of the supernatant. This observation demonstrates that the partitioning of weak multiprotic acids can be easily tuned by varying the pH, providing interesting opportunities for engineering applications, such as separation of small acids based on their pK A, .