The third constant cannot be chosen independently because its value is determined by the electroneutrality constraint (see Sec. S1.2). The exact salt concentration and pH in the supernatant are determined from an auxiliary simulation using the chosen values of equilibrium constants. Therefore, the reported values of salt concentration usually are not round numbers, as expounded in Ref. When considering a weak diprotic acid as an additional solute, we add acid-base dissociation reactions of the solute to the above set of reactions: