We fit the experimental data in two steps. In the first step, we determine the intrinsic dissociation constant pK a,acid by matching the apparent pK app a,acid from the theory with the experimentally measured pK app a,acid for the lowest salt concentration. This is shown in Fig. , where the ionization degree ζ a is plotted versus pH sys . The blue solid line shows that the ionization predicted by the theory reaches 0.5 for pH sys = 4.7 for an intrinsic pK a,acid = 3.6 in the theory.