We now assess the predictions of the equilibrium size that are generated with the linear solution as compared to the nonlinear case. In Fig. , we show the swelling for two different microgel suspensions containing microgels with weak acid sites (pK a,acid = 4.0) and N a = 20 and 50. The reservoir salt concentration is set to c res salt = 10 mM. Both, reservoir salt concentration and microgel valence, have been chosen to represent typical values that can be found in standard experiments. In Fig. , the degree of ionization of a microgel, ζ a , is shown as a function of pH res for the linear and the nonlinear solution. Furthermore, the ideal prediction from the Henderson-Hasselbalch equation, Eq. (10), is also shown. In the ideal case, the degree of ionization reaches a value of 50% for pH res = pK a,acid = 4.0. Contrasting with this, in the case of the microgels, we observe a shift in the ionization curve towards larger pH res values due to an extra penalty in the ionization that arises as a consequence of the electrostatic interactions. In our theory, this phenomenon is represented in a mean-field manner, disregarding fluctuation effects. Thus, we observe shifts in both the linear and the nonlinear case, with a more pronounced shift for the higher number of acid groups, due to the stronger electrostatic interactions. However, the linear approximation underestimates the shift as compared to the nonlinear case by about 0.5 units of pH.