Finally, we compare the theoretical model against experimental measurements of the microgel equilibrium size as a function of pH and salt concentration, for microgels containing weak acidic sites. The suspensions of interest consist of micrometer-sized monodispersed poly(methacrylic acid) (PMAA) microgels synthesized by precipitation polymerization with methylenebisacrylamide as the crosslinker. Via bulk titration, the apparent pK app a,acid of the microgels was determined to be 4.7 in water. Notice that the determined apparent pK app a,acid of the microgels is comparable to the individual MAA monomer, pK a,acid = 4.65 in water at 25°C. Below, we use the experimentally measured pK app a,acid to determine the intrinsic monomer pK a,acid as the input for the theory. In the experiments, the suspensions contained 0.1 mM citrate buffers with varying pH from 2.5 to 6.6 and with 0.001, 0. The swelling ratio is calculated from the microgel radius of gyration, R g , relative to the microgel size at c res salt = 0.1mM: α = R g /R g (c res salt = 0.1)mM. The fitting parameters in the theoretical model are Z max,a = 700, 900, 1000 and a 0 = 12σ, 18σ, 22σ for N = 20, 30, 40, respectively. Dotted lines corresponds to fitting with parameters form Fig. : Z max,a = 700, 900, 1000 and a 0 = 6.5σ, 6.5σ, 6σ for N = 20, 30, 40, respectively. Here, we assume pH res = 6 for pK a,acid = 4. Other system parameters as in Fig. . added NaCl. On the basis of the monomer molecular weight and the microgel mass, the average number of monomers per microgel was determined to be ≈ 1.9 × 10 11 . All mea-surements were conducted for 0.1mg/mL suspensions of microgels. Considering the reported dry microgel mass and dry microgel volume, we estimate a dry microgel concentration of ϕ dry ≈ 10 -4 .