Solubility and Acid-Base properties and Activity Coefficients of Chitosan in Different Ionic Media and at different ionic strengths, at T = 25 °C.

Studies on the acid-base properties and solubility of a polyammonium polyelectrolyte (chitosan) withdifferent molecular weights (MW 310 and 50 kDa), were performed at T=25 °C, in the pH range 2.5–7. Theprotonation of chitosan was investigated by potentiometry ([H+]-glass electrode) in NaCl, NaNO3 and mixedNaNO3+Na2SO4 ionic media, at different ionic strengths. Protonation constants were calculated as a functionof dissociation degree α by means of two different models, namely, a simple linear model and the modifiedHenderson–Hasselbalch equation. Experimental data were also fitted using a model independent of α(Diprotic-like model), according to which the acid-base properties can be simply described by twoprotonation constants in all the acidic pH range. The dependence on ionic strength of protonation constantsin NaCl aqueous solution was modelled by Specific ion Interaction Theory (SIT). The ion pair formationbetween protonated chitosan and Cl−, NO3− and SO4 2− was also considered, and the relative formationconstants are reported.Solubility investigations were performed in NaCl aqueous solutions in a wide range of ionic strength (0.1bI/mol L−1b3.0), with the aim to determine the activity coefficients of neutral species and the Setschenowcoefficient of chitosan 310 kDa.