Acid-base properties of synthetic and natural polyelectrolytes: experimental results and models for the dependence on different aqueous media

Protonation constants of several natural and synthetic humates and fulvates were determined by ISE-H+potentiometry in different ionic media (alkali metal halides and tetraethylammonium iodide) at differentionic strengths and T ) 298.15 K. Experimental data obtained in previous studies of different synthetic(polyacrylates, polymethacrylates, polyacrylate-co-maleate) and naturally occurring (alginate, humicsubstances) polycarboxylates were also taken into account in the general analysis of acid-base propertiesof polyelectrolytes. Protonation constants were expressed as a function of the dissociation degree (R) usingthree models, namely, a simple linear model, the Ho¨gfeldt three-parameter equation, and the modifiedHenderson-Hasselbalch equation. Moreover, a model, independent of R, according to which acid-baseproperties of polyelectrolytes in the whole acidic pH range can be described by two protonation constants(Diprotic-like model), was also tested. This model allows us to analyze protonation and complex formationdata as for simple low molecular weight ligands. In general, all the models taken into consideration givesatisfactory results with fitting errors in the order Ho¨gfeldt three-parameter equation < Diprotic-like model< modified Henderson-Hasselbalch equation < simple linear model. For each type of polyelectrolyteinvestigated, a detailed discussion is reported. The parameters involved in the different models are strictlycorrelated, and canonical correlation analysis is reported. A statistical analysis on the protonation constantsof all the polyelectrolytes investigated was made confirming that very similar results can be obtained usingany considered model and, as a consequence, that the Diprotic-like model is a valid alternative simpleapproach to study acid-base properties of polyelectrolytes.