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 different ionic strengths and T = 298.15 K. Experimental data obtained in previous studies of different synthetic (polyacrylates, polymethacrylates, polyacrylate-co-maleate) and naturally occurring (alginate, humic substances) polycarboxylates were also taken into account in the general analysis of acid - base properties of polyelectrolytes. Protonation constants were expressed as a function of the dissociation degree (α) using three models, namely, a simple linear model, the Hogfeldt three-parameter equation, and the modified Henderson - Hasselbalch equation. Moreover, a model, independent of α, according to which acid - base properties 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 formation data as for simple low molecular weight ligands. In general, all the models taken into consideration give satisfactory results with fitting errors in the order Högfeldt three-parameter equation < Diprotic-like model < modified Henderson - Hasselbalch equation < simple linear model. For each type of polyelectrolyte investigated, a detailed discussion is reported. The parameters involved in the different models are strictly correlated, and canonical correlation analysis is reported. A statistical analysis on the protonation constants of all the polyelectrolytes investigated was made confirming that very similar results can be obtained using any considered model and, as a consequence, that the Diprotic-like model is a valid alternative simple approach to study acid - base properties of polyelectrolytes.