Thermodynamic Behavior of Polyalcohols and Speciation Studies in the Presence of Divalent Metal Cations

The acid-base properties and complexing ability of some biologically relevant polyalcohols (erythritol, sorbitol, maltol, and ethylmaltol) toward bivalent metal cations have been determined by potentiometric measurements with an ISE-H+ glass electrode in NaNO3 aqueous solutions at I = 0.151 mol·kg-1 and T = 298.15 K. For the erythritol system, the investigations have been carried out in the ionic strength range 0.1 ≤ I/mol·kg-1 ≤ 1.0, and the dependence on the ionic strength of the protonation and stability constants has been modeled by the specific ion interaction theory (SIT). The sequestering ability of the different ligands toward the considered metal cations (Ca2+, Zn2+, and Sn2+) has been evaluated by the pL0.5 parameter. For example, for M2+/maltol systems, at T = 298.15 K in NaNO3(aq), I = 0.151 mol·kg-1, and pH 7.4, the sequestering ability toward Zn2+ (pL0.5 = 4.60) is higher than that toward Sn2+ (pL0.5 = 3.94) and Ca2+ (pL0.5 = 1.27).