Potentiometric Determination of Some Solution Thermodynamic Parameters of Three Hydroxypyrone Derivates

In this paper, a potentiometric study on the thermodynamic acid base properties and solubility of three hydroxypyrones (successively indicated as maltols), namely maltol (Hma), ethylmaltol (Hema) and kojic acid (H2ka), is reported. The acid base properties of these O-donor ligands have been investigated in NaCl (0.10 ≥ I / mol L-1 ≥ 5.00), KCl (0.15 ≤ I / mol L-1 ≤ 2.90) and (C2H5)4NI (0.15 ≤ I / mol L-1 ≤ 1.10) aqueous solutions in the temperature range 283.15 ≤ T / K ≤ 318.15. According to the nature of the side chain of maltols, log KH (Hema) > log KH (Hma) >> log KH (H2ka). The values at I = 0 mol L-1 and T = 298.15 K are log KH = 8.764±0.002, 8.686±0.002 and 7.933±0.005 for Hema, Hma and H2ka, respectively. The dependence on ionic strength of protonation constants was interpreted with the Debye-Hückel and the Specific ion Interaction Theory (SIT) equations, in this last case specific interaction coefficients were also provided. In general, protonation constants decrease with increasing temperature and ionic strength until I = 1.5 mol L-1. The differences between the protonation constants of the maltols in the three ionic media were interpreted in terms of activity coefficients variation and weak complexes formation (with Na+ and K+ cations). Tetraethylammonium iodide has been considered as a baseline electrolyte. In NaCl and KCl media, the NaL and the KL (L = ligand) species were found and the interaction of the three ligands with sodium is stronger than that with potassium, as an example, for maltol: = -0.12 ± 0.05 and 0.001 ± 0.030 at T = 298.15 K and I = 0 mol L-1. The interaction between maltols and the cation of the supporting electrolyte lowers the apparent protonation constants and the trend is: log KH ((C2H5)4NI) > log KH (KCl) > log KH (NaCl). Enthalpy changes for the protonation of these ligands have been also determined and resulted negative in all cases ( = -15.0±0.1, -14.2±0.2 and -12.1±0.3 kJ mol-1 for Hema, Hma and H2ka, respectively at T = 298.15 K and I = 0 mol L-1). Between the three ligands the trend of the values is opposite to that observed for the protonation constants, (Hema) < (Hma) << (H2ka). The major contribution in the protonation reaction is entropic in nature. Solubility and distribution measurements at T = 298.15 K in NaCl, allowed us to determine the salting coefficient (Setschenow parameter) of the neutral species and then to calculate the activity coefficients of the HL species of the three hydroxypyrone derivatives. The values of Setschenow parameters are km = 0.10±0.01, 0.13±0.01 and 0.04±0.01 for maltol, ethylmaltol and kojic acid, respectively, therefore salting out effect was evidenced for the three maltols.