Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

The acid base properties of 1,2-dimethyl-3-hydroxypyridin-4-one (also known as deferiprone, def, figure 1), together with the solubility and the distribution ratio have been studied potentiometrically at different temperatures and ionic strengths in NaCl, KCl and in (CH3)4NCl aqueous solutions. The total solubility of deferiprone is fairly high (0.100 mol dm 3 in pure water) and decreases with increasing salt concentration (salting out effect); this behaviour is greater in NaCl than in (CH3)4NCl aqueous solutions. From the analysis of the solubility and the distribution measurements it was possible to determine the Setschenow and the activity coefficients of the neutral species. Deferiprone shows two protonation steps, whose protonation constants are log KH1 1/4 10:088 and log KH2 1/4 3:656 at infinite dilution and T = 298.15 K. The ionic strength dependence of the protonation constants was interpreted both in terms of variation of the activity coefficients, using the Debye Huckel, the SIT (Specific ion Interaction Theory) and the Pitzer approaches, or considering the formation of weak species with the ions of the supporting electrolyte (e.g. Na+, K+ and Cl ). Moreover, temperature gradients were provided for the two protonation constants. The stepwise protonation enthalpy values are negative in all cases (e.g. DH1 = 19.2 kJ mol 1 and DH2 = 13.8 kJ mol 1 at infinite dilution and T = 298.15 K) and become more negative increasing both temperature and ionic strength. It was observed that the proton binding process is mainly entropic in nature for the first protonation step and enthalpic for the second. The results are in good agreement with literature data.