Thermodynamics (Solubility and Protonation Constants) of Risedronic Acid in Different Media and Temperatures (283.15–318.15 K)

Potentiometric measurements were carried out to study the protonation constants of risedronic acid (RA) in NaCl(aq), (CH3)(4)NCl(aq) and (C2H5)(4)NI(aq) at different ionic strengths and temperatures (283.15 ae T/K ae 318.15). In the same conditions, solubility measurements were also performed. Calorimetric measurements were done in NaCl to determine the protonation enthalpy values at I = 0.15 mol center dot dm(-3) and 298.15 K. Generally, the proton binding process was endothermic and the driving force was entropic in nature. The values of the protonation constants determined in NaCl(aq) are lower than those obtained in the two tetraalkylammonium salts. The medium effect was interpreted using different thermodynamic models in terms of variation of the activity coefficients with ionic strength (Debye-Huckel type and SIT), or formation of weak complexes between risedronate (Ris(4-)) and the ions of the supporting electrolytes. Specific interaction coefficients (epsilon) and the stability of five (CH3)(4)N+/Ris(4-) (at different temperatures and ionic strengths) species are reported. The total solubility of risedronic acid is higher in NaCl(aq) than in the other two ionic media and, in all cases, increases with increasing temperature. Setschenow and activity coefficients of the neutral species were also computed in all ionic media.