Thermodynamics for Proton Binding of Pyridine in Different Ionic Media at Different Temperatures

The acid-base behavior of pyridine (py) was studied at different temperatures and ionic strengths in LiCl, NaCl, KCl, RbCl, CsCl, NaNO 3, (CH3)4NCl, (C2H5) 4NI, MgCl2, and CaCl2 aqueous solutions. The study was developed by means of potentiometric titrations with a ISE-H + electrode and by the reanalysis of literature data. It was found that the protonation process of pyridine is enthalpic-driven and the values of the protonation enthalpy are in agreement with literature findings. Different models were used to fit the experimental data, namely, the Debye-Hückel equation, the specific ion interaction theory (SIT), and the Pitzer equations. The variation of the apparent protonation constant in the different ionic media was also interpreted in terms of formation of weak complexes between pyridine and the ions of the supporting electrolyte. The formation of (CH 3)4Npy+, (C2H5) 4Npy+, Mgpy2+, Capy2+, and HpyCl species was found. The modeling ability were comparable for all considered models, although the Pitzer equations can be regarded as the most reliable. SIT and Pitzer coefficients were provided for the interaction between the Hpy + and the Cl-, and the activity coefficient of the neutral species has been reported. A good agreement was found between experimental data obtained in this work and literature findings.