Some Thermodynamic Properties of Aqueous 2-Mercaptopyridine-N-Oxide (Pyrithione) Solutions

Abstract

The thermodynamic properties of 2-mercaptopyridine-N-oxide (pyrithione, PT) were studied potentiometrically in NaCl aqueous solutions at different ionic strengths and temperatures. A set of protonation constants is provided, together with distribution (water/2-methyl-1-propanol) and solubility data. The total and the specific solubility (solubility of neutral species) values of pyrithione were determined and, for example, are 0.0561 and 0.0518 mol·dm−3 at cNaCl = 0.244 mol·dm−3 and T = 298.15 K. By fitting the distribution and solubility results against the ionic strength, the Setschenow coefficient and the activity coefficients of the neutral species were determined. In pure water, the specific solubility is log10\( S_{m 0}^{0} = \, {-} 1. 20 \, \pm \, 0.0 4 \). To determine the activity coefficient of the charged species and the protonation constant at infinite dilution, the data were analyzed by different models, namely the Debye–Huckel type equation, the SIT (Specific ion Interaction Theory) and the Pitzer approach. The interaction coefficient of the deprotonated pyrithione species was determined [e(Na+, PT−) = −0.105 ± 0.002]. The protonation enthalpy was also determined, is slightly positive, and the protonation process is entropic in nature. At infinite dilution and T = 298.15 K, log10KH0 = 4.620 ± 0.002, ΔG0 = –26.4 ± 0.1 kJ·mol−1, ΔH0 = 2.1 ± 0.5 kJ·mol−1 and TΔS0 = 28.5 ± 0.5 kJ·mol−1. The electrochemical behavior of pyrithione was studied in NaCl solutions at T = 298.15 K. It was found that voltammetry can be used to study the binding ability of pyrithione towards metal cations. The results of this work are in agreement with literature findings and improve the knowledge of the chemistry of pyrithione in aqueous solutions.