Thermodynamics of Zn2+ 2-Mercaptopyridine-N-oxide and 2-Hydroxypyridine-N-oxide interactions: stability, solubility, activity coefficients and medium effects

In this paper, a study, at T = 298.15 K, on the thermodynamic properties and the complexing ability towards Zn2+ of 2-mercaptopyridine-N-oxide (pyrithione, PT) and 2-hydroxypyridine-N-oxide (HPNO) is reported. For this purpose, different analytical techniques were used, potentiometry, voltammetry, atomic absorption and UV–visible spectroscopy. The equilibrium constants of the H+/HPNO and Zn2+/HPNO systems, together with the solubility of the Zn(HPNO)2 specieswere determined in NaNO3 aqueous solutions at different ionic strengths (0 b I/mol·dm−3 ≤ 3.2). For pyrithione, since reliable protonation constants have been recently published, the studywas focused on its complexing ability towards Zn2+, and to the determination of the total and intrinsic solubility of the neutral Zn(PT)2 species in NaCl at different ionic strengths (0 b I/mol·dm−3 ≤ 3.1). The speciation schemes of the two investigated Zn2+/L systems are similar, and consist of the ZnL+ and ZnL2 species. Generally, the formation constant values of the Zn2+/PT species are higher than those of the Zn2+/HPNO ones, for example, the stability of the ZnL+ species is log K=5.07 and 6.26 (at I=0.1 mol·dm−3) for HPNO and PT, respectively. At infinite dilution, the total solubility of the ZnL2 species is: log S0 T =−4.291 ±0.007 and−1.241± 0.004 for the Zn(PT)2 and Zn(HPNO)2, respectively. For these two species, the solubility product, KS0,was also determined. The dependence on ionic strength of the equilibrium constants was modeled by the extended Debye–Hückel type equation (EDH) and the Specific ion Interaction Theory (SIT). Using this last model, the activity coefficients of all the species involved in the protonation and complex formation reactions were calculated. Furthermore, the formation of the mixed complex Zn(PT)(HPNO) was evidenced by means of spectrophotometric titrations, with a stability constant log β = 11.36 ± 0.06. In the case of pyrithione, the speciation study was performed also in a multicomponent solution simulating the composition of soaps.