Solvent Effects on Protonation and Complexation of Penicillamine and Thallium(I) in Different Aqueous Solutions of Methanol

The protonation equilibria of penicillamine and its complex formation with the Tl(I) ion were studied over a wide range of pH (1 to 11), using a combination of spectrophotometric and potentiometric methods at constant temperature, 25 °C, different methanol−water mixtures [(0 to 45) % v/v], and constant ionic strength (0.1 mol·dm−3 sodium perchlorate). Least squares regression calculations are consistent with the formation of TlH2L+, TlHL, and TlL− species, where L2− represents the fully dissociated ligand. The protonation of penicillamine and the formation constants of the formed complexes in different media were analyzed in terms of Kamlet, Abboud, and Taft (KAT) parameters. A single-parameter correlation of the formation constants versus α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity), and π* (dipolarity/polarizability) are relatively poor in all solutions, but multiparameter correlations represent significant improvements with regard to the single-parameter model. Linear correlation is observed when the experimental log β xyz values are plotted versus calculated ones, while all of the KAT parameters are considered. Finally, the results are discussed in terms of the effect of solvent on protonation and complexation.