Thermodynamic Protonation Parameters of some Sulfur-Containing Anions in NaClaq and (CH3)4NClaq at t = 25 °C

Protonation constants of one thiocarboxylate (thioacetate) and four sulfurcontaining carboxylates (2-methylthioacetate, thiolactate, thiomalate, 3-mercaptopropionate) were determined by potentiometric measurements in a wide ionic strength range [0 ≤ I ≤ 5 mol·L−1 in NaCl and 0 ≤ I ≤3 mol·L−1 in (CH3)4NCl] at t = 25 °C. For two of these ligands (2-methylthioacetate and thiolactate), the protonation enthalpies were also determined by calorimetric measurements in NaCl ionic medium [0 ≤ I ≤ 5 mol·L−1] at t = 25 °C. Individual UV spectra of the protonated and unprotonated 3-mercaptopropionate species, together with values of the protonation constants, were obtained by spectrophotometric titrations. Results were analyzed in terms of their dependence on the ionic medium by using different thermodynamic models [Debye-Hückel type, SIT (Specific ion Interaction Theory) and Pitzer’s equations]. Differences among protonation constants obtained in different media were also interpreted in terms of weak complex formation.