Acid-base and thermodynamic properties of d-gluconic acid and its interaction with Sn2+ and Zn2+

In this paper, new potentiometric, calorimetric and voltammetric measurements are reported to model the behavior of D-gluconic acid in aqueous NaCl(aq) and NaNO3(aq) ionic media at different ionic strengths and temperatures (283.15 ≤ T / K ≤ 318.15) also in the presence of Sn2+ and Zn2+. The protonation constants of D-gluconic acid (Gluc- in its deprotonated form) in NaCl(aq) and NaNO3(aq) are very similar. The dependence on ionic strength of the protonation constant was modeled by means of the extended Debye-Hückel type equation, the Specific ion Interaction Theory (SIT) and a weak interaction model. At infinite dilution and T = 298.15 K, the thermodynamic parameters for the proton binding reaction, determined by direct calorimetric titrations and temperature gradients of protonation constants, are: log KH0 = 3.709 ± 0.004, ΔG0 = -21.18 ± 0.01 kJ mol-1, ΔH0 = -4.56 ± 0.04 kJ mol-1 and TΔS0 = 16.6 ± 0.1 kJ mol-1, indicating that the reaction is exothermic and entropic in nature. Potentiometric and voltammetric measurements on the Sn2+ and Zn2+/Gluc- systems allowed us to determine the MGluc+ species for both systems at different ionic strengths, together with the Sn(Gluc)2 and Sn(OH)Gluc species. The values of the formation constant of the SnGluc+ species are higher than the corresponding ZnGluc+. During the experiments, the precipitation of insoluble species was evidenced at pH ≥ 4.0 for Sn2+/Gluc- and ≥ 7.0 for Zn2+/Gluc-. The stoichiometry of the precipitates was established by means of thermogravimetric analysis, and the solubility of the two salts, Sn(OH)Gluc(s) and Zn(Gluc)2(s), was determined in pure water and in NaCl and in NaNO3 aqueous media. In pure water, the total solubility is ST = 0.262 ± 0.001 and 0.082 ± 0.006 mol kg-1 for the ZnGluc2 and Sn(OH)Gluc, respectively. Calorimetric measurements were performed to determine the enthalpy changes of the gluconate protonation and Sn2+ complex formation constants at different ionic strengths. The effect of two anions, as fluoride and citrate, on the stability of the Sn2+/Gluc- species was studied by means of potentiometric measurements at I = 0.15 and 1.02 mol kg-1, and it was found that the formation of mixed complexes is thermodynamically favored.