Investigation of the Specific Ion Interactions and Determining Protonation Constant of 3,5-Dihydroxy-2-(3,4,5-trihydroxybenzoyl)oxy-6-[(3,4,5-trihydroxybenzoyl)oxymethyl] oxan-4-yl] 3,4,5-trihydroxybenzoate at Different Ionic Strength

In this research, the protonation constant value of tannic acid was determined at 25 °C and different ionic strengths (0.1 to 0.7 mol dm-3 NaCl) using potentiometric titration technique. The dependence of protonation constant on ionic strength was modeled and discussed by a Debye-Hückel type equation. Then, based on the obtained data from experiments, the minimizing sum of squares error (SSE) was done by Microsoft's Excel solver program and the most suitable model was chosen. After optimization, the protonation constant of tannic acid at different ionic strengths was calculated and curves of log Kcal and log Kexp were compared. Finally, Daniele constants of C and D were measured and the Specific Ion interaction Theory (SIT) was assessed for this weak acid. The obtained results show that the protonation constants of tannic acid decrease with increasing ionic strength.