Thermodynamic Studies on Protonation Constant of Quercetin at Different Ionic Strengths

Stepwise protonation constants of quercetin were determined in aqueous solution at different conditions of ionic strength (0.1–0.7 mol·dm−3 NaCl) and temperature (25.0–40.0 °C) using a combination potentiometric–spectrophotometric method. The results obtained show that the first and the third protonation constants decrease with increasing ionic strength whereas the second protonation constant has a minimum around 0.6 mol·dm−3 NaCl. The values of protonation constants were also calculated using the models of Debye–Hückel, Guggenheim, Scatchard, Davies, and the SIT equations at 25.0 °C and different ionic strengths. Also, the protonation constants of quercetin decrease with increasing temperature indicating an exothermic process. The thermodynamic parameters of the protonations were calculated at different ionic strengths using a modified van’t Hoff equation and are discussed.