The dependence on ionic strength of the protonation of water-insoluble flavonoids chrysin and daidzein in NaClO4 non-aqueous solutions

The acid-base equilibria of two water-insoluble flavonoids, chrysin and daidzein, were studied using combined spectrophtometric/potentiometric measurements at constant temperature (25.0 ± 0.1) °C. The investigation was performed over a wide range of ionic strengths of 0.101-3.503 mol kg− 1 NaClO4 medium in ethanol-water mixture (1:1 v/v). The Gran's method was used for glass electrode calibration to obtain pH readings based on the concentration scale (pcH). Data process for calculation of the protonation constants was done using the STAR computer program. The dependence on ionic strength of the constants was successfully modeled by means of the Brönsted-Guggenheim-Scatchard Specific Ion Interaction Theory (SIT). Other than the specific interaction parameters of the ionic species, the protonation constants at infinite dilution (zero ionic strength) were obtained. The thermodynamic functions ΔG0 (standard Gibbs free energy change) for the protonation processes were also calculated at different ionic strengths.