Thermodynamic Study on the Acid-Base Properties of Antioxidant Compound Ascorbic Acid in Different NaClO₄Aqueous Ethanol Solutions

Abstract

The proton dissociation equilibria of ascorbic acid were investigated using a combination of spectrophotometric/potentiometric techniques at constant temperature 25.0 ± 0.1 °C. The study was carried out over a wide range of ionic strengths of 0.1-3.0 mol dm -3 NaClO 4 in ethanol-water mixture (1:1). The Gran's method was used for glass electrode calibration to obtain pH readings based on the concentration scale (p c H). Data process for calculation of the protonation constants was done using the STAR (stability constants by absorbance reading) computer program. The dependence on ionic strength of the constants was analyzed by means of the SIT approach (specific ion interaction theory). In addition to the specific interaction parameters of the ionic species, the protonation constants at infinite dilution (zero ionic strength) were obtained. Finally, the thermodynamic functions ΔG 0 (standard Gibbs free energy change) for the protonation processes were also calculated at different ionic strengths.

Phase transitions of ascorbic acid and sodium ascorbate in a polymer matrix and effects on vitamin degradation

“…The pH is also known to be a factor in vitamin C degradation (Emese & Peter, ; Golubitskii et al, ). Since pH is directly related to the degree of ionization, it is important to document the difference between solution pH and the pKa of AA (4.04) (Figueroa‐Méndez & Rivas‐Arancibia, ; Jabbari & Khosravinia, ). The effects of PVP and vitamin:polymer ratio on the pHs of solutions prior to freeze drying were determined, and an assumption was made that the general pH trend in the VPSDs would be similar to the trend found in the solutions.…”

Understanding the Effect of Differently Charged Surfactants on the Protolytic Equilibria of the Drug Naproxen in Micellar Solution

“…To the protolytic equilibria, the standard molar Gibbs free energy of the protonation process, Δ p G m ° , at a constant temperature, can be evaluated according to the following well-known thermodynamic equation where R stands for the universal gas constant (8.3145 J K –1 mol –1 ), T is the absolute temperature, and p K a is the negative logarithm of K a .…”

“…Consequently, the current research mainly concerns the protonation equilibria of naproxen in aqueous cosolvent mixtures containing various surfactants and studying the dependence of these equilibria on the surfactant type and concentration. Potentiometric titration is accepted as a high-precision, powerful, simple, and commonly used electroanalytical technique to determine the p K a values of acidic and basic substances. − …”