Protonation of l-Ascorbic Acid in an Aqueous Solution at T = 298.2 K, p = 0.1 MPa, and I = 0.10–5.0 mol L^–1 (NaCl)

Abstract

Ascorbic acid is one of the essential compounds for living organisms. The reductive properties of ascorbate depend significantly on the dissociation of ascorbic acid, which, in turn, is influenced by the ionic strength value. The protonation of ascorbate dianion was studied in an aqueous solution at T = 298.2 K; p = 0.1 MPa; and I = 0.10, 0.25, 0.5, 1.0, 2.0, 3.0, and 5.0 mol L–1 (NaCl) using the spectropotentiometric technique. Protonation constants were calculated from the spectral data. The analysis of ionic strength influence on the ascorbic acid dissociation constants was performed using the modified Davies equation (the so-called Vasil’ev equation) and specific ion interaction theory.

Copper (II)-Catalyzed Oxidation of Ascorbic Acid: Ionic Strength Effect and Analytical Use in Aqueous Solution

“…The secondary salt effect leading to the change in ascorbic acid dissociation degree can be neglected for two reasons. First, the variation in the dissociation constants of vitamin C with the ionic strength alteration up to 0.25 mol L −1 (NaCl) is negligible [22]. AAH 2 0 and AAH − coexist in roughly equimolar quantities when the ascorbic acid is dissolved in the distilled water [22].…”

“…First, the variation in the dissociation constants of vitamin C with the ionic strength alteration up to 0.25 mol L −1 (NaCl) is negligible [22]. AAH 2 0 and AAH − coexist in roughly equimolar quantities when the ascorbic acid is dissolved in the distilled water [22]. Second, the monoascorbate anion undergoes oxidation much more readily than ascorbic acid [24].…”

“…Only freshly prepared solutions of ascorbic acid were used. The concentration of ascorbic acid was determined spectrophotometrically using the literature values of the molar extinction coefficients [22]. All solutions were prepared using bidistilled water (κ = 3.6 µS/cm, pH = 6.7).…”

“…The dependence of absorbance on time was then recalculated using the literature values of molar extinction coefficients of ascorbic acid [22], and the resulting data 'ascorbic acid concentration' vs. 'time' were processed using Kinet software [64] to obtain the apparent rate constant of the ascorbic acid oxidation. All kinetic experiments were at least triplicated; the error bars in Figure 1, Figure 2, and Figure 4 represent the standard deviations calculated for the corresponding sample size.…”

“…However, another valuable property of ascorbic acid (also related to the antioxidant activity) is of special interest to us. Ascorbate monoanion (AAH − )-which exists predominantly in neutral media considering pK a1 = 4.14 and pK a2 = 11.28 [22]-has a low reduction potential (0.19 V for the pair of DHA/AAH − at pH 3.5). However, the rate of redox reaction in absence of catalysts (for example Cu 2+ [23] or Fe 2+ ions) is very low as the reaction is spin-forbidden [24].…”