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Solvent Effects on Protonation Constants of Betanin in Different Aqueous Solutions of Methanol at T = 25 ºC

Authors: 

Fatemeh Zabihi, Farhoush Kiani*, Mojtaba Yaghobi and Seyed Ahmad Shahidi, Fardad Koohyar*

Source title: 
Journal of the Mexican Chemical Society, 63(2): 94-102, 2019 (ISI)
Academic year of acceptance: 
2019-2020
Abstract: 

The protonation constants of the betanin (pKa1, pKa2, and pKa3) were determined in mixed solvent of water and methanol containing 0, 10, 20, 30, 40, 50, 60, 70, and 80 % (v/v) methanol, using a combination of the spectrophotometric and potentiometric methods at T = 25 ºC and constant ionic strength (0.1 mol.dm-3 NaClO4). The obtained protonation constants were analyzed using Kamlet, Abboud, and Taft parameters. KAT parameters are α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity), and π* (dipolarity/polarizability). In this study, a good linear relationship was obtained between protonation constants (on the logarithmic scale) and dielectric constant (ɛ) of the water-methanol mixed solvents. It was found that the dual-parameter correlation between log10K,s and π*, β  give us the best result in various volume fractions of methanol for water-methanol mixed solvent. Finally, the results are discussed in terms of the effect of the solvent on the protonation constants.