Solvent Effects on Protonation Constants of Betanin in Different Aqueous Solutions of Methanol at T = 25 ºC
DOI:
https://doi.org/10.29356/jmcs.v63i2.789Keywords:
Protonation Constants, Methanol, Spectrophotometric, Potentiometric, Solvent.Abstract
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.
Resumen. Las constantes de protonación de la betanina (pKa1, pKa2 y pKa3) se determinaron en un disolvente mixto de agua y metanol que contenía metanol al 0, 10, 20, 30, 40, 50, 60, 70 y 80% (v / v), utilizando una combinación de los métodos espectrofotométricos y potenciométricos a T = 25 ºC y fuerza iónica constante (0,1 mol.dm-3 NaClO4). Las constantes de protonación obtenidas se analizaron utilizando los parámetros de Kamlet, Abboud y Taft. Los parámetros KAT son α (acidez del donante de enlaces de hidrógeno), β (basicidad del aceptor de enlaces de hidrógeno) y π* (dipolaridad / polarizabilidad). En este estudio, se obtuvo una buena relación lineal entre las constantes de protonación (en la escala logarítmica) y la constante dieléctrica (ɛ) de los disolventes mixtos agua-metanol. Se encontró que la correlación de doble parámetro entre log10K, s y π*, β nos da el mejor resultado en varias fracciones de volumen de metanol para el disolvente mixto agua-metanol. Finalmente, los resultados se discuten en términos del efecto del solvente en las constantes de protonación.
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