THERMOCHEMICAL STUDY OF ACID-BASE REACTIONS IN AQUEOUS SOLUTION OF PYRIDOXINE

Abstract

Vitamin B6 is one of the most important vitamins necessary for the life of living organisms. Vitamin B6, which in the body is represented by coenzyme forms - pyridoxal phosphate and pyridoxamine phosphate, is a part of more than 100 enzymes. As the object of study, we chose pyridoxine. The thermal effects of interaction of the solution of pyridoxine with solutions of nitric acid and potassium hydroxide at 298.15K and ionic strength values of 0.25; 0.5 and 0.75 against the background of electrolyte-supporting potassium nitrate were measured with direct calorimetric method. The measurements were carried out in a calorimeter with an isothermal shell equipped with a 60 cm3 reaction vessel and electric calibration at T = 298.15 ± 0.01 К and P = 100.5 ± 0.7 kPa and automatic recording of the temperature-time curve. Relative measurement error for heats of dissolution of a standard substance was -0.1-0.3%. Calculation of the equilibrium composition of the system taking into account processes a step dissociation and water dissociation was carried out according to the KEV program. The thermal effects of dissociation of pyridoxine in a standard solution were found by extrapolating the heats of stepwise dissociation at fixed values of ionic strength to zero ionic strength. Since the thermal effects of dilution were measured in three concentration ranges of KNO3, the number of experiments in each series was at least three. To calculate the confidence interval of the average value of DН the Student's criterion was taken at a confidence interval of 0.95. Standard thermodynamic characteristics (DrH°, DrG°, DrS°) of reactions of acid-base interaction in aqueous solutions of pyridoxine were calculated. With an increase in the background electrolyte concentration, the endothermicity of stepwise pyridoxine ionization processes increases.