EVALUATION OF PROTONATION CONSTANTS OF MERCAPTOSUCCINIC ACID IN AQUEOUS SOLUTIONS OF PROPYLENE GLYCOL AND DIOXAN

Abstract

The protonation constants values of mercaptosuccinic acid were determined in propylene glycol- and dioxan-water mixtures (0-60% v/v) at 303.0 K at an ionic strength of 0.16 M using pH-metric technique. The protonation constants were calculated with the computer program MINIQUAD75 and selection of the best fit chemical models was based on the statistical parameters. The log K values were found to increase with increase of the organic solvent content. The linear variations of the protonation constants with the reciprocal of the dielectric constant of the medium have been attributed to the dominance of electrostatic forces. Distribution of species, protonation equilibria and effect of influential parameters on the protonation constants have also been presented. 16 1.2 Alkalimetric Titrations The pH measurements of proton-ligand system were carried out in aqueous media containing varying compositions of organic solvent (propylene glycol or dioxan) in the range of 0-60% v/v maintaining an ionic strength of 0.16 M with sodium nitrate at 303.0 ± 0.1 K using a Digital pH meter MK- VI type (readability 0.01) with mechanical stirring carried by a teflon stirrer. The pH metric titration assembly consisted of a double walled spoutless pyrex glass vessel of 100 ml capacity fitted with a perspex lid through which the glass combination pH electrode, gas inlet-outlet tubes and burette tip were admitted. Water from a thermostat was pumped at constant rate through the annular space between the walls of the titration vessel. The titrand in the double walled spoutless pyrex glass vessel was maintained inert by bubbling dried and pure nitrogen gas throughout the course of the titration to purge carbon dioxide and oxygen. Potassium hydrogen phthalate (0.05 mol) and borax (0.01 mol) solutions were used to calibrate the pH meter. In each titration, the titrand consisted of approximately 1 mmol of nitric acid in a total volume of 50 ml. The amounts of MSA in the titrands ranged between 0.25 and 0.50 mmols. The glass electrode was equilibrated in a well stirred organic solvent-water mixture containing inert electrolyte for several days. At regular intervals, the strong acid was titrated against alkali to check the complete equilibration of the glass electrode. The details of the experimental procedure and titration assembly used in our laboratory have been given elsewhere. 17