Effect of temperature on dissociation constant of (+)-(S)-2-(6-methoxynaphthalen-2-yl) propanoic acid in methanol+water binary solvent by potentiometry

The dissociation constant is an important physic-chemical parameter of a substance, and the knowledge of this parameter is of fundamental importance in a wide range of applications and research areas. Thus, the dissociation constants of (+)-(S)-2-(6-methoxynaphthalen-2-yl) propanoic acid at 0.1 M (Me4NCl) ionic strength and 25, 37 and 45 °C under argon atmosphere in aqueous medium were determined by the potentiometric method using the BEST computer program. Log β values found were 4.4711, 4.423, and 4.3736 for 25, 37 and 45°C respectively. Results show that the logβ values decrease with increasing temperature. This can simply be explained as a result of proton release from ligand [1, 2]. The amount of protons released depends on the strength of the intermolecular hydrogen bond. In other words, it can be said that when temperatures are increased the strength of the intermolecular hydrogen bonds between ligand and solvent molecules is decreased. The thermodynamic parameters such as, Gibb’s free energy change (∆G°), entropy change (∆S°) and enthalpy change (∆H°) associated with the dissociation of the ligand were calculated. The negative ΔG° values indicate that dissociation of the ligand process is spontaneous [3]. A decrease in proton-ligand dissociation constant (logβ) with an increase in temperature and the negative value of enthalpy change (ΔH°) for the dissociation suggests that this reaction is exothermic, favorable at lower temperature. ΔS° is positive revealing that entropy is favorable for the dissociation of ligand [4].