Changes in thermodynamic parameters in the exchange of alkaline earth metals with sodium ions in Dowex-50Wx 8 and Amberlyst-15 resins in aqueous medium

Abstract

Standard free energy, ∆<i>G</i>°, enthalpy, ∆<i>H</i>°and entropy, ∆<i>S</i>°, changes have been evaluated for Mg²⁺/Na⁺ , Ca²⁺/Na<i>⁺ </i> +, Sr²⁺<i>I</i>Na⁺<i>, </i>Ba²⁺<i>I</i>Na⁺ exchange systems on strong polystyrenesulfonic (PSS) acid type cation exchange resins, Dowex-50WX8 and Amberlyst-15 (macroreticular) in dilute aqueous solutions from equilibrium ionic distribution and heat of exchange measurements calorimetrically at 298K. The alkaline earth metals are preferred by both the resins and the preference increases from magnesium through barium. The selectivity is lower for all the metals on Amberlyst-15 resin. The exchanges take place with absorption of heat. The endothermicity is more on Amberlyst-15 and it decreases with increasing size of the metal ion on both the exchangers. The uptake of divalent ions is governed by increase in entropy. The difference in entropies of hydration of alkaline earth metal and sodium ions is considered to be a major factor responsible for increase in standard entropy. The lower selectivity of alkaline earth metals on Amberlyst-15 is attributed to decrease in the hydration numbers of the cations in the resin since it imbibes more water than Dowex-50WX8.