Synthetic inorganic ion-exchange materials XXX. Ion-exchange equilibria of alkaline earth metal ions and Pb 2+/hydrogen ions on crystalline antimonic(V) acid

Abstract

Ion-exchange equilibria of alkaline earth metal ions (Mg 2+, Ca 2+, Sr 2+ and Ba 2+) and of Pb 2+/H + systems have been studied in M(NO 3) 2HNO 3 media with ionic strength of 0.1 at 30, 45 and 60°C on crystalline antimonic(v) acid (CSbA) as a cation exchanger. The selectivity coefficients (logarithmic scale) vary with the equivalent fraction ( X M ) of alkaline earth metal ions in the exchanger and give linear functions of X M for Ca 2+/H + and Sr 2+/H + systems in a range from 0.2 to 0.8. The results on the Ba 2+/H + system reveal that there is a range of X M (0–0.3) in which ion-exchange reaction takes place without steric hindrance. The selectivity sequence Mg 2+ < Ba 2+ < Sr 2+ < Ca 2+ < Pb 2+ shows from the overall thermodynamic equilibrium constants. The thermodynamic data and chemical analysis indicate that the large ΔS° contribution in the Ca 2+/H + and Sr 2+/H + systems is due to water transfer from exchanger to aqueous solution with progressive exchange of the metal ions.