Single Ion Free Energies and Entropies of Aqueous Ions

Single ion free energies of hydration were assigned by Latimer, Pitzer, and Slansky by choosing a set of ion radii which made both negative and positive ions fall on the theoretical Born curve in a plot of ΔF against 1/r′. The r′ values were the crystal ion radii plus 0.1 A for negative ions and plus 0.85 A for positive ions. It is now shown that this assignment of single ion free energies is consistent with the single ion entropies of hydration if S̄H+ is —2.1 (the Eastman value). The ΔS values are proportional to Z/r′ and the tentative suggestion is made that it is the temperature coefficient of r′ which leads to the Z/r′ function. It is proposed that the ΔF values be used to calculate a consistent set of radii of aqueous ions which will take into account coordination numbers and polarization.