Thermodynamic Clarification of the Curious Ferric/Potassium Ion Exchange Accompanying the Electrochromic Redox Reactions of Prussian Blue, Iron(III) Hexacyanoferrate(II)

The recent Glasser−Jenkins method for lattice-energy prediction, applied to an examination of the solid-state thermodynamics of the cation exchanges that occur in electrochromic reactions of Prussian Blue, provides incisive thermodynamic clarification of an ill-understood ion exchange that accompanies particularly the early electrochromic cycles. A volume of 0.246 ± 0.017 nm3 formula unit-1 for the ferrocyanide ion, FeII[(CN)6],4- is first established and then used, together with other formula unit−volume data, to evaluate the changes of standard enthalpy, entropy, and Gibbs energy in those ion-exchange reactions. The results impressively show by how much the exchange of interstitial Fe3+ ions by alkali metal ions, usually exemplified by K+, is thermodynamically favored.