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 nm(3) formula unit(-1) for the ferrocyanide ion, Fe-II[(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.