Mechanism of caesium ion exchange on potassium cobalt hexacyanoferrates(II)

The caesium uptakes by K2[CoFe(CN)6] and non-stoicheiometric compounds K2–xCox/2[CoFe(CN)6] were found to correlate directly with the specific surface areas of the products with x < 1. The exchange process is assumed to involve only the outermost surface layer of their crystals, which have a cubic lattice, i.e. only potassium (or cobalt) ions inside the elementary cubes closest to the surface of the crystals are exchanged for caesium ions. Compounds with x > 1 are mixtures of cubic potassium cobalt hexacyanoferrate(II) and tetragonal Co2Fe(CN)6. The thermodynamic equilibrium constant of the caesium exchange on K2[CoFe(CN)6] was found to have a high value of 125.