The association of hexacyanoferrate(II) ions and group IIA cations

Abstract

A thermodynamic study has been made of the equilibrium

$$M^{2 + } {\text{ + }}Fe(CN)_6^{4 - } {\text{ }} \rightleftharpoons {\text{ }}MFe(CN)_6^{2 - } $$

in dilute aqueous solution by a potentiometric method using a divalent-cation electrode where, M²⁺ is a group IIA cation. Ion pairing was found to be quite extensive: Thus, at 25°C, the association constants extrapolated to zero ionic strength were 5840, 4730, 4560, and 6080M ⁻¹ for M²⁺=Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, respectively. In each case, ion pairing is slightly endothermic, the corresponding ΔH ^o values being 18.0, 13.0, 7.8, and 17.5 kJ-mole⁻¹. However, the values of ΔS ^o are larger and positive, thus favoring ion pairing: the values are 133, 115, 96, and 131 J^o-K⁻¹-mole⁻¹ for the same series of cations. There was also some indication of a small amount of ion triplet formation, M₂Fe(CN)₆. Comparisons were made with the predictions of the Fuoss theory, and these support the view that the bonding in the ion is largely electrostatic and not covalent.