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 ΔHo values being 18.0, 13.0, 7.8, and 17.5 kJ-mole−1. However, the values of ΔSo are larger and positive, thus favoring ion pairing: the values are 133, 115, 96, and 131 Jo-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.