Conductometric and UV–Visible Spectroscopic Studies on Association in Dilute Aqueous Solutions of Dq2Fe(CN)6

Electrical conductivities and electronic absorption spectra of Dq2Fe(CN)6 (where Dq2+ is diquat, the N,N′-ethylene-2,2′-bipyridine cation) were measured in dilute solutions in water. Experimental data were interpreted in terms of a molecular model which includes the presence of 1:1 and 2:1 associates. Initially, electrical conductivities and absorption spectra were treated using the Quint–Viallard conductivity equations and modified Hemmes algorithm in a limited concentration range, when only minor 2:1 complex formation is expected. The thermodynamic association constant of the 1:1 complex (\( K_{ 1}^{{^\circ }} \) = 17,800 ± 1800 dm3·mol−1) and the corresponding contact distance (r min = 5.31 ± 0.07 A) were evaluated from the spectral data. Analysis of the conductivity data resulted in \( K_{ 1}^{{^\circ }} \) = 16,500 ± 1500 dm3·mol−1 (r min = 5.38 ± 0.07 A). Both contact distances were found to agree with the Fe···N distances in crystalline DqH2Fe(CN)6·4H2O and Dq2Fe(CN)6·6H2O. This agreement allowed us to use the same r min value to estimate the association constant of the 2:1 complex in accordance to the Fuoss–Krauss theory. Both \( K_{ 1}^{{^\circ }} \) and \( K_{ 2}^{{^\circ }} \) were used to calculate electrical conductivities and optical absorbancies in the concentration range up to 5 mmol·dm−3. Satisfactory agreement was achieved between calculated and experimental data.