Electrical conductivities and electronic absorption spectra of Dq 2 Fe(CN) 6 (where Dq 2? is diquat, the N,N 0 -ethylene-2,2 0 -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 = 17,800 ± 1800 dm 3 Ámol -1 ) and the corresponding contact distance (r min = 5.31 ± 0.07 Å ) were evaluated from the spectral data. Analysis of the conductivity data resulted in K 1 = 16,500 ± 1500 dm 3 Ámol -1 (r min = 5.38 ± 0.07 Å ). Both contact distances were found to agree with the FeÁÁÁN distances in crystalline DqH 2 Fe(CN) 6 Á4H 2 O and Dq 2 Fe(CN) 6 Á6H 2 O. 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 and K 2 were used to calculate electrical conductivities and optical absorbancies in the concentration range up to 5 mmolÁdm -3 . Satisfactory agreement was achieved between calculated and experimental data.
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