Association of the picrate ion with cations of various nature in solvents of medium and low relative permittivity. An UV/Vis spectroscopic and conductometric study

Abstract

The behavior of tetra- n-butylammonium picrate, TBAPic, in acetone– n-butyl acetate (AC–BA) solvent system, with the relative permittivity changing gradually from ε r = 20.56 to 5.10, was studied at 25.0 °C using UV/Vis spectroscopy and conductometry. Interactions of the Pic − anion with the TBA + cation in solution result in changes in its electronic absorption spectrum, though not so markedly as in the case of its association with metal ions, such as Li +, Na +, Mg 2+, La 3+, etc. The association constants (TBA + + Pic −⇄ ion pair, K A) and the limiting molar conductivities were determined using the Lee–Wheaton equation. The literature K A value in pure AC is refined. The state of TBAPic, an electrolyte with relatively poor solvated cations and anions, in solvents of medium and low polarity is discussed. At an AC content of 25 mass % and less, the 1:1 association model is unable to satisfactorily describe the conductance data. Hojo's method, modified to some extent by us, was used to fit the phoreograms by taking into consideration symmetrical ion triplets and quadruples, with equilibrium constants K T and K Q, respectively. Our approach presumes also the possible extrapolation of the plots of the Walden product logarithm, log( Λ o η), as well as that of log K A vs. ε r − 1 to required low ε r value. This allows to refine the Λ o value for pure BA and to verify the K A value estimated by Hojo's procedure in this solvent.