Conductometric Study of Ion Association between Tris(oxalato)chromate(III) and Alkali Metal Ions at Temperatures from 0 to 50 °C

Abstract

Abstract Conductivities for aqueous solutions of alkali metal salts of tris(oxalato)chromate(III) ([Cr(ox)3]3−) were measured at various temperatures from 0 to 50 °C. Ion-association constants (KA) between the complex ion and alkali metal ions increased in the order Li+ << Na+ < K+ < Rb+ < Cs+ and had minimum values (KA(min)) at characteristic temperatures (tmin) in the observed temperature range except for the lithium ion. The appearance of minima of KA suggested the formation of contact ion pairs besides that of solvent-separated ones. The contact ion-pair formation was considered to be assisted by weak hydration of alkali metal ions and enhanced with decreasing their hydration strength. The lithium ion was presumed to form only solvent-separated ion pairs on the basis of extremely small ion-association constants. Temperature dependence of log KAfor each ion association could be represented by the equation: log KA = p(t − tmin)2 + log KA(min). A common relationship between p and tmin was found out for the ion-association systems of [Cr(ox)3]3−, [Co(NH3)6]3+, and [Co(en)3]3+ (en = ethylenediamine), and p was expressed by an empirical quadratic equation in tmin. The standard enthalpy changes of ion association (ΔHas°) at any temperature for these complex systems could be reproduced within ±0.2 kJ mol−1 by use of these equations with the experimental tmin. Hydration behavior of [Cr(ox)3]3− was also discussed based on limiting molar conductivities and their temperature dependence.