Thermodynamic Parameters for the Complexation of 18-Crown-6 with Ag⁺, Pb²⁺, Cd²⁺, Cu²⁺, Co²⁺ and Ni²⁺ in Methanol-Water Solvents by Conductometry PDF

Keywords

Methanol-Water, conductometry, solvents

Abstract

The complexation of 18-crown-6 with Ag⁺, Pb²⁺, Cd²⁺, Cu²⁺, Co²⁺ and Ni²⁺ in methanol-water solvents has been studied using a conductometric technique. Log K, ∆H°, ∆S° values are determined in 0%- 90 w% methanol-water at several temperatures in the range 288-308K. The measured stability constants and thermodynamic parameters were found to be solvent dependent. There is an inverse relationship between the stability constants of the complexes and Gutmann donicity of the solvents. The measured molar conductivities, Λ, Pb²⁺, Ag⁺, and Cd²⁺ complexes of 18-crown-6, were observed to decrease significantly for mole ratios, [L]₀/ [Mⁿ⁺]₀, close to unity while those of Cu²⁺, Co²⁺ and Ni²⁺ complexes were decreased less significantly. The measured molar conductivities have been analysed using a 1:1 stoichiometric model. The stability constant K, and the limiting molar conductivity, Λc, for each 1:1 complex have been determined from the measured molar conductivities by using a non-linear least-squares analysis. In 90% methanol-water solvents used, the binding sequence, based on the value of log K at 298 K, was found to be Pb²⁺ > Ag⁺ > Cd²⁺ > Cu²⁺ > Co²⁺ > Ni²⁺. The significance of the determined values of the present thermodynamic parameters has been discussed in terms of solvent medium and relative sizes of cations and crown ether cavity.