Thermodynamic Analysis of Ion-Pair Partition of Methyl Orange Anion with Alkali Metal Cations in an Octanol-Water System

Abstract

The ion-pair partition of methyl orange anion with the alkali metal cations Na⁺, K⁺ and Cs⁺ in an octanol-water system was studied at various temperatures. The ion-pair partition process could be divided into two processes; ion-pair formation and transfer of the ion-pair. The results showed that the degree of ion-pair formation increased with temperature, while the degree of partition of an ion-pair decreased with temperature for all alkali metal cations. The ion-pair partition of methyl orange anion was shown to be greatest with Cs⁺. The thermodynamic parameters ΔG^θ, ΔS^θ and ΔH^θ for these two processes were determined. The values are discussed in relation to the degree of hydration of these alkali metal cations.