Voltammetric study of complexing sodium cation with dibenzo-18-crown-6 in nitrobenzene phase of two-phase water-nitrobenzene extraction system

The facilitated transfer of the Na⁺ cation through the water (w)/nitrobenzene (nb) interface affected by dibenzo-18-crown-6 dissolved in the organic phase was studied by the cyclic voltammetry in the temperature range of 5–65°C. The transfer was found to be a very fast process at each studied temperature (5, 20, 35, 50, and 65°C); the process is controlled by diffusion of the electroneutral crown ionophore from the volume of the nitrobenzene phase to the interface. Changes of thermodynamic functions characterising the studied equilibrium reaction Na⁺ (nb) + L(nb) ? NaL⁺ (nb), where L = dibenzo-18-crown-6, were determined for a temperature of 25°C: ΔG = ?36.0 kJ mol^?1, ΔH = 38.9 kJ mol^?1, and ΔS = ?9.7 J mol^?1 K^?1. Further, the change of the standard Gibbs energy of the transfer of the NaL⁺ complex from the aqueous phase into the nitrobenzene one, ΔG^{0,w → nb}_{tr, NaL⁺} = ?20.1 kJ mol^?1, was calculated together with the value of the standard Galvani potential difference of the NaL⁺ cation between the nitrobenzene and the aqueous phases, Δ^nb_wφ⁰_NaL⁺ = 0.208 V, and the value of Briggs' logarithm of individual extraction constant, log Kⁱ_Nal⁺ = 3.5 (L = dibenzo-18-crown-6), for the two-phase water-nitrobenzene system.