Metal–ion complexation reactions in the presence of surfactants. Part 2.—Mechanism of pH-dependent reaction between nickel(II) and murexide in aqueous solution and application of the reaction to study of micellar phenomena

Abstract

The thermodynamics and kinetics of the reaction between Ni²⁺(aq) and the metal ion indicator murexide (Mu^–) have been studied over the pH range 4.0–7.3 in aqueous solution. Rates and equilibrium concentrations are strongly pH-dependent. Rate constants were obtained using pressure-jump and stopped-flow methods and results from both techniques are in excellent agreement. All data are consistent with the reaction scheme: Ni²⁺+ Mu^– [graphic omitted] (NiMu)⁺ [graphic omitted] (NiMu)_dep+ H⁺.

The reaction (involving charged reagents) is studied in the absence of added inert electrolyte and, therefore, under conditions of varying ionic strength. A general treatment for handling rate and equilibrium data in this situation is described. By the inclusion of appropriate activity co-efficients the following results (at 298.2 K) are obtained: K₁=k₁/k_–1= 3.8(± 0.2)× 10³ dm³ mol^–; K_H= 1.8(± 0.2)× 10^–6 mol dm^–3, k₁= 7.8(± 0.5)× 10³ dm³ mol^–1 s^–1; k_–1= 2.1 s^–1. Such indicator reactions involving murexide are very convenient for the study of various aspects of micellisation and their application to the detection of micelles formed by anionic detergents is described.