Formation kinetics of an aluminium(III)–ethylenedinitrilotetraacetate–fluoride mixed ligand complex‡

Abstract

The formation kinetics of the [Al(edta)F]^2– mixed ligand complex has been studied using potentiometric and ¹⁹F NMR methods. The rate equation is –dc_F/dt = d[Al(edta)F^2–]/dt = k₁[Al(edta)^–][F^–] + k₂[Al(edta)^–][HF], where k₁ = 20.7 ± 0.3 M^–1 s^–1 and k₂ = 471 ± 93 M^–1 s^–1 at 298 K, the activation parameters for the main reaction path (k₁) being ΔH ^‡ = 49.2 ± 0.9 kJ mol^–1, ΔS ^‡ = –54.6 ± 2.8 J K^–1 mol^–1 studied in the range of pH 4.6–6.0, T = 283–328 K and I = 1 M NaNO₃ medium. An associative interchange (I_a) mechanism can be proposed for the reaction. The stability constant, K_AledtaF = [Al(edta)F^2–]/[Al(edta)^–][F^–], was redetermined from the kinetic curves: log K = 4.63 ± 0.01 at 298 K, ΔH = –25.1 ± 0.5 kJ mol^–1 and ΔS = 4.6 ± 1.5 J K^–1 mol^–1. The initial rate of the reaction can be used as a kinetic method to determine the aluminium concentration using a fluoride-selective electrode.