Voltammetry of Dissolved Iron(III)–Nitrilotriacetate–Hydroxide System in Water Solution

Abstract

AbstractThe investigation of the dissolved iron(III)–nitrilotriacetate–hydroxide system in the water solution (I=0.1 mol L−1 in NaClO4; pH 8.0±0.1) using differential pulse cathodic voltammetry, cyclic voltammetry, and sampled direct current (DC) polarography, was carried out on a static mercury drop electrode (SMDE). The dissolved iron(III) ion concentrations varied from 2.68×10−6 to 6×10−4 mol L−1 and nitrilotriacetate concentrations were 1×10−4 and 5×10−4 mol L−1. By deconvoluting of the overlapped reduction voltammetric peaks using Fourier transformation, four relatively stable, dissolved iron(III) complex species were characterized, as follows: [Fe(NTA)2]3−, mixed ligand complexes [FeOHNTA]− and [Fe(OH)2NTA]2−, showing a one‐electron quasireversible reduction, and binuclear diiron(III) complex [NTAFeOFeNTA]2−, detected above 4×10−4 mol L−1 of the added iron(III) ions, showing a one‐electron irreversible reduction character. The calculations with the constants from the literature were done and compared with the potential shifts of the voltammetric peaks. Fitting was obtained by changing the following literature constants: log β2([Fe(NTA)2]3−) from 24 to 27.2, log β1([FeNTA]−) from 8.9 to 9.2, log β2([Fe(NTA)2]4−) from 11.89 to 15.7 and log β2([Fe(OH)2NTA]3−) from 15.63 to 19. The determination of the electrochemical parameters of the mixed ligand complex [FeOHNTA]−, such as: transfer coefficient (α), rate constant (ks) and formal potential (E°') was done using a sampled DC polarography, and found to be 0.46±0.05, 1.0±0.3×10−3 cm s−1, and −0.154±0.010 V, respectively. Although known previously in the literature, these four species have now for the first time been recorded simultaneously, i.e. proved to exist simultaneously under the given conditions.