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Electrochemical characterisation of Iron(III)-Glycine-Nitrilotriacetate mixed ligand complexes and their stability in aqueous solution

Electrochemical characterisation of iron(III)-glycine-nitrilotriacetate (iron(III)-Gly-NTA) mixed ligand complexes and determination of their stability constants and retention time in aqueous solution (I = 0.1 mol dm-3 in NaClO4, pH = 8.0+-0.1 at 25+-1 oC) using differential pulse cathodic voltammetry (DPCV), cyclic voltammetry (CV) and direct current (d.c.) polarography with a static mercury drop electrode (SMDE), were performed. Iron(III) concentrations were varied from 5×10-6 to 6×10-4 mol dm-3, NTA total concentrations varied from 2×10-5 to 1×10-3 mol dm-3 and glycine total concentrations were 0.2, 0.02 and 0.002 mol dm-3. Iron(III) redox reaction in the mixed ligand system (with the techniques employed) was found to be the one-electron reversible process. For the total concentration ratios of 1 : 800 : 2 for iron(III), glycine and NTA, respectively, the iron(III)-Gly-NTA mixed ligand complexes are dissolved and stable (>18 hours) in the aqueous solution. The complexes were formed either by the addition of NTA into the iron(III) and glycine aqueous solution or by the addition of iron(III) to the mixture of glycine and NTA. Under these conditions, iron(III) hydrolysis was suppressed to the great extent. By fitting of experimental data following stability constants for mixed ligand complexes, not found in the literature so far, in 0.1 mol dm-3 NaClO4 aqueous solution were calculated: for iron(III) log beta1([FeGlyNTA]-) = 27.23+-0.69, log beta2([Fe(Gly)2NTA]2-) = 30.29+-0.77 ; for iron(II) log beta1([FeGlyNTA]2-) = 14.13+-0.43 and log beta2([Fe(Gly)2NTA]3-) = 18.51+-0.51.

iron(III); mixed ligand complexes; nitrilotriacetate (NTA); glycine; stability constants; voltammetry; aqueous solution

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