UV–visible spectroscopic and electrochemical study of the complex formation between Fe(II) and 5-amino-1,10-phenantroline (5-Aphen) in aqueous solution

The system Fe(II)–5-Aphen–H2O was studied. The spectroscopic and electrochemical results show that only one stable complex between Fe(II) and 5-Aphen forms, having a 1:3 stoichiometric ratio. The spectrophotometry study allowed determination of the formation constant of the complex (log β3 = 23.42 ± 0.06). Also, the stability of the complex was evaluated as a function of pH; it was found that it decomposed at low pH values depending on the concentration and a pseudo-first order kinetics constant associated with k′ = 0.011 min−1. The results are in agreement with the electrochemical behaviour observed in the system, which indicated that at pH 1.33 the destruction of the complex [Fe(5-Aphen)3]2+ took place as a function of time; however, when the experiments were carried out at pH 6.19 the complex was stable. The thermodynamic data obtained through the use of MEDUSA allowed construction of predominance zone diagrams of the system Fe(II)–5-Aphen–H2O under the experimental conditions used. The thermodynamic results represented in the PZD describe the experimental behaviour reported in this work.