Interaction study of paraquat and silver electrode using electrochemical impedance spectroscopy: Application in milk and tomato samples

This paper reports on the use of electrochemical impedance spectroscopy (EIS) for analytical determination of paraquat herbicide at a silver electrode. The electrochemical technique was applied to aqueous solution, at pH 7.0, containing different concentrations of paraquat. The impedance measurements were carried out in the frequency range from 100 mHz to 100 KHz at a potential of 0.0 mV versus Ag/AgCl. The Nyquist plots were modeled with a Randle equivalent circuit, by identifying the charge transfer resistance as the relevant concentration dependent parameter. The silver surface adsorbs paraquat and the transfer process is controlled by diffusion. The response of the working electrode was attributed to the electron-exchange mechanism at the electrode–contact interface and the ion exchange at the electrode–solution interface. The proposed methodology offers good repeatability (RSD = 1.03%) with (n = 8) and impedimetric response to paraquat in the range from 1.0 × 10−14 to 8.0 × 10−4 mol L−1. The detection limit (DL, 3σ) and quantification limit (QL, 10σ) were 7.37 × 10−16 mol L−1 and 24.59 × 10−16 mol L−1 respectively. The method was satisfactorily applied to determine paraquat in milk and tomato samples.