Parameters affecting the determination of paraquat at silver rotating electrodes using differential pulse voltammetry

The electrochemical determination of aqueous paraquat PQ(II) by differential pulse voltammetry at a solid rotating silver electrode (RSE) is described. The aim of this work is to optimize all factors that can influence this determination. Potential wave forms, potential scan parameters and deposition time were examined for their effect on the paraquat peak shape and intensity. The best responses were obtained with differential pulse voltammetry in 0.1 mol L⁻¹ Na₂SO₄ as supporting electrolyte using amplitude 50 mV, scan increment 5 mV, deposition time 120 s, frequency 50 s⁻¹ and step amplitude 0.05 V. Electrochemical and mechanical surface cleaning, aimed at removing the amount of paraquat deposited onto the silver surface, were necessary for obtaining a good performance of the electrode. Response linearity, repeatability, accuracy and detection limit were also evaluated. The obtained detection limits were 7.1 × 10⁻⁹ mol L⁻¹ and 2.8 × 10⁻⁹ mol L⁻¹ for peak 1 and peak 2 respectively. The relative standard deviation (RSD) was found to be 1.19% in 1.0 × 10⁻⁴ mol L⁻¹ paraquat. The applicability of the RSE for PQ(II) determination in milk samples, without any sample pretreatment, was successfully demonstrated.