SQUARE-WAVE VOLTAMMETRIC DETERMINATION OF PARAQUAT AND DIQUAT IN AQUEOUS-SOLUTION

A rapid, sensitive method for the determination of paraquat and diquat in aqueous media using square wave voltammetry (SWV) is discussed on the basis of the analyte behaviour in cyclic voltammetry. Cation-radic als formed upon the one-electron reduction of the analyte(s) adsorb on the mercury electrode surface with both flat and vertical orientation s. With low concentrations only the flat oriented deposit occurs and t he adsorption becomes progressively diffusion-controlled, especially a t high scan rates. SWV experiments were based on this diffusion-contro lled adsorption process. Effects on square wave frequency f, staircase step height Delta E(s), scanning direction, presence of surfactants a nd supporting electrolyte concentration were examined for the optimiza tion of experimental conditions. The optimal parameters found were f = 100 Hz and Delta E(s) = 2 mV for scanning potentials from -0.85 to -0 .25 V in the presence of 0.003% gelatin and with supporting electrolyt e concentrations ranging from 0.02 to 0.05 M. Calibration curves were linear in the range 5 x 10(-8) to 1 x 10(-5) M with a detection limit of 1.5 x 10(-8) M. SWV is a sensitive technique, more rapid than all o ther described procedures for determining these pesticides, and allows analysis to be performed in the presence of an ionic matrix. The meth od was extended to the analysis of paraquat and diquat in potatoes aft er digestion in hot sulphuric acid, neutralization and solid phase ext raction. The detection limit has been found to be 1 mu g g(-1) for bot h pesticides.