Estudos para a determinação amperométrica de nitrofosforados totais em águas em sistema de análise por injeção em fluxo

Abstract

An electrochemical determination methodology of Parathion was investigated based on the amperometric detection of multiple pulses in a flow injection analysis system (FIA). A propulsion system without pulsation and an electrochemical detector for flow analysis systems were built and investigated for this determination. Studies of the electrochemical behavior of Parathion on carbon electrodes were investigated in function of the pH and of the supporting electrolyte type. Glassy carbon electrode and citric acid 0.1 mol L-1 supplied the more favorable experimental conditions for the amperometric detection of Parathion. The studies of the amperometric detection of multiple pulses, in the generator-collector mode, under flow analysis conditions, showed that this detection mode increases the sensitivity and the selectivity of the electrochemical determination of Parathion. Stable amperometric signs and of greater sensitivity were obtained when pulse potentials were applied in the sequence: -0.5 V, +0.4 V and -0.1 V, during 30 ms each pulse. The application of these potential pulses involved the following electrode processes, respectively: reduction of RNO2 to RNHOH, oxidation of RNHOH to RNO and reduction of RNO to RNHOH. The relative standard deviation of the amperometric signs obtained at -0.5 V, +0.4 V and -0.1 V was, respectively: 1.5%, 2.0% and 1.4%. The studies showed that the amperometric detection of multiple pulses in the generator-collector mode also increases the selectivity of the nitrophosphates detection, because it allows to differentiate the amperometric sign due to these compounds through the amperometric sign of the electroative products formed at -0.5 V. An increase of the sensitivity of the analytical sign was reached through the algebraic sum, in module, of the amperometric signs obtained in -0.5 V, + 0.4 V and 0.1 V. Limit detection of 0.14 mg L-1 for Parathion and a linear range from 10 to 50 mg L-1 were obtained. These findings indicate that the methodology investigated in this work presents good repeatability, selectivity and sensibility, and it can be used for the determination of total nitrophosphates in natural waters. Recovery studies carried out with water samples of river (Rio Uberabinha) spiked with Parathion at level of 20 mg L-1 showed a recovery of 102%, with relative standard deviation of 1.7%.