Aplicação de técnicas eletroquímicas para a quantificação de amônia

Abstract

This study presents the development of an electrochemical device fabricated using platinum wires, modified with electrodeposited Pt microparticles, for the voltammetric determination of NH3 in natural and drinking waters. Ammonia plays a relevant role in the nitrogen cycle and has a negative environmental impact when present at high concentrations due to its ability to cause eutrophication in aquatic environments. The method proposed in this study employs differential pulse voltammetry in an alkaline medium, aiming to combine instrumental simplicity, low reagent consumption, and high analytical sensitivity. Initially, three miniaturized devices were fabricated and characterized using a Pt wire (⌀ = 0.5 mm) as the working electrode. Subsequently, optimization studies of experimental conditions were carried out, including the type of abrasive used for working electrode polishing, the concentration of the supporting electrolyte, and the presence of dissolved oxygen. It was found that the use of alumina as an abrasive and the presence of dissolved oxygen in solution provided improved voltammetric responses and greater repeatability, with RSD for ammonia oxidation peak currents lower than 10%. The modification of the working electrode surface with electrodeposited Pt microparticles increased sensitivity by approximately threefold, and lowered limits of detection (LOD = 0.57 µmol L-1) and quantification (LOQ = 1.92 µmol L-1) compared to the unmodified Pt electrode (LOD = 0.9 µmol L-1 e LOQ = 3.0 µmol L-1). The linear range obtained with the modified electrode was from 0.5 to 50 µmol L-1 (0.007 to 0.7 ppm N), which meets the Brazilian drinking-water regulatory limits (Resolution 357 of 2005). The applicability of the miniaturized device was assessed through the analysis of four water samples, showing satisfactory accuracy and precision for all samples in accordance with AOAC International criteria. NH3 recoveries ranged from 92 to 103%, meeting validation requirements according to the AOAC guide. It is concluded that the proposed miniaturized system is a promising tool for the determination of NH3 in aquatic environments, offering advantages such as high sensitivity, low sample and reagent consumption, reduced waste generation, and minimal sample preparation requirements.