Determinação voltamétrica de amônia em meio alcalino utilizando um eletrodo de Pt

Abstract

Electrochemical techniques have held a prominent place in modern Analytical Chemistry, as they often combine good analytical performance with low sample and reagent consumption, producing smaller amounts of waste and thus aligning with the principles of Green Chemistry. Therefore, in this work, the electrochemical behavior of ammonia in alkaline solutions was studied using a Pt electrode, and the analytical performance of different voltammetric techniques for this analyte was evaluated. A three-electrode electrochemical cell was used, with a Pt disk as the working electrode, a Pt wire as the auxiliary electrode, and an Ag/AgCl/KClsaturated reference electrode. The results obtained by cyclic voltammetry showed that the oxidation of ammonia generates an anodic peak at -0.3 V vs. Ag/AgCl/KClsaturated and that this process is irreversible and diffusion-controlled. Subsequently, the operational parameters of differential pulse voltammetry and square wave voltammetry techniques were optimized to obtain the most suitable experimental conditions for the determination of NH3. It was found that differential pulse voltammetry exhibited the best performance and was thus selected as the analytical technique to be applied. The possible interfering effect of SO42-, NO2-, Na+, K+, CH3COO-, NO3-, and Cl- ions was evaluated, with only sulfate ions showing statistically significant interference. Finally, four water samples were analyzed, three of which were from the Paranaíba River in Cachoeira Dourada, MG, and one from the city of Ituiutaba, MG. Among the samples analyzed, the determination of NH3 was considered accurate and precise for three of them, with recovery percentages ranging from 93 to 96%. For the remaining sample, a recovery percentage of 56% was obtained, requiring further studies to clarify the reasons for this low recovery percentage. The original concentrations of NH3 in the river water samples ranged from 0.09 to 0.13 ppm as N-ammonia, thus complying with the parameters allowed by CONAMA Resolution 357/2005. Therefore, it is concluded that the voltammetric method developed in this work is promising for the quantification of NH3 in natural and supply water samples.