New Quantitative Spectrophotometric Analysis in the Visible (VIS) Range of Nitrates, NO₃^-, from Fresh Groundwater Sources Intended for Domestic Consumption

The aim of this study was to conduct a precise analysis through a new spectrophotometric method that uses nitrate anions, NO₃^-, as species with significant toxic potential covertly existing in the composition of several samples of water intended for domestic consumption. Nitrates, NO₃^-, from the studied groundwater samples quantitatively reacted with phenol disulfonic acid, which led to the synthesis of an intense bright yellow nitro-phenol disulfonic acid compound in the presence of a solution of NaOH, 2 M, and ammonia, 10-12%, 2:1. Nitro-phenol disulfonic acid showed an absorption maximum at a wavelength of ʎ = 402 nm and was obtained in a quantitative proportion perfectly equivalent to the concentration of pure NO₃^- nitrate anions obtained from the water samples. Through spectrophotometrically dosing the bright-yellow intense nitro-derivative formed at λ = 402 nm in relation to double-distilled H₂O as a control, the concentrations expressed in mg/L of pure nitrate NO₃^- ions in the unknown water samples were directly determined. Following this method, the NO₃^- nitrate concentrations from all eight unknown groundwater sources were found to be below the maximum allowed reference limit of 50 mg/L. The method proposed was linear over the entire concentration range of the chosen standard solutions (4,00 µg/mL- 40 µg/mL). The linear regression coefficient was R² = 0.9995198, R² ≥ 0.9990, and the correlation coefficient was R = 0,9997599, R > 0.9990. The standard error of _the regression line was SE = 0,0059577, SE <<1. The detection limit was LOD = 0,8456 μg/ mL, LOD < 1, and the quantitation limit was LOQ = 2,8093 μg/ mL, LOQ < 3. The method used for the visible spectrophotometric analysis of nitrates was subsequently subjected to complete statistical validation.