Development of Green-Assessed and Highly Sensitive Spectrophotometric Methods for Ultra-Low-Level Nitrite Determination Using Rhodanine and 7-Hydroxycoumarin in Environmental Samples

Rapid, sensitive, and environmentally sustainable spectrophotometric methods for the determination of nitrite (NO2−) in environmental specimens are proposed. The presented procedures are grounded in the diazotization of sulphathiazole (STZ), followed by coupling with rhodanine (RDN) or 7-hydroxycoumarin (7-HC) in an alkaline medium, and the results were studied. This reaction gave an intense soluble red color at 504 nm and a pale red color at 525 nm for RDN and 7-HC, respectively. The conditions producing the maximum performance and other important analytical criteria in relation to the proposed procedures were investigated to enhance their sensitivity. Beer’s law was abided by for NO2− over the concentration ranges of 0.08–2.0 µg mL−1 and 0.04–2.4 µg mL−1 using RDN and 7-HC, respectively. The lower limit of detection (LLOD), lower limit of quantification (LLOQ), molar absorptivity (ε), and Sandell’s sensitivity were calculated as follows: 0.0303 µg mL−1, 0.0918 µg mL−1, 4.20 × 104 L mol−1 cm−1, and 1.63 × 10−6 µg cm−2 (in the case of RDN); and 0.0387 µg mL−1, 0.1172 µg mL−1, 6.90 × 104 L mol−1 cm−1, and 1.00 × 10−6 µg cm−2 (in case of 7-HC). Furthermore, the ecological implications were assessed using three green assessment methodologies: Analytical Eco-Scale (ESA), Analytical GREEnness metric (AGREE), and Green Analytical Procedure Index (GAPI). Thus, our proposed procedures are fully validated and implemented in order to carry out NO2− quantification in the selected ecological samples (water and soil samples).