Dispersive surfactant micelle-mediated extraction combined with a smartphone-based portable colorimeter: a cost-effective and simple approach for cobalt determination

In this research, we present a cost-effective, environmentally friendly methodology for the precise determination of trace levels of cobalt in various environmental matrices, based on a new surfactant micelle-mediated extraction combined with digital image analysis. Specifically, cetyltrimethylammonium bromide (CTAB) serves as the key surfactant. Prior to extraction, the conversion of Co2+ ions into hydrophobic species is achieved through the utilization of 1-nitroso 2-naphthol as a chelating agent. The procedure involves injecting a tetrahydrofuran (THF) solution of CTAB into water samples containing the target analytes and some added KI, resulting in the formation of a turbid solution due to CTAB dispersion within the medium. Following centrifugation, the resulting precipitate is re-dissolved in 1 mL of dimethylformamide and subjected to analysis using a self-constructed colorimeter, which is based on a mobile device. In the colorimeter, digital image analysis is conducted using the RGB color space, with the G channel value serving as the analytical signal. Our investigation encompasses the exploration and optimization of several critical parameters influencing the extraction and complex formation processes. Under optimal conditions, a linear range spanning 10-2.00 × 102 μg L-1 is achieved, exhibiting a correlation coefficient of 0.994. The detection limit (DL) is determined to be 4.1 μg L-1. The relative standard deviations for the determination of Co2+ at concentrations of 40 and 100 μg L-1 are found to be 7.0 and 6.6, respectively, for five replicates. Further assessments include an evaluation of the impact of common cations and anions on the proposed method, which subsequently qualifies it for the efficient preconcentration and quantification of cobalt in diverse environmental matrices.