Simple colorimetric assay using pectin hydrogel reagent coupled with camera-based photometry for trace arsenic determination

Humans mainly ingest arsenic through contaminated drinking water, causing serious health effects. The World Health Organization (WHO) has set the permissible limit of arsenic in drinking water at 0.01 mg/L and concentrations should be regularly determined to ensure a safe supply. In this study, a leucomalachite green (LMG) pectin-based hydrogel reagent was prepared that selectively reacted with arsenic over other metals including manganese, copper, lead, iron, and cadmium. Pectin, optimized at 0.2% (w/v), was used to form the hydrogel matrix. Arsenic reacts with potassium iodate in sodium acetate buffer medium to liberate iodine that then oxidizes LMG entrapped in pectin hydrogel to form a blue product. Camera-based photometry/ImageJ software was used to monitor the color intensity, eliminating the need for a spectrophotometer. The intensity of gray in the red channel was chosen as optimal for the red, green, and blue (RGB) analysis. The colorimetric assay revealed a dynamic detection range toward arsenic solution standards of 0.003-1 mg/L, covering the WHO recommendation of below 0.01 mg/L arsenic in drinking water. The assay gave recovery rates between 97 and 109% at a 95% confidence interval, with precision of 4-9%. Concentrations of arsenic in the spiked drinking water, tap water, and pond water samples monitored by the developed method agreed well with conventional inductively coupled plasma optical emission spectrometry. This assay showed promise for on-site quantitative analysis of arsenic in water samples.