Combination of a methylene blue-iodide system with a smartphone-based portable colorimeter for the on-site determination of nitrite

Abstract

In this study, a simple, inexpensive and rapid technique has been developed for the on-site determination of nitrite in different water samples by utilizing a methylene blue–iodide system and a smartphone-based portable colorimeter. The methylene blue–iodide system involves the bleaching of the bluish color of methylene blue dye as a result of triiodide action, which occurs due to the reaction between potassium iodide and nitrite in an acidic medium. In order to assess the intensity of the blue color of the bleached solution, a digital image of the solution was obtained using a smartphone and converted to the primary colors (Red (R), Green (G) and Blue (B)) using a free application. Then, the green channel value was considered as the analytical signal. The effects of different parameters influencing the methylene blue–iodide system were investigated and optimized. Under optimal conditions, the calibration graph was found to be linear in the range of 0.1–1 mg L⁻¹ with a limit of detection (LOD) of 0.028 mg L⁻¹. Relative standard deviation for the determination of 0.5 mg L⁻¹ of nitrite was found to be 2.2% for six replicates. The proposed method was successfully applied for the on-site determination of nitrite ions in snow and rainwater samples with a satisfactory recovery rate ranging from 90% to 97%. In addition, the experiments indicate that the results obtained by using a digital image analysis are consistent with those obtained by using a commercial UV-Vis spectrophotometer.