Bromophenol blue-based colorimetric reagent for on-site alkalinity monitoring in highly saline oilfield produced waters via smartphone-based digital imaging.

A bromophenol blue-based colorimetric reagent was developed and applied to a digital imaging methodology for alkalinity determination in high-salinity waters, such as produced water. The reagent formulation combines bromophenol blue, formic acid as the acidifying agent, and a controlled saline medium, aiming to impose a dominant chemical environment that ensures stability, reproducibility, and reduced sensitivity to matrix effects. The analytical principle is based on the acidification of alkaline species present in the sample, followed by indirect estimation of alkalinity through the chromatic response of the indicator, which exhibits a pH-dependent color transition. Different compositions of colorimetric solutions were systematically evaluated in terms of stability, analytical range, and ionic strength effects, resulting in an optimized system capable of maintaining constant ionic strength and preventing turbidity even in complex saline matrices. Colorimetric analysis was performed using digital images acquired by a smartphone coupled to a portable 3D-printed device, and data processing was carried out using a convolutional neural network to correlate color information with alkalinity levels. The proposed method requires only 100 µL of sample and showed good agreement with conventional acid-base titration, with deviations within 10%. The controlled ionic strength strategy adopted in the reagent formulation represents a relevant contribution to colorimetric analytical chemistry, enabling reliable alkalinity measurements in saline systems where conventional methods are often limited. The proposed method was successfully applied to highly saline oilfield produced waters. The reduced sample and reagent consumption, combined with the portability of the detection system, highlights its potential for microanalytical applications in complex matrices.