High-spatial-resolution characterization of hydroxide ion concentration in solutions: A novel strategy based on RGB color analysis.

In this work, a novel method for the quantification of hydroxide ion concentration ( [Formula: see text] ) in aqueous solutions was developed based on RGB (color saturation) analysis, which operates on a principle fundamentally different from the Lambert-Beer law. A theoretical equation correlating the solution color saturation (S) and [Formula: see text] was established as ( [Formula: see text] , m and n were constant), and the linear relationship between lg(1-S) and [Formula: see text] was validated by experimental data. The quantification results exhibited RSD ( [Formula: see text] ) 0.13% - 0.60% (n = 6), Er 1.22% - 3.34%, and LOD 0.11 μmolˑL-1. High-spatial-resolution distribution maps of [Formula: see text] near the cathode surface were acquired in a non-invasive, in-situ dynamic measurement way. Under the experimental conditions, the width of the concentration polarization region ( [Formula: see text] 35.14 - 39.50 μmolˑL-1) was approximately 0.6 mm. In the region 1.4 - 2.2 mm away from the electrode surface, [Formula: see text] decreased rapidly at a gradient of 21.31 μmolˑL-1ˑmm-1, and finally declined to the bulk concentration (<1.24 μmolˑL-1).