Geometry in CIELAB color space for protonation of pH indicators

Abstract Ultraviolet-visible spectroscopy provides color information on organic compounds during chemical reactions. Absorption spectra of a pH indicator during protonation show absorption bands from electronic transitions based on molecular structure. Information from the bands including the wavelength of maximum absorption and calculated molar extinction coefficient can provide specific parameters for the reactant (the indicator) and the product (the protonated indicator). However, these parameters never directly present what chemical reaction is occurring, which is typically only determined after identification of the compounds with other characterization methods. Here, we show that a geometric object, which was built out of the loci of chromaticity points for a polymer pH-indicator in CIELAB color space, indicates that the reaction is protonation. The object contains information about species, i.e., the pH-indicating portion and its protonated form. Each locus for different initial concentrations was a line segment, which corresponded to the protonation process. The segments for a polymer pH indicator with azobenzene as the pH-indicating dye portion at different initial concentrations created a planar triangle in CIELAB color space. For another polymer pH indicator of nitrophenol, the geometric space was a half line. The corresponding geometric equations may be specific to protonation including information about the chemical species.