CIELAB Color Space\nas a Field for Tracking Color-Changing\nChemical Reactions of Polymeric pH Indicators

Two types of cross-linked\npolymeric dye films that exhibit\nreversible\ncolor changes in response to pH were prepared. Upon protonation, films\nwith a nitrophenol dye moiety (NP) changed from yellow to colorless,\nwhereas films with an azobenzene dye moiety (AB) changed from yellow\nto reddish-purple. The colored light transmitted through these films\nfrom a D65 white-light source was measured colorimetrically, and the\ntransition of the chromaticity point in CIELAB upon protonation of\nthe polymeric dyes was observed. At low dye concentrations ([NP] <\n4.7 × 10<sup>–5</sup> and [AB] < 2.7 × 10<sup>–5</sup> mol L<sup>–1</sup>), linear loci of the chromaticity\npoints were displayed for both polymeric dye films, indicating perceptual\nlinearity. This linearity is caused by the avoidance of nonlinear\nperceptual phenomena such as the Abney effect and the Bezold–Brücke\nphenomenon. The perceptual linearity of space is a necessary condition\nfor achieving stoichiometric linearity, which requires an additional\nlinear relationship between the dye concentration and each component, <i>a</i>*, <i>b</i>*, and <i>L</i>*. A geometric\nequation that included the AB concentration and the protonation ratio\nof AB was used to successfully depict, on the basis of stoichiometric\nlinearity, the protonation of the polymeric AB films three-dimensionally\nin the restricted area of CIELAB.