Video colorimetry of single-chromophore systems based on vector analysis in the 3D color space: Unexpected hysteresis loops in oscillating chemical reactions

Colorimetry, the quantitative determination of color, usually of a digital image, has useful applications in diverse areas of research. Many methods have been proposed for translating the RGB data of an image to obtain concentration information. Among the many methods for RGB analysis, we focus on the vector projection method (VP), which is based on a vector analysis in 3D RGB color space. This method has the major advantages of being conceptually intelligible and generalizable to various systems. For solutions with variable concentrations of one chromophore, we will show that the analysis of the trace in RGB color space allows for a judgment about the reliability of the linear concentration dependence of the chromapostasi parameter. We discuss the theoretical underpinnings of the method in two test cases, a simple dye solution and a titration of an organic acid with phenolphthalein indicator. The VP method was then applied to the Ce-catalyzed Belousov-Zhabotinsky reaction with the expectation that the colorimetry would quantify [Ce⁴⁺] oscillations. Surprisingly, the 3D color space analysis revealed hysteresis loops and the origin and implications of this observation are discussed.