Colorimetric Oxygen Sensing by Analyzing Chromaticity Points for Polymer Cobalt Schiff Base in CIELAB

Abstract

Polymer membranes of a copolymer ligand (L) coordinated to tetra-coordinated N,N'-ethylene bis(salicylideneiminato) cobalt(II) at the fifth coordinating site of the center cobalt(II), Co(II)S-L, are prepared to measure colorimetrically the color changes in response to rapid reversible oxygen binding simultaneously with slow oxidation under moist conditions to yield Co(III)S-L. The chromaticity point observed in CIELAB is the endpoint of the vector sum of the unit vectors for Co(II)S-L, O₂-Co(II)S-L, and Co(III)S-L, scaled by their respective abundance ratios: (1 − m)(1 − n), m(1 − n), and n, where m is the ratio of O₂-Co(II)S-L to the sum of Co(II)S-L and O₂-Co(II)S-L and n is the ratio of Co(III)S-L to the total of the three cobalt complexes. The observed limited area in CIELAB as a stoichiometric Euclidean space is supported by the following two different reaction processes of the polymer membranes: an oxygen-binding reaction under several different partial pressures of oxygen supplied to the membranes in a short period, during which oxidation of the cobalt(II) complexes is negligible, and an oxidation reaction over a long period at a constant partial pressure of oxygen. The chromaticity points in both processes progress linearly in three different two-dimensional coordinates: a^*b^*, a^*L^*, and b^*L^*.