Determination of the relevant equilibrium constants working in pH Colorimetric Sensor Arrays (CSAs)

The rationalization of the behavior of pH Colorimetric Sensor Arrays, based on Bromophenol Blue embedded in tetraethyl orthosilicates-based matrices, was accomplished by calculating the thermodynamic constants of the reactions involved. The constants were determined in four matrices upon variation of the concentration of surfactant, indicator, and anionic species present in buffers and samples. Based on the values found, it was demonstrated that the variation of the surfactant concentration widened the working interval up to 1.80 and 5.30 pH units in the solution and one of the tested polymers, respectively. The competitive reaction between anionic species and cationic head-groups of the surfactant shifted the position of the calibration profile up to 1.50 pH units in solution. For the CSA, the shift was 0.35 pH units for a large concentration of surfactant (0.1 M) and negligible at low concentrations (<0.01 M). The proposed mechanism allowed for a very good curve fitting in all conditions.