Multi-wavelength spectrophotometric determination of pH using phenol red–doped sol–gel film sensors

Abstract

ABSTRACTDye leakage and the nonlinear relationship between absorbance and pH affect the pH measurement precision of absorption-based optic sensors and limit their application. Based on the preparation of a transparent sol–gel thin film with a phenol red indicator, this work assumed that the absorbancies had an exponential relationship to the concentrations of the indicator, and the multi-wavelength equation for the pH was derived as follows:where n is the exponential coefficient, e1, e2, and e3 are the extinction coefficient ratios, and R is the ratio of the absorbancies. The calculation parameters for an immobilized phenol red film were pKa = 8.577, e1 = 0.0191, e2 = 1.530, e3 = 0.2992, and n = 0.6322, while those for an immobilized phenol red/Triton X-100 film were pKa = 8.612, e1 = 0.0507, e2 = 1.501, e3 = 0.3687, and n = 0.3931. The correspondence between the calculated pKa and the pKa from the calibration curve verifies the equation. The calculation parameters remain stable as the indicator content is increased, but change with an increase in the Triton X-100 content. Using the equation, the calculated pH shows stable behavior over a long duration regardless of dye leakage, and the calculated precision is 0.0053. The method provides drift-free and precise water pH measurements for R that is not dependent on the total concentration of the indicator, and can be applied to online monitoring.