An optical pH sensor with a linear response over a broad range Original Research Article

A method was developed for achieving a linear response over a broad pH range for optical pH sensors by using mixtures of multiple pH indicators. Requirements on the pH indicators were investigated, including the ΔpKa between the indicators, and the colors and concentrations of the indicators. It is demonstrated that a linear response over a broad pH range can be obtained by using multiple indicators that have ΔpKa ≤ 1.7. Optimum ratios between the concentrations of the indicators can be calculated by a computer program. A linear response over a range of 4.5 pH units was obtained for a mixture of four indicators in solution. A sol–gel glass pH sensor with a linear response over a range of 3.5 pH units was developed by co-entrapping three indicators in sol–gel glass. In both cases, a standard deviation of pH equal to 0.03 was obtained.