Optical pH Sensor Based on Quinizarin for Alkaline pH Regions

The development of an optical pH sensor based on immobilization of quinizarin (1,4-dihydroxyanthraquinone) on a triacetylcellulose membrane was described. The resulting membrane exhibited fast color change from yellow to violet, while changing the pH in alkaline region. The immobilized quinizarin showed a broader dynamic range from (pH 9.4-11.4) with respect to free form (pH 8.8-10.4). The sensor response was fully reversible and fast (<20 s) under the batch conditions with pH between 7 and 12.5. The sensor showed excellent long term stability (more than three months) and high reproducibility (R.S.D. less than 2%). The Ka values for free and immobilized dye were evaluated and the chemical reactivity of the quinizarin in solution and after immobilization was also described. Prog.