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.