Optical pH Sensor Based on Quinizarin for Alkaline pH Regions.

The development of an optical ph sensor based on immobilization ofquinizarin (1,4-dihydroxyanthraquinone) on a triacetylcellulose membranewas described. the resulting membrane exhibited fast color change fromyellow to violet, while changing the ph in alkaline region. the immobilizedquinizarin 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.