A wide range pH optical sensor with mixture of Neutral Red and Thionin immobilized on an agarose film coated glass slide

A new pH optical sensor for a wide pH range was prepared by chemical immobilization of a mixture of two indicators of Neutral Red and Thionin on an agarose film coated glass slide (ACGS). For preparation of the sensor, the two dyes were immobilized on an epoxy activated agarose support and the effects of the coupling pH and the dyes ratio and concentrations were optimized. The sensor was mounted in a flow cell and successfully applied for on-line pH monitoring in a wide pH range of 0.5–12 with two linear calibration curve regions. The pH sensor could perform fast response (within 1–2 min) and long-term stability (continuous use for at least 3 months) with no evidence of dye leaching. The sample ionic strengths up to 0.5 mol L−1 did not significantly influence the responses. A reproducibly better than 0.20% (R.S.D.) was obtained for 6 replicated pH measurements.