An optical sensor for determination of low pH values based on covalent immobilization of Congo red on triacetyl cellulose films via epichlorohydrin

•An optode for determination of pH at low pH regions.•Epichlorohydrin is used as a bridge to connect the dye to the bed.•The stability of the optode is very high due to strong covalent bonds.

A new method for immobilization of some indicators with amino groups on triacetyl cellulose beds was applied for construction of an optical sensor for measurement of low pH values. Congo red was used as a pH indicator which was immobilized covalently on triacetyl cellulose via epichlorohydrin. The response of the optical sensor to pH of the solutions was found to be 0-4.5. The effect of influencing parameters on activation of the hydrolyzed film by epichlorohydrin and also on the reaction of the activated film with the indicator was studied and they were optimized. Under optimum experimental conditions, the obtained results showed that the constructed optical sensor has a good repeatability, reproducibility, short term and long term stability (more than 9 months) and a relatively short response time (about 150 s) for a relatively wide pH range.

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