AbstractA heterogeneous membrane pH sensor based on chitosan, polyvinyl chloride (PVC) and a suitable plasticizer was developed. The best overall performance was shown by a membrane with composition ratio of chitosan (ionophore) : PVC (binder) : 2-nitrophenyl octyl ether (2-NPOE) (plasticizer) at 20 : 30 : 30 (w/w) respectively. The potentiometric responses were linear within pH 2 to 9, limit of detection at pH 9.5, slope 56.6 ± 0.4 mV per decade at 25.0 ± 1.0 oC and response time of 20.0 ± 5.0 seconds. A life time of more than 3 months was obtained. The sensor showed good selectivity and reproducibility towards hydrogen ion in the presence of a number interfering cations. Results were in agreement with a commercial pH electrode for pH measurement in fruit juice samples. The sensor has been utilized in the flow injection analysis (FIA) of pH at standard solution. The slope of 57.1 ± 0.2 mV per decade was linear at pH range of 2 to 7 at 25.0 ± 1.0 oC was obtained..