A novel sensitive, selective, and reversible cobalt(II) ion optical sensor was prepared by the incorporation of 5-[o-carboxyphenylazo]2,4-dihydroxybenzoic acid [CPDB] and sodium tetraphenylborate (NaTPB) in a plasticized polyvinyl chloride (PVC) membrane containing dioctyl adipate (DOA) as a plasticizer. The influence of several parameters such as pH, base matrix, solvent mediator and reagent concentration was optimized. A comparison of the obtained results with those of previously reported sensors revealed that the proposed method, in addition to being fast and simple, provided a good linear range (0.05– 45.20 mM) and low detection limit (0.015 mM). Low detection and quantification limits and excellent selectivity in the presence of interfering ions such as Fe 3+ ,Cu 2+ ,Ni 2+ ,Ag + ,Au 3+ ,Cr , and SO 4 2
make it feasible to monitor Co 2+ ion content accurately and repeatedly in environmental samples with complicated matrices. The optode was regenerated successfully using 0.3 M nitric acid (HNO ) solution while its response was reversible with a relative standard deviation (RSD) lower than 1.9% for seven replicate determinations of 20 mMCo 3 2+ in various membranes. The optode was stable and was stored for at least 15 days without observing any change in its sensitivity.