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.