Development of a selective optical sensor for Cr(VI) monitoring in polluted waters

An optical sensor is described for the sensitive and selective determination of the Cr(VI) ion in aqueous solutions. The optode membrane is prepared by incorporating Aliquat 336 as an ionophore and a chromoionophore (4′,5′-dibromofluorescein octadecyl ester) in a poly(vinyl chloride) (PVC) membrane containing ortho-nitrophenyloctyl ether (o-NPOE) as a plasticizer. The response to Cr(VI) is indicated by co-extraction of the target species and of hydrogen ions into the bulk of the membrane yielding large absorbance changes which can easily be measured in the visible spectral range. The optode membrane shows a reversible response in the concentration range of 1.1 × 10−5–1.0 × 10−3 M and has been shown to be more selective towards the HCrO4− ion than other anions with a selectivity pattern HCrO4− > SCN− ≈ ClO4− > NO3− ≈ I− ≈ NO2− ≫ H2PO4− ≈ Cl− ≈ SO42−. The sensing method developed has successfully been applied to the determination of Cr(VI) in spiked river water as well as in electroplating rinse waters.