Co(II) triggered radical reaction between SO2 and o-phenylenediamine for highly selective visual colorimetric detection of SO2 gas and its derivatives

Rapid, reliable, and highly selective detection of SO 2 and its derivatives has an especially important practical significance in terms of environmental monitoring and human health. In this study, we describe a novel Co 2+ triggered radical reaction pathway for visual colorimetric detection of SO 2 and SO 3 2− . The reaction between Co 2+ and SO 2 or SO 3 2− with the assistance of dissolved oxygen produces highly active sulfate radical, leading to the oxidation of o-phenylenediamine (OPD). The oxidation products containing 2,3-dinitro-phenazinemethane, OPD dimer, and OPD trimer show strong UV–vis absorption. Consequently, SO 2 and SO 3 2− are quantitatively determined using the Co 2+ -OPD colorimetric detection system. This colorimetric assay exhibits a 60–650 μM and 30–70 ppm linear ranges for detection of SO 3 2− and SO 2 , respectively. The corresponding limit of detection is calculated to be 4.07 μM and 1.26 ppm for SO 3 2− and SO 2 . What is more, a distinct color change can be found at a SO 3 2− concentration of 50 μM by the naked-eye observation. The specific radical detection guarantees that the present colorimetric assay has a good anti-interference ability over common inorganic ions and gases.