An organopalladium-PVC membrane for sulphur dioxide optical sensing

The development of a sensing membrane for the determination of gaseous sulphur dioxide is described. An organopalladium complex, Pd2(dpm)2Cl2 (dichloro-bis-(diphenylphosphine)-methane dipalladium I), was immobilised in a poly(vinyl chloride) thin film plasticised with ortho-nitrophenyloctylether (o-NPOE). Several membranes were prepared, using 20, 25 and 30% of PVC; 1, 5 and 8% of palladium complex and enough o-NPOE to make a total of 100 mg, which was dissolved in 1.0 mL of THF. The sensing membranes were obtained by manual deposition of 10, 15 and 20 μL of the solution onto cellulose acetate films, which were left to dry for 24 h and then stored in a desiccator sheltered from ambient light. The membrane was placed in an acrylic flow cell, in which the common end of a bifurcated optical fibre bundle was adapted and placed 1 mm from the membrane. Reflectance measurements were performed from 400 to 800 nm, after exposing the membrane to 0 to 500 ppmv of SO2 in air, at a flow rate of 500 mL min−1. The membrane composed of 20% PVC, 72% o-NPOE and 8% palladium complex showed the best performance and the film prepared from 20 μl of solution provided the highest signal (ca 160 mV upon exposure to 500 ppmv SO2). Measurements made at 530 nm showed a linear response range up to 300 ppmv, with a detection limit of 3.5 ppmv. By employing a flow rate of 500 mL min−1, response times (t90%) of 3 and 2 min were obtained for reaction of the membrane with SO2 and for reverse response upon exposure to dry nitrogen, respectively. Although the palladium complex also reacts with carbon monoxide in solution, this interference was not observed for concentrations up to 1000 ppmv of CO. The membrane showed a lifetime of ca. 2 months when stored in a desiccator.