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 ppm v 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 ppm v SO2). Measurements made at 530 nm showed a linear response range up to 300 ppmv, with a detection limit of 3.5 ppm v. 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. © 2004 Elsevier B.V. All rights reserved.1071 SPEC. ISS.4752http://www.epa.gov/air/airtrends/sulfur2.html, accessed on 11/march/2004Kuratli, M., Pretsch, E., Sulfur dioxide-selective optodes (1994) Anal. Chem., 66, pp. 85-91Carballo, R., Dall'Orto, V.C., Lo Balbo, A., Rezzano, I., Determination of sulfide by flow injection analysis using a poly [Ni-(protoporphyrin IX)] chemically modified electrode (2003) Sens. Actuators B, 88, pp. 155-161Chiou, C.Y., Chou, T.C., Amperometric SO2 gas sensor based on solid polymer electrolytes (2002) Sens. Actuators B, 87, pp. 1-7Li, H., Wang, Q.J., Xu, J.M., Zhang, W., Jin, L.T., A novel nano-Au- assembled amperometric SO2 gas sensor: Preparation, characterization and sensing behavior (2002) Sens. Actuators B, 87, pp. 18-24Hodgson, A.W.E., Jacquinot, P., Hauser, P.C., Electrochemical sensor for the detection of SO2 in low-ppb range (1999) Anal. Chem., 71, pp. 2831-2837Matsuguchi, M., Tamai, K., Sakai, Y., SO2 gas sensors using polymers with different amino groups (2001) Sens. Actuators B, 77, pp. 363-367Guimarães, M.O., Detection of sulfur dioxide using piezoelectric quartz crystal microbalance (1997) Anal. Lett., 30, pp. 2159-2174Mohr, G.J., Draxler, S., Trznadel, K., Lehmann, F., Lippitsch, M.E., Synthesis and characterization of fluorophore-absorber pairs for sensing of ammonia based on fluorescence (1998) Anal. Chim. Acta, 360, pp. 119-128Raimundo Jr., I.M., Narayanaswamy, R., Simultaneous determination of relative humidity and ammonia in air employing an optical fibre sensor and artificial neural network (2001) Sens. Actuators B, 74, pp. 60-68Choi, M.M., Xiao, D., Oxygen-sensitive reverse-phase optode membrane using silica gel-absorbed ruthenium (II) complex embedded in gelatin film (1999) Anal. Chim. Acta, 387, pp. 197-205Miller, B., Hauser, P.C., Fluorescence optical sensor for low concentrations of dissolved carbon dioxide (1996) Analyst, 121, pp. 339-343Freeman, M.K., Bachas, L.G., Fiber optic sensor for NOx (1992) Anal. Chim. Acta, 256, pp. 269-275Nezel, T., Fakler, A., Zhylyak, G., Mohr, G.J., Spichiger-Keller, U.E., A highly sensitive NO2-selective optode membrane (2000) Sens. Actuators B, 70, pp. 165-169Stangelmayer, A., Klimant, I., Wolfbeis, O.S., Optical sensors for dissolved sulfur dioxide (1998) Fresenius J. Anal. Chem., 362, pp. 73-76De Marcos, S., Alcubierre, N., Galbán, J., Castillo, J.R., Reagentless system for sulphite based on polyaniline (2004) Anal. Chim. Acta, 502, pp. 7-13Razek, T.M.A., Miller, M.J., Hassan, S.S.M., Arnold, M.A., Optical sensor for sulfur dioxide based on fluorescence quenching (1999) Talanta, 50, pp. 491-498Lin, J.M., Qu, F., Yamada, M., Chemiluminescent investigation of tris(2,2′-bipyridyl)ruthenium(II) immobilized on a cationic ion-exchange resin and its application to analysis (2002) Anal. Bioanal. Chem., 374, pp. 1159-1164Papkovsky, D., Uskova, M.A., Ponomarev, G.V., Korpela, T., Kulmala, S., Guilbault, G.G., Optical sensing of sulfite with a phosphorescent probe (1998) Anal. Chim. Acta, 374, pp. 1-9Gimenez, I.F., Alves, O.L., Immobilisation of complexes containing metal-metal bonds in porous Vycor glass and subsequent insertion of gas molecules into the Pd-Pd bonds (2002) Glass Technol., pp. 166-169Benner, L.S., Olmstead, M.M., Hope, H., Balch, A.L., Reversible insertion of sulfur dioxide into palladium-palladium bonds. the preparation and structure of Pd2(μ-SO2) (Ph 2PCH2PPh2)2Cl2 (1978) J. Organomet. Chem., 153, pp. 31-35Benner, L.S., Balch, A.L., Novel reactions of metal-metal bonds. Insertion of isocyanides and carbon monoxide into the palladium-palladium bond of some Pd (I) dimmers (1978) J. Am. Chem. Soc., 13, pp. 6099-6106Balch, A.L., Benner, L.S., Olmstead, M.M., Novel reactions of metal-metal bonds. Adittion of sulfur dioxide and sulfur to Pd2(μ-SO2) (Ph2PCH 2PPh2)2Cl2 and the oxidation of coordinated sulfide (1979) Inorg. Chem., 18, pp. 2996-3003Brook, T.E., Narayanaswamy, R., Polymeric films in optical gas sensor (1998) Sens. Actuators B, 51, pp. 77-83Wang, E., Meyerhoff, M.E., Anion selective optical sensing with metalloporphyrin-doped polymeric films (1993) Anal. Chim. Acta, 283, pp. 673-682