Optodes based on bromothymol blue (BB) and bromocresol purple (BP) acid-base indicators were constructed. The dyes were chemically immobilised on the surface of a bifurcated glass fibre optical bundle, which was previously silanised with 3-aminopropyl-triethoxysilane. A PTFE/acrylic cell was specially constructed, allowing the use of the optodes as detector in flow analysis systems. The optodes were evaluated for their pH responses in different flow approaches, such as continuous flow, flow injection and monosegmented flow systems. A steady state signal was achieved in the continuous flow system. The effect of the flow rate and the injected volume on the signal amplitude were studied. Signals up to 69 and 77% for BB optode and up to 82 and 92% for the BP optode of the steady state signal can be obtained by employing flow injection and monosegmented flow systems, respectively. The sampling frequency can be improved and sample consume can be lowered by employing the monosegment flow approach. © 1998 Elsevier Science S.A. All rights reserved.511-3382390Blair, D.A., Cynkowski, T., Bachas, L.G., Fluorocarbon-based immobilization of a fluoroionophore for preparation of fiber optic sensors (1993) Anal. Chem., 65, pp. 945-947De Oliveira, W.A., Narayanaswamy, R., A flow-cell optosensor for lead based on immobilized dithizone (1992) Talanta, 39, pp. 1494-1503Carroll, M.K., Bright, F.V., Hieftje, G.M., Fiber-optic time-resolved fluorescence sensor for the simultaneous determination of Al 3+ and Ga 3+ or In 3+ (1989) Anal. Chem., 61, pp. 1768-1772Suzuki, K., Tohda, K., Tanda, Y., Ohzara, H., Nishihama, S., Inoue, H., Shirai, T., Fiber-optic magnesium and calcium ion sensor based on a natural carboxylic polyether antibiotic (1989) Anal. Chem., 61, pp. 382-384Serra, G., Schirone, A., Boniforte, R., Fibre-optic pH sensor for sea-water monitoring using a single dye (1990) Anal. Chim. Acta, 232, pp. 337-344Kirkbright, G.F., Narayanaswamy, R., Welti, N.A., Fibre-optic probe based on the use of an immobilised colorimetric indicator (1984) Analyst, 109, pp. 1025-1028Alabbas, S.H., Ashworth, D.C., Narayanaswamy, R., Design and performance features of an optical-fibre reflectance pH sensor (1989) Anal. Proc., 26, pp. 373-375Motellier, S., Toulhoat, P., Modified acid-base behaviour of resin-bound pH indicator (1993) Anal. Chim. Acta, 271, pp. 323-329Baker, M.E.J., Narayanaswamy, R., The modeling and control of the response of an immobilised indicator (1995) Sensors Actuators B, 29, pp. 368-373Alabbas, S.H., Ashworth, D.C., Bezzaa, B., Momin, S.A., Narayanaswamy, R., Factors affecting the response time of an optical-fibre reflectance pH sensor (1996) Sensors Actuators A, 51, pp. 129-134McGlip, J., Glass cages make sense (1991) Phys. World, 11, pp. 23-25Baldini, F., Bracci, S., Cossi, F., Bechi, P., Pucciani, F., Controlledpore glasses embedded in plastic optical fibers for gastric pH sensing purposes (1994) Appl. Spectrosc., 48, pp. 549-552Wolfbeis, O.S., Offenbacher, H., Fluorescence sensor for monitoring ionic strength and physiological pH values (1986) Sensors Actuators B, 9, pp. 85-91Shulman, S.G., Chen, S., Bai, F., Leiner, M.J.P., Weis, L., Wolfbeis, O.S., Dependence of the fluorescence of immobilized 1-hydroxypyrene-3,6,8-trisulfonate on solution pH: Extension of the range of applicability of a pH fluorosensor (1995) Anal. Chim. Acta, 304, pp. 165-170Jones, J.P., Porter, M.D., Optical pH sensor based on the chemical modification of a porous polymer film (1988) Anal. Chem., 60, pp. 404-406Werner, T., Huber, C., Heinl, S., Kollmannsberger, M., Daub, J., Wolfbeis, O.S., Novel optical pH-sensor based on a boradiaza-indacene derivative (1997) Fresenius J. Anal. Chem., 359, pp. 150-154Cardwell, T.J., Cattrall, R.W., Deady, L.W., Dorkos, M., O'Conell, G.R., A fast response membrane-based pH indicator optode (1993) Talanta, 40, pp. 765-768Kostov, Y., Tzonkov, S., Krysteva, L., Membranes for optical pH sensors (1993) Anal. Chim. Acta, 280, pp. 15-19Ruzicka, J., Hansen, E.H., Flow injection analysis. Part I. A new concept of fast continuous flow analysis (1975) Anal. Chim. Acta, 78, pp. 145-157Pasquini, C., Oliveira, W.A., Monosegmented system for continuous flow analysis. Spectrophotometric determination of chromium (VI), ammonia, and phosphorus (1985) Anal. Chem., 57, pp. 2575-2579Van Der Linden, W.E., Classification and definition of analytical methods based on flowing media (1994) Pure Appl. Chem., 66, pp. 2493-2500Woods, B.A., Ruzicka, J., Christian, G.D., Rose, N.J., Charlson, R.J., Measurements of rainwater pH by optosensing flow injection analysis (1988) Analyst, 113, pp. 301-306Woods, B.A., Ruzicka, J., Christian, G.D., Charlson, R.J., Measurement of pH in solutions of low buffering capacity and low ionic strength by optosensing flow injection analysis (1986) Anal. Chem., 58, pp. 2496-2502Holobar, A., Weigl, B.H., Trettnak, W., Benes, R., Lehmann, H., Rodriguez, N.V., Wollschlager, A., Wolfbeis, O.S., Experimental results on an optical pH measurement system for bioreactors (1993) Sensors Actuators B, 11, pp. 425-430Kirkbright, G.F., Narayanaswamy, R., Welti, N.A., Studies with immobilized chemical reagents using a flow-cell for the development of chemically sensitive fibre-optic devices (1984) Analyst, 109, pp. 15-17Posch, H., Leiner, M.J.P., Wolfbeis, O.S., Towards a gastric pH-sensor: An optrode for the pH 0-7 range (1989) Fresenius J. Anal. Chem., 334, pp. 162-165Werner, T., Wolfbeis, O.S., Optical sensor for the pH 10-13 range using a new support material (1993) Fresenius J. Anal. Chem., 346, pp. 564-568Perrin, D.D., Dempsey, B., (1974) Buffers for pH and Metal Ion Control, p. 156. , Chapman and Hall, LondonMalcome-Lawes, D.J., (1988) Microcomputers and Laboratory Instru-mentation, 2nd Edn, pp. 147-178. , Plenum Press, New YorkBergamin Fo, H., Medeiros, J.X., Reis, B.F., Zagatto, E.A.G., Solvent extraction in continuous flow injection analysis. Determination of molybdenum in plant material (1978) Anal. Chim. Acta, 101, pp. 9-16Norris, J.O.W., Current status and prospects for the use of optical fibres in chemical analysis (1989) Analyst, 114, pp. 1359-1372Narayanaswamy, R., Optical chemical sensors: Transduction and signal processing (1993) Analyst, 118, pp. 317-322Munkholm, C., Walt, D.R., Polymer modification of fibre optic chemical sensors as a method of enhancing fluorescence signal for pH measurement (1986) Anal. Chem., 58, pp. 1427-1430Bacci, M., Baldini, F., Scheggi, A.M., Spectrophotometric investigations on immobilized acid-base indicators (1988) Anal. Chim. Acta, 207, pp. 343-348