Reverse-phase optode. Design and characterization of an organic acid-selective optode membrane based on fluorescein sodium

Hydrophilic gelatin membranes incorporating water soluble fluorescein sodium immobilized at the tip of an optical fiber were developed as the first 'reverse-phase' optode membrane system for the determination of organic acids as exemplified by benzoic acid. In contrast to the traditional optode membrane application, the fluorescence detection of organic acids by the present system was carried out in an organic medium (hexane). Under experimental conditions, the optical fiber coated membrane exhibited a linear response in the range 4.72 × 10^-4-4.72 × 10^-2 mol 1^-1 benzoic acid hexane solution with a slope of 0.2 relative fluorescent intensity units per concentration decade. The selectivity, response time, reproducibility and lifetime of this optical device are discussed.