Development of sulphide-selective optode membranes based on fluorescence quenching

Sulphide-selective optode membranes have been fabricated by immobilizing tetraoctylammonium fluorescein mercury(II) acetate (TOAFMA), tetraoctylammonium hydroxide (TOAOH) and tributyl phosphate into ethyl cellulose and these membranes have been used in sensing devices for optical determination of sulphide ions in pH buffered solutions. The sensing scheme is based on the fluorescence quenching of TOAFMA by hydrosulphide ions, which can be related to the concentration of the sulphide ions present in solution. The response behaviour of the membrane is pH dependent and has a wider linear response range at high pH values. At pH 9.00, 10.0 and 11.0 the useful ranges are 1.0 to 20 μM, 0.10 to 10 μM and 0.10 to 100 μM, respectively. The detection limit is 56 nM sulphide ion at pH 11.0. Interferences, response time and reproducibility are discussed.