Fluorimetric optode membrane for sulfide detection

Sulfide-selective optode membranes have been fabricated by immobilizing tetraoctylammonium fluorescein mercuric acetate (TOAFMA), tetraoctylammonium hydroxide and tri-n-butyl phosphate into ethyl cellulose. These membranes were coated on overhead transparency films as solid supports and employed as sensing devices for fluorimetric determination of sulfide ions in pH buffered solutions. The sensing scheme is based on a ground state chemical reaction of TOAFMA and hydrosulfide leading to the formation of a non-fluorescent fluorescein mercuric sulfide, which can be related to the concentration of the sulfide ions present in solution. The response behaviour of the membrane is slightly pH dependent at pH values higher than 10.0. At pH 12.5, the useful detection range is 0.07 to 4.4 μmol dm^-3 of sulfide ions since this range has better response to Na₂S. The detection limit is calculated to be 0.07 μmol dm^-3. The concentrations of sulfide ions in spiked water samples determined by the sulfide-selective optode membranes at pH 12.5 were found to reasonably agree with the added concentrations. The optode membrane has a fast response time of less than 1 min. The fluorescence signal of the optode membrane can easily be recovered by rinsing with a solution of 0.42 mmol dm^-3 of sodium hypochlorite and 0.10 mmol dm^-3 of sodium acetate. Anions including bromide, nitrate, perchlorate and dichromate interfere seriously with the detection but other anions such as sulfite, oxalate, thiosulfate, iodide, sulfate, nitrite, acetate, fluoride, chloride and carbonate either do not interfere or interfere slightly.