On-line Determination of Sulfide by the ‘Methylene Blue Method’ With Diode-laser-based Fluorescence Detection

The application of an inexpensive, compact, solid-state, fluorescence-based detector for flow injection analysis to the determination of sulfide by the ‘Methylene Blue Method’, viz., production of Methylene Blue (MB) via the oxidative coupling of sulfide with N,N-dimethyl-p- phenylenediamine (DMPD) in the presence of iron(III), is described. The use of fluorescence-based detection allows the reaction to be performed on-line under less corrosive, albeit sub-optimum, reaction conditions. The detector uses a 670 nm diode laser as the excitation source and inexpensive photodiodes as detector elements; a color filter (used to block scattered laser light) is the only other optical component of the detector. The fluorescence signal resulting from the MB formed on-line is linear over the range 0.75–15.0 mg l–1 injected sulfide, with a limit of detection of 0.08 mg l–1 injected sulfide when 9.0 M H2SO4 is used in the DMPD carrier stream, and 1–2 mg l–1 when less acidic carrier streams are employed. Use of this method for analysis of sulfide unknowns in aqueous solution and in a simulated waste water matrix demonstrates that unknown sulfide samples can be analysed reproducibly.