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

Abstract

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.