Determination of hydrogen sulfide in seawater using flow

Flow injection analysis (FIA) and flow analysis (FA) were used to automate the calorimetric determination of hydrogen sulfide by the methylene blue method. A low-sensitivity FIA manifold for concentrations up to 200 PM H,S had a detection limit of 1.2 FM. A highsensitivity FIA manifold for concentrations up to 75 PM H,S had a detection limit of 0.12 PM. The C.V. was < 1% at concentrations > 10 PM. Sulfide standards were calibrated by calorimetric measurement of the excess triiodide ion remaining after reacting sulfide with iodine. The FIA method was tested onboard ship with samples from the Galapagos hydrothermal vents and compared with results obtained in situ using FA. Hydrogen sulfide frequently occurs in oxygen-deficient areas of the ocean such as coastal lagoons, stagnant basins, and organic-rich sediments where circulation is restricted. Rare occurrences in the open sea beneath areas with intense productivity have also been reported (Dugdale et al. 1977). Hydrogen sulfide is produced by bacterial reduction of sulfate during oxidation of organic carbon in these areas after dissolved oxygen, nitrate, nitrite, and manganese oxides have been consumed. It is also found in deep-sea hydrothermal solutions, where it is produced by high-temperature interactions between seawater and rock (Edmond et al. 1979; McDuff and Edmond 1982). Dissolved sulfide can be an important energy source to chemoautotrophic organisms near the boundaries of these regions where oxygen and sulfide coexist in thermodynamic disequilibrium. The oxic-anoxic interface is characterized by sharp gradients due to the reactivity of sulfide and oxygen and the frequently low mixing rates. For the high-resolution measurements at the interface necessary to adequately describe these regions it is desirable to automate the analysis of sulfide. The measurements are based on the highly sensitive and specific reaction of dimeth’ This work was supported by National Science Foundation grant OCE 8311256. yl-p-phenylenediamine with sulfide in the presence of Fe(III) in an acidic medium. The product of the reaction, methylene blue, is the tetramethyl derivative of Lauth’s violet (Strickland and Parsons 1972; Fonselius 1983). Variations of the methylene blue method have been used routinely for the manual determination of sulfide in seawater (Cline 1969; Fonselius 1983). Automation of the technique by segmented continuous flow analysis (AutoAnalyzers) has been reported (Grasshoff and Chan 197 1). We have adapted the methylene blue method to flow injection analysis (FIA) for several reasons. The high sampling rates and short lag times available with FIA result in high-resolution data in near-real time. Also, a modified FIA is amenable for use with a submersible chemical analyzer (SCANNER) developed in our laboratory (Johnson et al. 1986a). We report here our methods for the determination of sulfide by flow injection analysis onboard ship or in shorebased laboratories and by flow analysis for measurements in situ with the SCANNER. All solutions were prepared from reagent grade chemicals and distilled deionized water passed through a Millipore Milli-Q water system. Amine solution: Dissolve 0.48 g of N,Ndimethyl-p-phenylene diamine dihydrochloride in 100 ml 6 N HCl and dilute to 1 liter. Protect the solution from light. Prepare a fresh solution every 2 days. Ferric chloride solution: Dissolve 1.4 g of FeCl, (anhydrous) in 100 ml of 6 N HCl and dilute to 1 liter. Prepare a fresh solution every 2 days. Zodate standard: Dry KIO, at 115°C for about 1 h. Weigh exactly 356.7 mg (1.667 mmol) and make up to 1,000 ml. Stock sul&de solution: Deoxygenate 500 ml of water for 1 h with an inert gas in a glass aspirator bottle. Rinse crystals of Na,S . 9H,O (to remove any sodium sulfite) and wipe dry. Weigh 12.00 g of the clearest crystals and dissolve in the deoxygenated water,