A new amperometric microsensor for detection of dissolved H 2 S in aquatic environments was developed. The design of the microsensor is based on the same principle as the Clark-type oxygen microsensor. The sensor is equipped with a glass-coated platinum working electrode and a platinum guard electrode positioned in an outer glass casing (tip diameter 20-100 µm). Both working electrode and guard electrode were polarized at a fixed value in the range from +85 to + 150 mV with respect to a counter electrode. The outer casing is sealed with a thin silicone membrane and filled with a buffered electrolyte solution containing ferricyanide (K 3 [Fe(CN) 6 ]) as redox mediator. Hydrogen sulfide penetrates the silicone membrane and is oxidized by K 3 [Fe(CN) 6 ], resulting in the formation of elemental sulfur and ferrocyanide (K 4 -[Fe(CN) 6 ]). The latter is electrochemically reoxidized at the exposed end of the platinum working electrode, thereby creating a current that is directly proportional to the dissolved H 2 S concentration at the sensor tip. The sensor was characterized and calibrated in a flow-through cell combined with a coulometric sulfide generator. Difficult studies including the determination of H 2 S with high spatial and temporal resolution seem to be possible.

We developed a microsensor for the amperometric detection of dissolved hydrogen sulfide, H,S, in sedirnents and biofilms. The mlcrosensor exhibits a fast (tgO c0.2 to 0.5 S ) and linear response to H2S over a concentration range of 1 to >l000 pm01 H2S I-', and has a low stirring dependency of the microsensor signal (c1 to 2%). We used the new mlcrosensor to obtain the first microprofiles of H2S in an acidic lake sediment with a several cm thick flocculant surface layer. Despite the low pH of 4.6, a relative low SO,'-level in the lake water, and a broad O2 respiration zone of ca 6 mm, we were able to measure H2S depth profiles in the sediment at a good resolution, that allowed for calculation of specific sulfate reduction and H2S oxidation activities. Such calculations showed highest sulfate reduction activity in the anoxic sediment down to ca 20 mm depth A comparison of calculated area1 rates of O2 consumption and sulfate reduction Indicated that sulfate reduction accounted for up to 13 % of total organic carbon mineralization in the acidic sediment. All produced H2S was reoxldized aerobically with O2 at the oxic-anoxic interface. In addition to its good performance in acidic environments, the new H2S microsensor has proven useful for sulfide measurements in neutral and moderate alkaline (pH < 9) biofilms and sediments, and thus is a true alternative to the traditionally used potentiometric Ag/Ag2S microelectrode for most applications in aquatic ecology and biogeochemistry.

A galvanic cell for trace determination of hydrogen sulfide in gases is presented. The cell covers the range from 30 ppb to some hundred ppm. It is characterized by simple construction. The use of ferricyanide/ferrocyanide ions as a redox mediator system ensures very low cross sensitivity. The sensor is well-suited for mobile use.

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