Fig. 3. The polarographic sulfide sensor (PSS) signal compared to the standard chemical analysis of sulfide with 2,2 dipyridyl disulfide (2-PDS). A single injection of anoxic Na₂S stock brought the concentration to 100 µmol l^–1 in 3 ml air-saturated 20 mmol l^–1 Tris at pH 7.0. 50 µl aliquot samples of solution were removed at timed intervals and immediately added to the 2-PDS reaction mixture. Both methods reported the same change in sulfide concentration over time due to spontaneous sulfide oxidation and volatilization. Signals were not influenced by the presence of a polarographic oxygen sensor (POS) or PSS in the chamber.