Fig. 2. Calibration of the polarographic sulfide sensor, PSS. A typical calibration curve is derived from sequential injections (inset) of Na₂S stock into anoxic 20 mmol l^–1 Tris at pH 7.0. A linear regression fits the PSS signals at <200 µmol l^–1 sulfide, whereas a second order polynomial fits the PSS signals above this concentration.