Quantifying elemental sulfur (S0), bisulfide (HS-) and polysulfides (Sx2-) using a voltammetric method

Free polysulfides (S-4(2-)), elemental sulfur (S-0) and bisulfide (HS-) can be separated at discrete peak potentials using either cyclic voltammetry (CV) or linear sweep voltammetry (LS) with extremely fast scan rates. A scan rate of 1000 mV/s was found to be optimal for peak separation. Differences in the nucleophilic nature of S-x(2-) versus HS- and the electrochemical irreversibility of the S-0 in S-x(2-) allowed for peak separation. For 10 mu M solutions at a scan rate of 1000 mV/s, peak potentials were found to occur at -0.66 V for HS- and -0.69 V for S-0. S-4(2-) was found to give two discrete peaks at -0.70 and -0.81 V, representing a two step mechanism. In mixed laboratory standards, all the three sulfur species were uniquely identified. Flectrochemical analyses revealed similar peak separation patterns under different environmental conditions, ranging from estuarine pore waters to hydrothermal vents. In estuarine sediments, the sulfur speciation was found to change throughout a core profile, with S-0 dominant in the top layers (0-6 cm), S-x(2-) dominant in the transition zone (6-7 cm), and HS- dominant in the deeper sediment (>7 cm). In hydrothermal vent waters, different regions were observed to have different sulfur speciation. In diffuse flow regions away from chimneys containing trace O-2 concentrations, polysulfides, elemental sulfur and bisulfide were discretely identified. (C) 2000 Elsevier Science B.V. All rights reserved.