Differential pulse cathodic stripping voltammetric determination of nanomolar levels of dissolved sulfide applicable to field analysis of groundwater Original Research Article

Conventional batch mode analysis of dissolved sulfide by cathodic stripping voltammetry (CSV) is known to suffer from loss of sulfide in the cell to the waste mercury pool, compromising quantification of sulfide. Here we report a simple alternative approach to batch-mode differential pulse CSV (DPCSV). A fresh aliquot of sample is used for each voltammetric scan to minimize loss of sulfide through reaction with the mercury by limiting the time for sulfide–mercury contact, which is found to be more important in suppressing the sulfide signal than the amount of free mercury in the cell. Our improved batch-mode method exhibited a limit of detection of 1.3 nM, a relative standard deviation of 2.5% in NaOH supporting electrolyte and a linear response to as high a concentration as 1600 nM in a supporting electrolyte composed of Na2CO3/NaHCO3 (pH 8.3) mixed with an equal volume of oxic groundwater. A relative standard deviation of 4.5% was obtained for a groundwater sample in Na2CO3/NaHCO3 (pH 8.3) supporting electrolyte. These values are comparable to previously published results. Compared to other sensitive sulfide analytical techniques such as gas chromatography or high performance liquid chromatography (HPLC), DPCSV is preferred for sulfide analysis in the field due to its simple and portable instrumentation, lack of complex sample preparation, and short analysis time. The method was applied on site to analyze Fe-rich, reducing groundwater samples collected at a landfill site in Winthrop, Maine. Sulfide concentrations ranged from undetectable (<4 nM) to 7340 nM, generally increasing as the oxidation/reduction potential (ORP) of the water became more negative. We also demonstrate, for the first time, that the onset of sulfate reduction as indicated by the presence of small amounts of sulfide (tens to hundreds of nM) occurs in groundwater systems when the ORP value reaches −130 mV.