Trace sulfide determination in oxic freshwaters

A simplified method for determination of reduced sulfur species in natural waters is presented. Reduced sulfur species were separated from a natural water matrix, using purge-and-trap (PT), after reacting with acid (acid volatile sulfide—AVS) or Cr(II) in acidic medium (chromium reducible sulfur—CRS). Sulfide in the trapping medium (0.05 M NaOH), was analyzed spectrophotometrically after derivatization to form methylene blue (MB). AVS precision for Na₂S and zinc sulfide clusters in synthetic solutions was ≤8.5% RSD at concentrations ranging from 48 to 503 nM. Spike recoveries of zinc sulfide clusters were 75-98% in a variety of freshwaters using the AVS procedure. Spike recoveries of Cu sulfide colloids were 94-109% in the same freshwater samples using the CRS procedure. During the analytical procedure an interfering compound was produced due to the reaction of mixed diamine reagent with itself. Lowering the pH of the reaction mixture minimized the formation of this compound. Minimizing contamination from particulates was necessary to achieve sub-nanomolar detection limits. The detection limit for AVS in a 500 ml sample with a 10 cm spectrophotometer cell was 0.1-0.3 nM (3×S.D. blank). The CRS procedure was calibrated with a synthetic CuS colloid. Interferences to the CRS method included finely divided pyrite, polysulfide, thiosulfate, sulfite and some elemental sulfur. The Cr(II) reagent did not reduce sulfate under our experimental conditions. The degree to which zinc sulfide clusters were adsorbed on membrane filters during filtration was mitigated if the clusters were synthesized in the presence of natural organic matter. Examples of AVS and CRS concentrations determined in oxygenated waters using the PT method were comparable to those reported recently by other methods. This method offers greater simplicity than other methods for trace dissolved sulfide determination in natural waters.