Direct ultraviolet spectrophotometric determination of total sulfide and iodide in natural waters.

A technique is described that allows the determination of total dissolved sulfide in natural waters using direct ultraviolet detection of the HS- ion. The concentration of bisulfide is determined by measuring absorption from 214 to 300 nm and then deconvolution of the HS- spectra from the complex spectrum of natural fluids. A nonlinear least-squares fitting approach is used for the deconvolution. At a pH near 8, where >95% of total sulfide is present as HS-, the results are indistinguishable from total sulfide measured using the methylene blue method in a wide range of sample types and matrixes including freshwater from groundwater wells, marine hydrothermal vent fluids, and marine sediment porewaters. The method allows simultaneous determination of other UV-absorbing ions, including nitrate, bromide, and iodide, in samples with low total sulfide concentrations. Bisulfide concentrations can be determined in samples with low background absorption, such as well water and hydrothermal fluids, with a detection limit of < 1 microM. The detection limit for bisulfide in sediment porewaters that have a high organic loading, which produces background absorbances of approximately 0.5 A at 260 nm in a 1-cm cuvette, is 5 microM. The only chemical manipulation required is buffering acidic samples to pH > 7 and filtration of particulate-rich samples.