1.IntroductionThe chemical speciation of arsenic in natural waters determines its reactivity, toxicity, and transport in the environment.1–4Historically, the formation of soluble thioarsenic species has been recognized as an important factor governing arsenic chemistry in reducing environments.5–17This observation has been paralleled by efforts to identify and quantify the chemistry controlling the formation of thioarsenic species in nature, yet reliable analytical strategies for these species are not fully developed.18Speciation models derived from thermodynamic analysis of arsenic sulfide solubility in aqueous systems support the existence of thioarsenite species,9,12–15as do more recent molecular orbital theory calculations and Raman spectroscopic data.16,17,19Despite these extensive efforts, available data are limited for many practical applications because they provide an indirect quantification of thioarsenite stability and stoichiometry at conditions that are often unrepresentative of aquatic environments,i.e., at saturation with respect to an arsenic sulfide. Excluding some low-pH environments,20natural systems are usually found to be highly undersaturated with respect to arsenic sulfides such as orpiment.21,22Development of thermodynamic data for mineral and aqueous species is critical towards assessment of arsenic chemistry in sulfate-reducing environments. This geochemical setting is commonly encountered in organic-rich surface and groundwater systems,e.g., landfill leachate plumes,23hydrocarbon contaminant plumes,24and lacustrine to marine systems.25,26In this paper, we provide new experimental data that explore arsenic speciation in sulfidic waters and examine the results of previous solubility studies in light of the new experimental evidence. Reported herein is a direct analytical quantification of thioarsenite species formed under environmentally relevant conditions.