Abstract
The Raman spectra of thioarsenite and arsenite species in aqueous solution were obtained at room temperature. Solutions at constant ΣAs + ΣS of 0.1 and 0.5 mol kg-1 were prepared with various ΣS/ΣAs ratios (0.1–9.0) and pH values (~7–13.2). Our data suggest that the speciation of As under the conditions investigated is more complicated than previously thought. The Raman measurements offer evidence for at least six separate S-bearing As species whose principal bands are centered near 365, 385, 390, 400, 415 and 420 cm-1. The data suggest that at least two different species may give rise to bands at 385 cm-1, bringing the probable minimum number of species to seven. Several additional species are possible but could not be resolved definitively. In general, the relative proportions of these species are dependent on total As concentration, ΣS/ΣAs ratio and pH. At very low ΣS/ΣAs ratios we also observe Raman bands attributable to the dissociation products of H3AsO3(aq). Although we were unable to assign precise stoichiometries for the various thioarsenite species, we were able to map out general pH and ΣS/ΣAs conditions under which the various thioarsenite and arsenite species are predominant. This study provides a basis for more detailed Raman spectroscopic and other types of investigations of the nature of thioarsenite species.
Highlights
In moderately oxidizing to moderately reducing hydrothermal solutions the species H3AsO30 or one of its deprotonated equivalents generally is considered to be the dominant form of dissolved arsenic.[1,2,3,4,5,6,7,8,9] The As(V) species H3AsO40 or one of its dissociation products may be predominant under near-surface, oxidizing conditions.[6,10] Under extremely reducing conditions, dissolved arsine gas, AsH3(aq), may be an important species.[5]
The results of Helz et al.[24] strongly support the possibility that the degree of polymerization of thioarsenite species depends on the total As concentration in solution and that there may be a progression from monomers through dimers to trimers as total As increases towards saturation with respect to an arsenic sulfide phase
These results are preliminary in nature but they do provide some additional constraints on the nature of thioarsenite species and the conditions under which they are transformed into arsenite species
Summary
In moderately oxidizing to moderately reducing hydrothermal solutions the species H3AsO30 or one of its deprotonated equivalents generally is considered to be the dominant form of dissolved arsenic.[1,2,3,4,5,6,7,8,9] The As(V) species H3AsO40 or one of its dissociation products may be predominant under near-surface, oxidizing conditions.[6,10] Under extremely reducing conditions, dissolved arsine gas, AsH3(aq), may be an important species.[5]. Krupp[29] has suggested the possibility of a similar progression for thioantimony species It is apparent from this brief review of the literature that additional work is required to determine unambiguously the stoichiometries and thermodynamic stabilities of thioarsenite species. In this paper we present results of Raman spectroscopic measurements conducted on thioarsenite and arsenite species near 25 uC in near-neutral to alkaline solutions. These results are preliminary in nature but they do provide some additional constraints on the nature of thioarsenite species and the conditions under which they are transformed into arsenite species
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