Über Vorgänge bei der Bildung des Arsen(V)‐sulfids beim Ansäuern von Tetrathioarsenatlösungen

Abstract

I n h a l t s i i b e r s i c h t . Beim Ansauern von Na,AsS,-Losungen entsOehen reaktionsfahige GAsSH-Gruppen, die zuuachst unter intermolekularem Abspalten von H,S zu sehr unbestandigen (AsS,):--Ionen kondensieren. Aus dieser intensiv gelben Zwischenverbindung bildet sich bei weiterer Saurezugabe das polymere As(V)-Sulfid. A b s t r a c t . On acidifying of n'a,AsS, solutions reactive =AsSH groups are formed which condense under intermolecular elimination of H,S to form very instable (ASS,):ions. These intermediary, yellow species give rise to the formation of polymeric AsV sulphide on further acidification.

Thermodynamic model for arsenic speciation in sulfidic waters: A novel use of ab initio computations

“…Here, a different problem arises; tetrathioarsenate is completely deprotonated above pH 5.2 (Thilo et al, 1970), so H 2 AsS 4 À could not be significant in the pH 7-10 range covered by Wilkin et al Apparently,neither posited equilibrium Reaction (6) or (7), satisfactorily explains the observed concentration relationships between the S/As = 3 and 4 species. On the other hand, if thioarsenate sulfidations are acid catalyzed, like the analogous thiomolybdate reactions (Erickson and Helz, 2000;Vorlicek and Helz, 2002), then appearance in a relatively short time interval of a fully sulfidized (S/As = 4) species at pH 7 but not pH 10 is easily explained by kinetics.…”

“…Ionization constants for H 2 S, H 3 AsO 3 and H 3 AsO 4 are reasonably well known; values in Table 1 have been calculated from a critical compilation of thermodynamic data. For many of the thioarsenates, ionization constants are available from Thilo et al (1970). For the remaining constants, we depend on estimates.…”

A spectrophotometric study of the formation and deprotonation of thioarsenite species in aqueous solution at 22°C

“…The presence of As(HS) 4 -could also be interpreted as due to the partial oxidation with their reported As-S distances (i.e. for As bound to four sulfurs) being consistent with those of an As(V) species such as AsS 3À 4 , the stable thioarsenate species in the pH range studied (Thilo et al, 1970). We note that James-Smith et al (2010) have described the photooxidation of As(III) in aqueous solutions containing other redox-sensitive species (e.g sulfur) during X-ray absorption spectroscopic measurements, even at moderately low photon flux densities.…”

“…The characteristic absorption of thioarsenate in the yellow region of the visible spectrum (570-580 nm) (e.g. Berzelius, 1826a;Guerin and Palazzi, 1967;Thilo et al, 1970) was not observed. Nor were any absorption bands due to polysulfides (see for example Giggenbach, 1972) detected.…”

“…The three deprotonation reactions for arsenic acid, H 3 AsO 4 , are also well defined at 25°C (e.g. Raposo et al, 2002) and the potentiometric study of Thilo et al (1970) provides apparently reliable estimates of the protonation constants for a number of thioarsenate and oxythioarsenate species at ambient conditions. However, the deprotonation(ionization) equilibria of thioarsenite species in aqueous solution as a function of reduced sulfur activity and pH have been little studied.…”