Redox reactions of arsenic

Arsenic exists in two oxidation states Asv and As111. In natural waters these oxidation states occur in the triprotic arsenic acid, H3ASO4, and the monoprotic arseneous acid, H3ASO3. 

For arsenic acid, the three dissociation reactions are H3As04 o H2AsC 4 + H+ p Kal = 2.24 

Flence the only As111 species of any importance over the pH ranges of most natural systems is arsenous acid, H3As03. 

The reduction of Asv to As111 in soils can be described by the following two reactions  

The Eh-pH predominance diagram for Asv and As111 species is shown in Fig. 5.9. The upper and lower boundaries represent the stability field for water. 

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Vagliasindi E. G. A. and Benjamin M. M. (2001) Redox reactions of arsenic in As-spiked lake water and their effects on As adsorption. J. Water Supply Res. Technology-Aqua 50, 173-186. 

The redox reaction of arsenic hydride with gold chloride is more sensitive than with silver nitrate. The metallic gold formed by the reaction ... 

The reactions discussed above show that arsenic(fV) is of redox amphoteric character and a stronger reducing agent than arsenic(in), but at the same time it is a stronger oxidant than arsenic(V). Partners of the oxidation-reduction reactions of arsenic(fV) known so far can be seen in Table 13. It follows from the redox amphoteric character that the oxidation potentials of couples involving arsenic species are in the order... 

Decomposes when heated above melting point, 536°F/280°C, producing toxic fumes of arsenic, lead. Lead arsenates may be subject to redox reactions. Both arsenic and lead are known human carcinogens. PLUMBOUS ACETATE (6080-56-4) Pb(CjH302)2 3H,0 Contact with acids forms acetic acid. Incompatible with oxidizers, bases, acetic acid alkalis, alkylene oxides, ammonia, amines, bromates, carbonates, citrates, chlorides, chloral hydrate cresols, epichlorohydrin, hydrozoic acid, isocyanates, methyl isocyanoacetate, phenols, phosphates, salicylic acid sodium salicylate, sodium peroxyborate, potassium bromate resorcinol, salicylic acid, strong oxidizers, sulfates, sulfites, tannin, tartrates, tinctures trinitrobenzoic acid, urea nitrate. On small fires, use dry chemical, Halon, or CO2 extinguishers. 

The course of the reaction has not been elucidated. Probably redox reactions involving cerium(IV) and arsenic(III) are catalyzed by iodide ions and organic iodine compounds with methylene blue acting as a redox indicator. 

Redox reactions may cause mobile toxic ions to become either immobile or less toxic. Hexavalent chromium is mobile and highly toxic. It can be reduced to be rendered less toxic in the form of trivalent chromium sulfide by the addition of ferrous sulfate. Similarly, pentavalent (V) or trivalent (III) arsenic, arsenate or arsenite are more toxic and soluble forms. Arsenite (III) can be oxidized to As(IV). Arsenate (V) can be transformed to highly insoluble FeAs04 by the addition of ferrous sulfate. 

The determination of ammonium, arsenic, thiosulfate, allyl alcohol, and iodide has been achieved with a bromine redox mediator. Tomcik etal. [156] employ interdigitated microelectrodes at which bromine is generated at one set of electrodes and collected at a second set of electrodes. The reaction of the bromine with the analytes allows quantitative determination down to a micromolar level. 

Once arsenic dissolves in natural water, it may remain in solution for an extended period of time or participate sooner in abiotic or biotic reactions that remove it from solution. Depending upon the pH, redox conditions, temperature, and other properties of an aqueous solution and its associated solids, dissolved arsenic may precipitate or coprecipitate. Arsenic may also sorb onto solid materials, usually through ion exchange. Due to their importance in understanding the behavior of arsenic in natural environments (Chapter 3) and their applications in water treatment (Chapter 7), the sorption, ion exchange, precipitation, and coprecipitation of arsenic have been the subjects of numerous investigations. 

Following consumption of dissolved O2, the thermodynamically favored electron acceptor is nitrate (N03-). Nitrate reduction can be coupled to anaerobic oxidation of metal sulfides (Appelo and Postma, 1999), which may include arsenic-rich phases. The release of sorbed arsenic may also be coupled to the reduction of Mn(IV) (oxy)(hydr)oxides, such as birnessite CS-MnCb) (Scott and Morgan, 1995). The electrostatic bond between the sorbed arsenic and the host mineral is dramatically weakened by an overall decrease of net positive charge so that surface-complexed arsenic could dissolve. However, arsenic liberated by these redox reactions may reprecipitate as a mixed As(III)-Mn(II) solid phase (Toumassat et al., 2002) or resorb as surface complexes by iron (oxy)(hydr)oxides (McArthur et al., 2004). The most widespread arsenic occurrence in natural waters probably results from reduction of iron (oxy)(hydr)oxides under anoxic conditions, which are commonly associated with rapid sediment accumulation and burial (Smedley and Kinniburgh, 2002). In anoxic alluvial aquifers, iron is commonly the dominant redox-sensitive solute with concentrations as high as 30 mg L-1 (Smedley and Kinniburgh, 2002). However, the reduction of As(V) to As(III) may lag behind Fe(III) reduction (Islam et al., 2004). 

Since crushed basalt has been recommended as a major backfill component (1), experiments were completed to evaluate the rate of dissolved oxygen consumption and the redox conditions that develop in basalt-water systems under conditions similar to those expected in the near-field environment of a waste package. Two approaches to this problem were used in this study (l)the As(III)/As(V) redox couple as an indirect method of monitoring Eh and (2) the measurement of dissolved oxygen levels in solutions from hydrothermal experiments as a function of time. The first approach involves oxidation state determinations on trace levels of arsenic in solution (4-5) and provides an estimate of redox conditions over restricted intervals of time, depending on reaction rates and sensitivities of the analyses. The arsenic oxidation state approach also provides data at conditions that are more reducing than in solutions with detectable levels of dissolved oxygen. 

Since rates of arsenic redox reactions are slow at room temperature (5), it is assumed that the oxidation state data represent adjustment of arsenic species to the electron activity of the solution at 300°C. A quantitative assessment of the Eh of the basalt-water system at 300°C requires high-temperature thermochemical data for aqueous arsenic species. Such data are not available and, therefore, approximations were used to calculate Eh at 300°C. 

Another specialized form of potentiometric endpoint detection is the use of dual-polarized electrodes, which consists of two metal pieces of electrode material, usually platinum, through which is imposed a small constant current, usually 2-10 /xA. The scheme of the electric circuit for this kind of titration is presented in Figure 4.1b. The differential potential created by the imposition of the ament is a function of the redox couples present in the titration solution. Examples of the resultant titration curve for three different systems are illustrated in Figure 4.3. In the case of two reversible couples, such as the titration of iron(II) with cerium(IV), curve a results in which there is little potential difference after initiation of the titration up to the equivalence point. Hie titration of arsenic(III) with iodine is representative of an irreversible couple that is titrated with a reversible system. Hence, prior to the equivalence point a large potential difference exists because the passage of current requires decomposition of the solvent for the cathode reaction (Figure 4.3b). Past the equivalence point the potential difference drops to zero because of the presence of both iodine and iodide ion. In contrast, when a reversible couple is titrated with an irreversible couple, the initial potential difference is equal to zero and the large potential difference appears after the equivalence point is reached. 

Redox reactions in soils are affected by a number of parameters, including temperature, pH (see Chapter 7), and microbes. Microbes catalyze many redox reactions in soils and use a variety of compounds as electron acceptors or electron donors. For example, aerobic heterotrophic soil bacteria may metabolize readily available organic carbon using NO3, NOj, N20, Mn-oxides, Fe-oxides and compounds such as arsenate (As04 ) and selenate (Se04 ) as electron acceptors. Similarly, microbes may use reduced compounds or ions as electron donors, for example, NH4, Mn2+, Fe2+, arsenite (AsCXj), and selenite (SeO ). 

Arsenic pentachloride was an elusive compound until it was finally obtained by the ultraviolet irradiation of an ASCI3/CI2 mixture at -105 °C. Above —50°C, a redox reaction occurs affording AsCb and molecular chlorine (see equation 14 above). The identity of AsCls was confirmed by elemental analysis and by comparison of its Raman spectrum with those of liquid PCI5 and SbCls. ... 

Phenyltetrafluorophosphorane was first obtained by the reaction of phenyldichlorophosphine with antimony(V) fluoride or a mixture of antimony(V) chloride and antimony-(III) fluoride. In another method of preparation, phenyl-tetrachlorophosphorane was fluorinated with antimony(III) fluoride. Sulfur(IV) fluoride was used to fluorinate both phenylphosphonic acid and phenylphosphonic difluoride under autogenous pressure. Finally, it was found that phenyltetrafluorophosphorane is formed upon reaction of phenyldichlorophosphine with antimony(III) fluoride, by a simultaneous redox and fluorination reaction. " The last reaction is described below. It is very general in scope and has been employed in the synthesis of a wide variety of tetrafluorophosphoranes. " It may be noted that arsenic-(III) fluoride can be employed similarly as the fluorinating agent instead of antimony(III) fluoride. ... 

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