Arsenic compounds reduction

Semiconductor Applications. A limited but important demand for metaUic arsenic of 99.99% and greater (exceeding 99.999 + %) purities exists in semiconductor appHcations (see Semiconductors). This high purity arsenic may be prepared by the reduction of a highly purified arsenic compound using a high purity gaseous or soHd reductant. 

In 1933 Challenger et al. discovered that trimethylarsine was synthesized from inorganic arsenic compounds by molds (93). Recently, McBride and Wolfe (94), have reported the synthesis of dimethylarsine from arsenate by cell extracts of the methanogenic bacterium M. O. H. Methylcobalamin is the alkylating coenzyme for this synthesis which requires reduction of arsenate to arsenite, methylation of arsenite to methylarsonic acid, reduction and methylation of methylarsonic acid to dimethylarsinic acid, and finally a four electron reduction of dimethylarsinic acid to dimethylarsine (Fig. 13). 

The simplest analytical method is direct measurement of arsenic in volatile methylated arsenicals by atomic absorption [ 11 ]. A slightly more complicated system, but one that permits differentiation of the various forms of arsenic, uses reduction of the arsenic compounds to their respective arsines by treatment with sodium borohydride. The arsines are collected in a cold trap (liquid nitrogen), then vaporised separately by slow warming, and the arsenic is measured by monitoring the intensity of an arsenic spectral line, as produced by a direct current electrical discharge [1,12,13]. Essentially the same method was proposed by Talmi and Bostick [10] except that they collected the arsines in cold toluene (-5 °C), separated them on a gas chromatography column, and used a mass spectrometer as the detector. Their method had a sensitivity of 0.25 xg/l for water samples. 

Methylation of arsenic is an important pollution problem because of the widespread use of arsenic compounds in insecticides and because of the presence of arsenate in the phosphate used in household detergents.421 422 After reduction to arsenite, methylation occurs in two steps (Eq. 16-45). Additional reduction steps result in the formation of dimethylarsine, one of the principal products of action of methanogenic bacteria on arsenate. The methyl transfer is shown as occurring through CH3+, with an accompanying loss of a proton from the substrate. However, a CH3 radical may be transferred with formation of a cobalt(II) corrinoid.423... 

Laschtschenko therefore suggested that the amorphous forms may be of colloidal origin and may thus represent stages in the continuous passage from the colloidal to the crystalline state. The mode of preparation of the brown form by reduction of arsenic compounds in solution is favourable to sol formation, and the view that the various forms differed only in degree of dispersion of the particles has already been mentioned (p. 30). X-ray investigations6 show that the precipitated... 

The reduction of arsenic compounds to arsine by nascent hydrogen may also be effected in alkaline medium thus in the presence of caustic alkali with zinc,10 aluminium11 or sodium amalgam,12 and also in ammonia or ammonium chloride with zinc.13... 

In natural waters, arsenic may exist as one or more dissolved species, whose chemistry would depend on the chemistry of the waters. Over time, arsenic species dissolved in water may (1) interact with biological organisms and possibly methylate or demethylate (Chapter 4), (2) undergo abiotic or biotic oxidation, reduction, or other reactions, (3) sorb onto solids, often through ion exchange, (4) precipitate, or (5) coprecipitate. This section discusses the dissolution of solid arsenic compounds in water, the chemistry of dissolved arsenic species in aqueous solutions, and how the chemistry of the dissolved species varies with water chemistry and, in particular, pH, redox conditions, and the presence of dissolved sulfides. Discussions also include introductions to sorption, ion exchange, precipitation, and coprecipitation, which have important applications with arsenic in natural environments (Chapters 3 and 6) and water treatment technologies (Chapter 7). 

Zakharyan, R.A. and Aposhian, H.V. (1999) Enzymatic reduction of arsenic compounds in mammalian systems the rate-limiting enzyme of rabbit liver arsenic biotransformation is MMA(V) reductase. Chemical Research in Toxicology, 12(12), 1278-83. 

Magnuson, M.L., Creed, J.T. and Brocklioff C.A. (1997) Speciation of selenium and arsenic compounds by capillary electrophoresis with hydrodynamically modified electroosmatic flow and on-line reduction of selenium (VI) to selenium (IV) with hydride generation inductively coupled plasma mass spectrometric detection. Analyst, 122, 1057-1061. 

Arsenic-based antioxidants, such as arsenic pentoxide and arsenic thioarsenate, had been used extensively in the past to retard oxidation. In a polyimide adhesive formulation, for example, arsenic compounds were found to improve thermal resistance. At 315°C no loss in strength was exhibited after 1000 h and substantial strength (1300 psi) was retained after 2000-h exposure. Without the arsenic additive there was marked reduction after only 200 h at 315°C. 

Iron(III)-arsenate compounds are stable under oxidizing conditions (Fig. 12.15). Assuming that redox conditions in the stratum become reductive, iron(III) converts to iron(II) and arsenate becomes arsenite (AsO3- or AsO ). As conditions reduce further, arsenic solubility is regulated by sulfides and pH (arsenic MCL is set at 0.05 mg L 1, arsenosulfides exhibit a solubility near 1 mg L 1). Arsenic redox reactions can be carried... 

Pentavalent arsenic compounds such as tryparsamide appear to require reduction to the trivalent state for antiparasitic activity (Figure 10.9C). Various arsenicals containing pentavalent arsenic have only slight in vitro antiprotozoal activity, whereas compounds containing trivalent arsenic are highly active. 

The phenomenon is not observed in an acid environment (at pH levels below 2) the most stable solutions are achieved when phosphoric(V) acid at a concentration of 10 mM is used with EDTA addition. Solutions are stable for 4 days at room temperature in the presence of manganese and iron at a concentration of 100 mg Reduction in temperature to 4 °C (but without freezing) and protection from light considerably prolong the period of sample storage to 28 days after 3 months the change in the content of arsenic compounds does not exceed 10 %... 

Even as little as 20 years ago, arsenic compounds were mainly analyzed by LLE. Organic and inorganic forms were separated by arsenic reduction in its inorganic combinations, leading to formation of halogen metal acids or heteropoly acids extracted with toluene or trichloromethane. Organic arsenic was assayed as the difference between the total quantity and the quantity extracted by performing the... 

Lewisite in soil may rapidly volatilize or may be converted to lewisite oxide due to moisture in the soil (Rosenblatt et al, 1975). The low water solubility suggests intermediate persistence in moist soil (Watson and Griffin, 1992). Both lewisite and lewisite oxide may be slowly oxidized to 2-chlorovinylarsonic acid (Rosenblatt et al, 1975). Possible pathways of microbial degradation in soil include epoxidation of the C=C bond and reductive deha-logenation and dehydrohalogenation (Morrill et al, 1985). Due to the epoxy bond and arsine group, toxic metabolites may result. Additionally, residual hydrolysis may result in arsenic compounds. Lewisite is not likely to bioaccumulate. However, the arsenic degradation products may bioaccumulate (Rosenblatt et al, 1975). 

It also is formed by reduction of soluble arsenic compounds by zinc in acidic solution. This reaction is the basis of an important and sensitive test for arsenic, the Marsh test (Fig. 21-5). The arsenic is deposited as a steel-gray or black mirror from the burning gas onto a cold glazed porcelain dish held in the flame. Antimony produces a velvety brown or black deposit, which is not soluble in sodium hypochlorite solution, whereas the arsenic deposit is. The antimony deposit, but not that of... 

Arsenic compounds of therapeutic vjilue are formed when sodium a,rs( nit(t rea<(ts with a pyridiiw oxidation with hydrogen peroxide. 

The differentiation of inorganic As(III) and As(V) can be achieved by exploitation of the pH sensitivity of the reduction of arsenic compounds by sodium tetrahydroborate(III), as adapted to analyses by the AgDDC spectrophotometric procedure. ... 

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