Reductions of arsenic compounds

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... 

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. 

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. ... 

Derivation Reaction of aluminum arsenide with water or hydrochloric acid, electrochemical reduction of arsenic compounds in acid solutions. 

The functions of the reagents used in the reduction of arsenic compounds, such as Zn, HCl, NiCla, SnCh, and KI are discussed below in connection with the Gutzeit method. Sodium borohydride also has been proposed as the reducing agent [39-42]. Sequential spectrophotometric determination of As(III) and As(V) is possible by using borohydride [40]. Hydrogen sulphide, which interferes in the reaction, is separated from arsine on cotton wool impregnated with lead acetate. 

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). 

Wolfolk CA, Whiteley HR. 1962. Reduction of inorganic compounds with molecular hydrogen by Micrococcus lactilyicus. I. Stoichiometry with compounds of arsenic, selenium, tellurium, transition and other elements. J Bacteriol 84 647-58. 

As stated in Section I, arsenate and phosphate are very similar. Hence, organisms have difficulty in assimilating phosphate without taking up arsenate, and this will uncouple their metabolism (Section II). They make use of an important difference between arsenate and phosphate to avoid this, i.e., the vastly greater ease of reduction of arsenate to arsenite than of phosphate to phosphite. By itself such a reduction would be no help, since arsenite is intensely toxic, but further processes can follow, such as alkylation to produce organoarsenic compounds (6), or extrusion of arsenite from the organism (e.g., 36, 37), driven by hydrolysis of ATP (38). Extruded arsenite may then be rendered less toxic by oxidation to arsenate by the arsenite oxidase mentioned in section III,A. 

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... 

The most generally applied method for determination of an arsenical is by atomic absorption spectrometry (AAS) after reduction of the compound to AsH3. However, this only provides an indication of the presence of the element as against a natural background. Lewisite rapidly hydrolyzes to 2-chlorovinylarsonous acid (CVAA see Figure 7) in an aqueous environment such as blood plasma, and analytical methods have focused mainly on the determination of CVAA (see Chapter 16). 

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 %... 

This in fact would correspond to a reduction of arsenic yielding the intermediate compound with an As—As bond. As this homocyclic species is even less stable towards oxygen, it readily takes up O2 to generate (MeAsO) (arsonousacid anhydride, also called arsoxane or arsenoxomethyl) which finally, in the presence of moisture, gives MeAsO(OH)2 (arsonic acid) (equations 7 and 8) . 

Reduction of pentavalent compounds has been the most active area of electrochemistry of organic derivatives of arsenic due to the previously widespread application of penta-... 

Major environmental trends that we see for land, air, water, and transportation of environmentally hazardous materials are shown in Box 9. These trends require that we get ahead of these issues and lead the chemical industry in the reduction of toxic metal (e.g., Sb, Sn, As) compounds, greenhouse gases, mercury emissions, and sulfur from gasoline and diesel, and find ways to control and sequester C02. Reduction of arsenic, as well as nitrates and ammonia, in drinking water is necessary. It is also imperative in these days of terrorism that we reduce transportation and storage of hazardous materials and continue our drive to develop inherently safer processes. 

In the photometric determination of arsenic, arsenic trihydride formed by reduction of the compounds of arsenic (mostly of arsenate) with nascent hydrogen reacts in the medium with pyridine with silver diethyldithiocar-bamate to produce a red colour. Absorbance is measured at 560 nm. The method is used for concentrations of arsenic > 0.05 mg 1 . For the determination of lower concentrations the sample should be first concentrated by evaporation [13]. 

Peraza et al. (2003) studied toxicity and metabolism of inorganic arsenic in kidney at low level subcytotoxic concentrations. Human renal proximal tubule epithelial cells (HK-2) were used as model in their study. The authors found that HK-2 cells were capable of biotransforming inorganic arsenic compounds in a pathway involving reduction of arsenate to arsenite. 

Poisoning by arsenic compounds is occasionally encountered in the laboratory. The toxic action of arsenic is due to its inhibition of sulphydryl enzymes. It can be detected in body fluids by the Reinsch test which is based on the reduction of arsenic to its elemental form by metallic copper in the presence of acid. The arsenic is deposited on the coppier as a dark film. 

An important difference between P and As is the ease of interconversion between the (V) and (III) oxidation states. In nature almost all phosphorus compounds are in the (V) oxidation state (or 5 valent state). Arsenic compounds are usually in either the (V) or (III) state. Arsenic metal has (0) oxidation state assigned to it. To put some numbers on the relative ease of oxidation/reduction of arsenic over phosphorus, the following half cell potentials are instructive. Thus, whereas arsenate is easily reduced to arsenite in acid solution, the reverse is true, easy oxidation of arsenite to arsenate, in base. 

Arsenic, Antimony, and Bismuth Arsenic is obtained by heating arsenic-containing metal sulfides. For example, FeAsS yields FeS and As(g). The As(g) deposits as As(s), which can be used to make other compounds. Some arsenic is also obtained by the reduction of arsenic(III) oxide with CO(g). Antimony is obtained mainly from its sulfide ores. Bismuth is obtained as a by-product of the refining of other metals. 

---