Arsenite oxidation solution

Power et al. (2005) show the effeet of pH and initial As(III) coneentration on the kineties of arsenite oxidation at bimessite-water interfaees, when a competitive metal (e.g., Zn) is present in an adsorbed or nonadsorbed state (Fig. 16.5). Two well-defined trends in the As(III) oxidation reactions can be distinguished (1) the extent of As(III) oxidation decreases with increasing pH from 4.5 to 6.0 and (2) oxidation on a percent basis is suppressed with increasing initial As(III) concentration from 100 to 300 dM. The pH effects on As(III) oxidation may have been influenced by competitive adsorption reactions between As(III) and reaction products (e.g., Mn(II)) and were not influenced by arsenic solution speciation. The suppressed As(III) oxidation rate constant may be a result of differences in the amount of Mn(II) release, which compete with dissolved As(III) species for unreacted Mn(IV) surface sites, and of Mn(II) adsorption, which inhibit the reaction between As(III) and Mn(IV) surface sites. 

An electrolytic method for the manufacture of calcium arsenate is as follows. A solution of arsenious oxide in caustic soda (As203 XaOH = 198 250) is electrolysed between iron electrodes. Hydrogen and a small quantity of metallic arsenic are liberated at the cathode and very little oxygen at the anode, the basic arsenite in solution being oxidised to arsenate (see also p. 221). When this oxidation is complete, any arsenic is filtered off and the solution treated with milk of lime. A basic arsenate of extremely low solubility is precipitated and, after removal, is washed and dried.3 The reactions involved are the following ... 

Pettine, M., Campanella, L. and Millero, F.J. (1999) Arsenite oxidation by H202 in aqueous solutions. Geochimica et Cosmochimica Acta, 63(18), 2727-35. 

Potassium iodide solution in the presence of concentrated hydrochloric acid, iodine is precipitated upon shaking the mixture with 1-2 ml of chloroform or of carbon tetrachloride, the latter is coloured blue by the iodine. The reaction may be used for the detection of arsenate in the presence of arsenite oxidizing agents must be absent. 

M Pettine, L Campanella, El MiUero. Arsenite oxidation by H2O2 in aqueous solutions. Geochim Cosmochim Acta 63 2727-2735, 1999. 

In addition to plasmid arsenic resistance that is well understood and for which clusters of genes have been isolated and sequenced, there are bacterial arsenic metabolism systems that involve oxidation of arsenite to arsenic. Arsenite oxidation by aerobic pseudomonads was first found with bacteria isolated from cattle dipping solutions where arsenicals were used as agents against ticks around the time of World War I. They were subsequently isolated by Turner and Legge... 

In 1953, Quastel and Scholefield (10) observed bacterial oxidation of arsenite to arsenate in soil perfusion experiments, using Audus s modification (11) of the airlift column designed by Lees and Quastel (12). They noted that sodium arsenite in aqueous solution at 2.5 X 10 M concentration when perfused through soil from Cardiff (Wales) became oxidized to arsenate. In the initial perfusion, a lag was observed before arsenite oxidation occurred. No lag was observed on reperfusion of the same soil column. Addition of sulfanilamide increased the initial lag in arsenite oxidation but not the oxidation on reperfusion. The oxidation was inhibited when they added 0.1% sodium azide to the arsenite solution. The initial lag and the effect of the azide indicated to the investigators that the oxidation was biological, but they made no attempt to isolate the arsenite-oxidizing organisms from the soil. They did show that ammonia was not oxidized in these columns. 

During arsenite oxidation the pH decreases and carbon dioxide is gradually expelled. When the solution is completely oxidized, it contains practically no carbon dioxide. In preparing the solution, it is important to maintain a ratio of a least 2 atoms of sodium to I atom of arsenic. If insufficient sodium is present, the pH can drop below 6.7, and part of the arsenic in the thio-arsenate can revert to its lower valence. Under these conditions, a mixture of arsenic trisulfide and sulfur precipitates. For these reasons, the process operates best within a pH range of 7.5 to 8.0. Ammonia can be substituted for the sodium carbonate without changing the characteristics of the process. 

In a 1-litre three-necked flask, mounted on a steam bath and provided respectively with a separatory funnel, mechanical stirrer and double surface condenser, place 165 g. of bromoform (96 per cent.). Add 10 ml. of a solution of sodium arsenite made by dissolving 77 g. of A.R. arsenious oxide and 148 g. of A.R. sodium hydroxide in 475 ml. of water. Warm the mixture gently to start the reaction, and introduce the remainder of the sodium arsenite solution during 30-45 minutes at such a rate that the mixture refluxes gently. Subsequently heat the flask on the steam bath for 3-4 hours. Steam distil the reaction mixture (Fig. 11, 41, 1) and separate the lower layer of methylene bromide (79 g.). Extract the aqueous layer with about 100 ml. of ether a further 3 g. of methylene bromide is obtained. Dry with 3-4 g. of anhydrous calcium chloride, and distil from a Claisen flask with fractionating side arm. The methylene bromide boils constantly at 96-97° and is almost colourless. 

In a 1-litre three-necked flask, fitted with a mechanical stirrer, reflux condenser and a thermometer, place 200 g. of iodoform and half of a sodium arsenite solution, prepared from 54-5 g. of A.R. arsenious oxide, 107 g. of A.R. sodium hydroxide and 520 ml. of water. Start the stirrer and heat the flask until the thermometer reads 60-65° maintain the mixture at this temperature during the whole reaction (1). Run in the remainder of the sodium arsenite solution during the course of 15 minutes, and keep the reaction mixture at 60-65° for 1 hour in order to complete the reaction. AUow to cool to about 40-45° (2) and filter with suction from the small amount of solid impurities. Separate the lower layer from the filtrate, dry it with anhydrous calcium chloride, and distil the crude methylene iodide (131 g. this crude product is satisfactory for most purposes) under diminished pressure. Practically all passes over as a light straw-coloured (sometimes brown) liquid at 80°/25 mm. it melts at 6°. Some of the colour may be removed by shaking with silver powder. The small dark residue in the flask solidifies on cooling. 

Concurrently with the preparation of the phenyldiazonium chloride solution, prepare a cold suspension of sodium arsenite. Place 250 ml. of water in a 3-htre round-bottomed flask equipped with a mechanical stirrer. Heat the water to boding, add 125 g. of anhydrous sodium carbonate, and, as soon as the carbonate has dissolved, introduce 62 5 g. of pure arsenious oxide and 3 g. of crystallised copper sulphate with stirring. When all the solids have dissolved, cool the solution with stirring under a stream of tap water until the temperature has fallen to 15°. 

Alternatively, prepare the sodium meta-arsenite solution by dissolving 39 6 g. A.R. arsenious oxide and 32 g. of A.R. sodium hydroxide in 600 ml. of water. 

Arsenic Oxides and Acids. The only arsenic oxides of commercial importance are the trioxide and the pentoxide. These are readily soluble in alkaline solution, forming arsenites and arsenates, respectively. 

Arsenite can be oxidized by manganese dioxides in soils. The rate constants for the depletion of As(III) by bimessite and cryptomelane are much higher than those by pyrolusite due to the difference in the crystallinity and specific surfaces of the Mn oxides (Oscarson et al., 1983). The ability of the Mn dioxides to sorb As(III) and As(V) is related to the specific surface and the point-of-zero charge of the oxides. The one-to-one relationship between the amount of As(III) depleted and the amount of As(V) appearing in solution was reported by Oscarson and colleagues (1983). 

Lynn et al. [71] demonstrated the damaging effect of arsenite on DNA. It has been shown that arsenite at low concentrations increased DNA oxidative damage in vascular smooth muscle cells (VSMCs) that can be a cause of arsenite-induced atherosclerosis. Bruskov et al. [72] found that heat induced the formation of 8-oxoguanine in DNA solution at pH 6.8, which was supposedly mediated by oxygen radicals. 

Copper compounds are used routinely and widely to control freshwater snails that serve as intermediate vectors of schistosomiasis and other diseases that afflict humans (Hasler 1949 NAS 1977 Rowe and Prince 1983 Winger etal. 1984 Al-Sabri etal. 1993). These compounds include copper sulfate, copper pentachlorophenate, copper carbonate, copper-tartaric acid, Paris green (copper arsenite-acetate), copper oxide, copper chloride, copper acetyl acetonate, copper dimethyl dithiocar-bamate, copper ricinoleate, and copper rosinate (Cheng 1979). Also, many species of oyster enemies are controlled by copper sulfate dips. All tested species of marine gastropods, tunicates, echinoderms, and crabs that had been dipped for 5 seconds in a saturated solution of copper sulfate died if held in air for as little as a few seconds to 8 h mussels, however, were resistant (MacKenzie 1961). 

The oxidized form of As, arsenate, As(V), which is present as HAs04 at neutral pH (p f values in Table 7.8), is sorbed on soil surfaces in a similar way to orthophosphate. The reduced form arsenite, As(lll), which is present in solution largely as H3As03(p fi = 9.29), is only weakly sorbed, hence mobility tends to increase under reducing conditions. Mobility will also increase without reduction of As(V) because, as for phosphate, reductive dissolution of iron oxides results in desorption of HAs04 into the soil solution. Under prolonged submergence As(lll) may be co-precipitated with sulfides. 

Iodine in aqueous solution may be measured quantitatively by acidifying the solution, diluting it, and titrating against a standard solution of sodium thiosulfate, sodium arsenite or phenyl arsine oxide using starch indicator. The blue color of the starch decolorizes at the end point. The indicator must be added towards the end of titration when the color of the solution turns pale yellow. Prior to titration, iodine in the dilute acidic solution is oxidized to iodate by adding bromine water or potassium permanganate solution. Excess potassium iodide is then added. The liberated iodine is then titrated as above. 

Arsenates have been described in one case exploiting the fact that the zirconyl cation forms a water-soluble arsenite but insoluble arsenate. By adding nitric acid to a solution of zirconyl chloride and sodium arsenite, the arsenite was oxidized to arsenate by the nitric acid, precipitating the insoluble zirconyl arsenate [32]. As for phosphates (and probably more readily), arsenates might be reduced to arsenides. 

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