Arsenic compounds, oxidation

Arsenic V) oxide, AS2O5, is a rather indefinite compound (AS2O3 plus cone. HNO3) which loses O2 and forms arsenates(V) with alkalis. 

On concentrating the solution, a solid of formula As40jq. SH O (which may be composed by hydrated arsenic(V) acid) is obtained, and this, on fairly prolonged heating to 800 K, loses water and leaves arsenic(V) oxide. No compounds corresponding to the other acids of phosphorus are formed, but salts are known. 

Arsenic trioxide [1327-53-3] (arsenic(III) oxide, arsenic sesquioxide, arsenous oxide, white arsenic, arsenic), AS2O2, is the most important arsenic compound of commerce. The octahedral or cubic modification, arsenoHte [1303-24-8], 298 1313.9 kJ/mol (—314 kcal/mol) 214 J/(mol-K)... 

The selective epoxidation of ethylene by hydrogen peroxide ia a 1,4-dioxane solvent ia the presence of an arsenic catalyst is claimed. No solvent degradation is observed. Ethylene oxide is the only significant product detected. The catalyst used may be either elemental arsenic, an arsenic compound, or both. 

Heteropolyacids (HPA) are the unique class of inorganic complexes. They are widely used in different areas of science in biochemistry for the precipitation of albumens and alkaloids, in medicine as anticarcinogenic agents, in industry as catalysts. HPA are well known analytical reagents for determination of phosphoms, silica and arsenic, nitrogen-containing organic compounds, oxidants and reductants in solution etc. 

Trimethylarsine gives a 98% yield of trimethylarsine difluoride when treated with xenon difluoride [102] in fluorotrichloromelhane. and tnsfpentafluorophen-yl)arsine gives a 94% yield of tris(pentafluornphenyl)arsme difluoride after reaction with dilute fluorine in fluorotnchloromethane at 0 C [106] Other trivalent arsenic compounds have also been fluorinated with xenon difluoride [103] In addition, arsines have been oxidatively fluorinated by iodine pentafluoride [107] or electrochemically in 26-34% yield [108]... 

Other examples are the use of osmium(VIII) oxide (osmium tetroxide) as catalyst in the titration of solutions of arsenic(III) oxide with cerium(IV) sulphate solution, and the use of molybdate(VI) ions to catalyse the formation of iodine by the reaction of iodide ions with hydrogen peroxide. Certain reactions of various organic compounds are catalysed by several naturally occurring proteins known as enzymes. 

Sublimation. This process is employed to separate volatile substances from non-volatile impurities. Iodine, arsenic(III) oxide, ammonium chloride and a number of organic compounds can be purified in this way. The material to be purified is gently heated in a porcelain dish, and the vapour produced is condensed on a flask which is kept cool by circulating cold water inside it. 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

As seen in the above equations, the aqueous oxidation processes convert sulfur in the feed to dissolved sulfate, while arsenic is oxidized and precipitated as ferric arsenate compounds. So, problems of the emission of sulfur and arsenic oxides caused by roasting are avoided in the aqueous oxidation processes. The two different industrial methods which achieve the oxidation reactions are pressure oxidation and biological oxidation. 

Although the element is a metalloid, the long, brittle, crystals have a metallic shine. The white, tasteless oxide (arsenic trioxide As203) has been famous and notorious ("inheritance powder") even after centuries traces can be found in bodies. The arsenic compound "Salvarsan" was first used by Paul Ehrlich for the treatment of syphilis — the start of chemotherapy. Popular today as a semiconducting material. Component of LEDs (light-emitting diodes) and lasers. Arsenic hardens lead, used earlier in letter-press printing, today only for lead shot. 

Tris(dimethylamino)arsine (d2o 1.1248 nd 1.4848)3 is a colorless liquid which is readily hydrolyzed to form arsenic (III) oxide and dimethylamine when brought into contact with water. The compound is soluble in ethers and hydrocarbons. The product is at least 99.5% pure (with respect to hydrogen-containing impurities) as evidenced by the single sharp peak at —2.533 p.p.m. (relative to tetramethylsilane) seen in the proton nuclear magnetic resonance spectrum of the neat liquid. 

The mobility of arsenic compounds in soils is affected by sorp-tion/desorption on/from soil components or co-precipitation with metal ions. The importance of oxides (mainly Fe-oxides) in controlling the mobility and concentration of arsenic in natural environments has been studied for a long time (Livesey and Huang 1981 Frankenberger 2002 and references there in Smedley and Kinniburgh 2002). Because the elements which correlate best with arsenic in soils and sediments are iron, aluminum and manganese, the use of Fe salts (as well as Al and Mn salts) is a common practice in water treatment for the removal of arsenic. The coprecipitation of arsenic with ferric or aluminum hydroxide has been a practical and effective technique to remove this toxic element from polluted waters... 

Arsenical compounds that can form during pressure-oxidation of refractory gold ores in the autoclaves include Phase-3, Phase-4, Type 2 and Fe0HS04 (Fig. 1). In addition, scorodite can form in autoclave with sulfate substituting up to 20% arsenate. Jarosite is another phase with limited arsenate substituting for sulfate (Paktunc Dutrizac 2003). 

Odanake et al. [1] have reported the application of gas chromatography with multiple ion detection after hydride generation with sodium borohydride to the determination of mono and dimethyl arsenic compounds, trimethyl arsenic oxide and inorganic arsenic in soil and sediments. Recoveries in spiking experiments were 100-102% (mono and dimethyl arsenic compounds and inorganic arsenic) and 72% (trimethyl arsenic oxide). 

G. J. Stridh. On the Slow Oxidation of Arsenic(llT) Oxide to ArsenidV) Oxide in Aqueous Solution by Elemental Oxygen and its Effect on the Precise Determination of Energies of Combustion of Organic Chloro- and Bromo-compounds. J. Chem. Thermodynamics 1975, 7, 703-705. 

With Arsenic —Hydrogen does not directly combine with arsenic, but if an arsenic compound is in solution in a liquid in which hydrogen is being generated, i.e. hydrogen in the nascent state, chemical union takes place. Thus, if arsenious oxide is dissolved in dilute hydrochloric acid and a piece of metallic zinc is added, the hydrogen produced by the action of the add on the zinc will combine with the arsenic, in accordance with the following equation —... 

On his way home from the Netherlands he studied mining and metallurgy in the Harz, and in 1727 he was placed in charged of the chemical laboratory at the Bureau of Mines in Stockholm, which was then in poor financial condition. After the laboratory was sold, Brandt and his students Henrik Teofil Scheffer and Axel Fredrik Cronstedt carried on their epoch-making researches at the Royal Mint, and in 1730 Brandt became assay master of the Mint. Three years later he published a systematic investigation of arsenic and its compounds in which he showed that arsenic is a semi-metal and that white arsenic [arsenious oxide] is its calx (35). 

Arsenic compounds are thought to exert their toxic effects by several modes of action. Interference with enzyme function may result from sulfhydryl group binding by trivalent arsenic or by substitution for phosphate. Inorganic arsenic or its metabolites may induce oxidative stress, alter gene expression, and interfere with cell signal transduction. Although on a... 

In general, the direct-oxidation processes employ a redox couple that has sufficient oxidation potential to convert H2S into elemental sulfur but insufficient potential to oxidize sulfur to higher states. Examples of materials that have this redox potential are vanadium compounds, arsenic compounds, iron compounds, and certain organic species. Typically, the redox materials, dissolved in a hot potassium carbonate solution with the species in its oxidized form, contacts the I S-laden gas and the H2S dissolves as the hydrosulfide. This sulfur reacts with the redox couple, forming elemental sulfur and the reduced state of the couple. Airblowing of the solution reoxidizes the couple and removes the elemental sulfur from solution as a product froth. 

In chemical combination, arsenic can exist in oxidation state III or V and can have a coordination number of 3, 4, 5, or 6. In marine samples, arsenic is mainly found in the V oxidation state, although, usually as a consequence of biological factors, arsenic (III) compounds can also occur and may at times be predominant. The properties and analysis of the various arsenic-containing compounds of significance in marine arsenic research are briefly discussed, and information is provided on their synthesis. For ease of reference, the arsenic compounds frequently mentioned by name (or abbreviation/acronym) are listed in Table IV together with their structure numbers. 

Arsenic compounds with one to four methyl groups attached to the arsenic atom are common constituents of marine samples. The relevant species are monomethylarsonic acid (MMA) (3), dimethylarsinic acid (DMA) (4), trimethylarsine oxide (TMAO) (5), and tetramethylarson-ium ion (TeMA) (6). Of these, MMA and DMA are readily separated... 

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