Arsenic elemental, reactions

When heated in air at 800°C AS4S4 vapors begin to dissociate to AS2S2 which then ignites to form arsenic oxides. Ignition in chlorine produces arsenic chloride. Reaction with fluorine forms arsenic trifluoride. It is stable in water and also in the air at ambient temperatures. It does not react with hot concentrated HCl but is decomposed by nitric acid. It forms thioarsenite ion, AsS3 and elemental arsenic when warmed with caustic soda solution. Similar reaction occurs with sodium sulfide. 

Lanthanum combines with nitrogen, carbon, sulfur and phosphorus at elevated temperatures, forming binary salts. Also, with metalloid elements such as boron, silicon, selenium, and arsenic, similar reactions occur at high temperatures forming similar binary compounds. 

Elemental arsenic undergoes reaction with oxygen to yield the trioxide (equation 19). 

New chemical reactions have been discovered during studies of spot tests. Two recently found instances will serve to illustrate such advances in our knowledge. The formation of arsenic sulfide just described, suggested that sodium thiosulfate would behave in the same way as sulfur towards elemental arsenic. This was shown to be the case when it was found that a warm concentrated ammoniacal solution of sodium thiosulfate dissolves elemental arsenic. The reaction may be written ... 

A complete set of trihalides for arsenic, antimony and bismuth can be prepared by the direct combination of the elements although other methods of preparation can sometimes be used. The vigour of the direct combination reaction for a given metal decreases from fluorine to iodine (except in the case of bismuth which does not react readily with fluorine) and for a given halogen, from arsenic to bismuth. 

Elemental selenium has been said to be practically nontoxic and is considered to be an essential trace element however, hydrogen selenide and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. 

Rea.ctivity ofLea.d—Ca.lcium Alloys. Precise control of the calcium content is required to control the grain stmcture, corrosion resistance, and mechanical properties of lead—calcium alloys. Calcium reacts readily with air and other elements such as antimony, arsenic, and sulfur to produce oxides or intermetaUic compounds (see Calciumand calciumalloys). In these reactions, calcium is lost and suspended soHds reduce fluidity and castibiUty. The very thin grids that are required for automotive batteries are difficult to cast from lead—calcium alloys. 

Arsenic Peroxides. Arsenic peroxides have not been isolated however, elemental arsenic, and a great variety of arsenic compounds, have been found to be effective catalysts ia the epoxidation of olefins by aqueous hydrogen peroxide. Transient peroxoarsenic compounds are beheved to be iavolved ia these systems. Compounds that act as effective epoxidation catalysts iaclude arsenic trioxide, arsenic pentoxide, arsenious acid, arsenic acid, arsenic trichloride, arsenic oxychloride, triphenyl arsiae, phenylarsonic acid, and the arsenates of sodium, ammonium, and bismuth (56). To avoid having to dispose of the toxic residues of these reactions, the arsenic can be immobi1i2ed on a polystyrene resia (57). 

Arsenic trichloride (arsenic(III) chloride), AsQ. is the most common and important haUde of arsenic. It may be formed by spontaneous combination of the elements and, in addition, by the following reactions (/) chlorine with arsenic trioxide (2) sulfur monochloride, 82(11, or a mixture of S2CI2 chlorine, with arsenic trioxide and (J) arsenic trioxide with concentrated hydrochloric acid or with a mixture of sulfuric acid and a chloride. 

Trialkyl- and triarylarsine sulfides have been prepared by several different methods. The reaction of sulfur with a tertiary arsine, with or without a solvent, gives the sulfides in almost quantitative yields. Another method involves the reaction of hydrogen sulfide with a tertiary arsine oxide, hydroxyhahde, or dihaloarsorane. X-ray diffraction studies of triphenylarsine sulfide [3937-40-4], C gH AsS, show the arsenic to be tetrahedral the arsenic—sulfur bond is a tme double bond (137). Triphenylarsine sulfide and trimethylarsine sulfide [38859-90-4], C H AsS, form a number of coordination compounds with salts of transition elements (138,139). Both trialkyl- and triarylarsine selenides have been reported. The trialkyl compounds have been prepared by refluxing trialkylarsines with selenium powder (140). The preparation of triphenylarsine selenide [65374-39-2], C gH AsSe, from dichlorotriphenylarsorane and hydrogen selenide has been reported (141), but other workers could not dupHcate this work (140). 

The reactions can also be initiated tlrrough the application of a shock wave from a mechanical source, and syntlresis of GaAs has been achieved without any external heating, which would lead to arsenic evaporation, being applied to the mixture of the elements. 

The reaction is a sensitive one, but is subject to a number of interferences. The solution must be free from large amounts of lead, thallium (I), copper, tin, arsenic, antimony, gold, silver, platinum, and palladium, and from elements in sufficient quantity to colour the solution, e.g. nickel. Metals giving insoluble iodides must be absent, or present in amounts not yielding a precipitate. Substances which liberate iodine from potassium iodide interfere, for example iron(III) the latter should be reduced with sulphurous acid and the excess of gas boiled off, or by a 30 per cent solution of hypophosphorous acid. Chloride ion reduces the intensity of the bismuth colour. Separation of bismuth from copper can be effected by extraction of the bismuth as dithizonate by treatment in ammoniacal potassium cyanide solution with a 0.1 per cent solution of dithizone in chloroform if lead is present, shaking of the chloroform solution of lead and bismuth dithizonates with a buffer solution of pH 3.4 results in the lead alone passing into the aqueous phase. The bismuth complex is soluble in a pentan-l-ol-ethyl acetate mixture, and this fact can be utilised for the determination in the presence of coloured ions, such as nickel, cobalt, chromium, and uranium. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Methyl arsenic, like methyl mercury, is generated from inorganic forms of the element by methylation reactions in soils and sediments. However, the mechanism is evidently different from that for mercury, depending on the attack by a methyl car-bonium ion rather than a methyl carbanion (Craig 1986, Crosby 1998). Methylation... 

This hematite is not soluble in the cyanide solution. The oxidative pretreatment of gold ores thus reduces the cyanide consumption. Some impurity elements inhibit leaching reactions, examples include elements, carbon, sulfur and arsenic in gold ores are such impurities, but these can be removed by heating in air. 

Reactions similar to these provide convenient syntheses of hydrides of such elements as phosphorus, arsenic, tellurium, and selenium, because these elements do not react directly with hydrogen and the hydrides are unstable. 

One of the most common techniques for preparing Zintl phases is by the reaction of a solution of the alkali metal in liquid ammonia with the other element. However, many of these materials are obtained by heating the elements. For example, heating barium with arsenic leads to the reaction... 

Kuroda and Tarui [498] developed a spectrophotometric method for molybdenum based on the fact that MoVI catalyses the reduction of ferric iron by divalent tin ions. The plot of initial reaction rate constant versus molybdenum concentration is rectilinear in the range 0.01-0.3 mg/1 molybdenum. Several elements interfere, namely, titanium, rhenium, palladium, platinum, gold, arsenic, selenium, and tellurium. 

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