Oxidation of arsenic

Before this treatment, the cassiterite content of the ore is increased by removing impurities such as clay, by washing and by roasting which drives off oxides of arsenic and sulphur. The crude tin obtained is often contaminated with iron and other metals. It is, therefore, remelted on an inclined hearth the easily fusible tin melts away, leaving behind the less fusible impurities. The molten tin is finally stirred to bring it into intimate contact with air. Any remaining metal impurities are thereby oxidised to form a scum tin dross ) on the surface and this can be skimmed off Very pure tin can be obtained by zone refining. 

When sulphur is melted viscosity changes occur as the temperature is raised. These changes are due to the formation of long-chain polymers (in very pure sulphur, chains containing about 100 (X)0 atoms may be formed). The polymeric nature of molten sulphur can be recognised if molten sulphur is poured in a thin stream into cold water, when a plastic rubbery mass known as plastic sulphur is obtained. This is only slightly soluble in carbon disulphide, but on standing it loses its plasticity and reverts to the soluble rhombic form. If certain substances, for example iodine or oxides of arsenic, are incorporated into the plastic sulphur, the rubbery character can be preserved. 

Fire Hazards - Flash Point Not flammable Flammable limits in Air (%) Not flammable Fire Extinguishing Agents Not pertinent Fire Extinguishing Agents Not To Be Used Not pertinent Special Hazards of Combustion Products Not pertinent Behavior in Fire May form toxic oxides of arsenic when heated Ignition Temperature Not pertinent Electrical Hazard Not pertinent Burning Rate Not pertinent. 

The "arsenic poison" referred to in true-crime dramas is actually the oxide of arsenic, As203, rather than the element itself. Less than 0.1 g of this white, slightly soluble powder can be fatal. The classic symptoms of acute arsenic poisoning involve various unpleasant gastroin-... 

Oxidation of arsenic(III) by chromate in alkaline medium was studied by Kolt-hoff and Fineman. They found the reaction to be first order with respect to both chromate and arsenic(rir). At pH greater then 9.1 the rate coefficient is independent of the hydrogen-ion concentration. The average value of the rate coefficient at 30° in solutions of pH 9.1 and ionic strength 1.75 was found to be (1.61+0.08) x 10 l.mole sec . ... 

Chromium VI) as inductor in the induced oxidation of arsenic III) by molecular... 

Abel has assumed that the reaction between arsenite and molecular oxygen is catalyzed by a chromium intermediate. He suggested that chromium(IV) is converted by oxygen into chromium(VI) which causes the excess oxidation of arsenic(III). However, this mechanism is also devoid of experimental support. 

This scheme accounts for both the induced oxidation of iodide (where ci = 2) and that of manganese(ri) (where ci = 0.5) without including step (19). Furthermore it can be seen that in the presence of iodide the rate of disappearence of chromate will not be altered, whilst the rate of oxidation of arsenic(III) will be reduced to one-third of its original value, as found experimentally. 

Both iron(III) and copper(II) inhibit the induced chain oxidation and, when present in sufficient quantity, iXp -t-X s becomes 1.0, the effect of oxygen is completely eliminated and only the induced oxidation of arsenic(III) by peroxydi-... 

The stoichiometry of the induced reaction depends, as in the Fe(II)-S20 system, on the iron(II)/iron(III) ratio and on the pH. Therefore, it can be expected that under identical experimental conditions (actor, inductor, and hydrogen ion concentration) the induction factors for the two systems should be identical. The data obtained show that this expectation is fulfilled. For the photo-induced oxidation of arsenic(III) the value of ks /k -j was found to be 2, while in the present system k Jk -j = 4. (Comparing these values with the value of k jk = 21, it can be concluded that the SO4 radical, formed by reaction (43), is not removed by the reaction... 

In the absence of oxygen the mechanism of the induced oxidation of arsenic(lll) involves the steps... 

In the presence of oxygen the chain-oxidation of arsenic(lll) consists of... 

In the absence of oxygen the addition of arsenic(Iir) has only a slight effect on the rate of reduction of peroxydisulphate. In the presence of air the rate of reduction of persulphate increases nearly fortyfold (Table 12). The oxidation of arsenic(III) by SO4 from reaction (62) is not a chain process, thus it need not be considered in the iron(III)- and copper(II)-catalyzed reaction between peroxydisulphate and arsenic(III). 

Further convincing evidence was found by Catherino - for the formation of arsenic(IV) during the electro-oxidation of arsenic(III) in perchloric acid solution. 

During the induced reduction of chlorate a considerable oxygen effect was observed. The air oxidation of arsenic(ril) is also an induced reaction, the extent of which decreases with increasing acid concentration and is increased by decreasing the rate of primary reaction. The induced oxidation caused by air also can be observed during the osmium tetroxide-catalyzed chlorate-arsenic(III) reaction. 

Arsenous Acid Oxidation Investigated in Micro Reactors Organic synthesis 92 [OS 92] Oxidation of arsenous acid by iodate... 

Figure 4.101 Formation of zones due to the change of reaction mechanism by applying an electrical field during the oxidation of arsenous acid by iodate ( = 2.0 V cm" ). Numbers show the time intervals after the electric field was switched on. Intermediate product iodine (dark) and iodide (white) [68. ...

Neppolian B, Doronila A, Grieser F, Ashokkumar M (2009) Simple and efficient sonochemical method for the oxidation of arsenic (III) to arsenic (V). Environ Sci Technol 43 6793-6798... 

Oxidation of arsenic, antimony, copper, potassium, tin or zinc proceeds with incandescence. 

Varies depending on the specific agent but may include HC1 and HCN. Several produce diphenylarsenious oxide. Other organic oxides of arsenic, antimony, or lead may also be present. 

Volatile decomposition products may include HC1, HCN, nitrogen oxides (NO ), benzene, and oxides of arsenic, antimony, or lead. 

Experiments were performed in the laboratory to investigate the oxidation process under the given conditions [7]. The oxidation of arsenic(III) was found to be slow and incomplete. After about 100 hours a steady state on a level of about 40 % of the starting concentration was determined. 

In this method lg of soil is refluxed with 2M sodium hydroxide after centrifuging to remove solids. The clean extract is digested with perchloric acid-nitric acid at 265 °C. The iodine content of the extract is determined by the catalytic action of iodine on the oxidation of arsenic III ions with cerium IV ions. Between 96 and 97% recovery of added iodine spikes to soil were obtained by this method. 

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. 

The metallic arsenic is obtained primarily from its mineral, arsenopyrite. The mineral is smelted at 650 to 700°C in the absence of air. However, the most common method of production of the metal involves reduction of arsenic trioxide, AsOs with charcoal. Arsenic trioxide is produced by oxidation of arsenic present in the lead and copper concentrates during smelting of such concentrates. The trioxide so formed, readily volatilizes and is collected in a dust flue system where further treatment and roasting can upgrade the trioxide content. The trioxide vapors are then condensed and further purified by pressure leaching and recrystallization techniques. It is then reduced with charcoal to give metallic arsenic. 

Hanna, A., Saul, A., and Showalter, K. (1982). Detailed studies of propagating fronts in the iodate oxidation of arsenous acid. J. Am. Chem. Soc., 104, 3838-44. 

The anode mud is now a much more important source of tellurium than any of the minerals. The mud is generally roasted in a current of air, when tellurium dioxide is formed. This condenses, together with oxides of arsenic and antimony, as small white crystals in the flues and cooling chambers, and is reduced to tellurium by smelting with charcoal at a low temperature.1... 

This view is supported by the following facts. If the hydrogen is removed periodically, the oxidation of arsenic becomes nearly quantitative if, on the other hand, the apparatus is filled initially with hydrogen at 30 atm., the quantity of arsenic oxidised is diminished from 58 per cent, to 15 per cent. The amount of quinquevalent arsenic produced is, in general, approximately one-sixth of the amount of tervalent arsenic formed. 

Arsenic Suboxide ( ), As20, was considered by Berzelius 1 to constitute the superficial dark brown film which forms on arsenic owing to oxidation. Retgers 2 stated that the brown deposit which accompanies the mirror formed by sublimation of commercial arsenic was the suboxide. It was looked upon 3 as an intermediate product in the oxidation of arsenic, but the material so described was undoubtedly a mixture of arsenic and arsenious oxide 4 or, in some cases, merely the amorphous variety of arsenic. 

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