Arsenious acid, oxidation

Arsenic(III) (arsenious) acid, H3ASO3.—When arsenic(III) oxide is dissolved in water the corresponding acid is formed ... 

The demand for metallic arsenic is limited and thus arsenic is usually marketed in the form of the trioxide, referred to as white arsenic, arsenious oxide, arsenious acid anhydride, and also by the generally accepted misnomer arsenic. 

Chlorate Analysis. Chlorate ion concentration is determined by reaction with a reducing agent. Ferrous sulfate is preferred for quaHty control (111), but other reagents, such as arsenious acid, stannous chloride, and potassium iodide, have also been used (112). When ferrous sulfate is used, a measured excess of the reagent is added to a strong hydrochloric acid solution of the chlorate for reduction, after which the excess ferrous sulfate is titrated with an oxidant, usually potassium permanganate or potassium dichromate. 

Nitrophenylarsonic acid has been prepared by heating p-nitrobenzenediazonium chloride with arsenious acid in hydrochloric acid, by the action of -nitrobenzenediazonium chloride on sodium arsenite, by the action of sodium arsenite on sodium -nitrobenzeneisodiazo oxide, by the diazotization of -nitro-aniline in acetic acid in the presence of arsenic chloride and cuprous chloride, and by the reaction of -nitrobenzenediazonium borofluoride with sodium arsenite in the presence of cuprous chloride. ... 

To illustrate case a, ii we may refer to the reduction of chlorate by arsenious acid induced by different 1-equivalent oxidizing reagents. The effect of the oxidizing agent is to form arsenic(IV). Chlorate will be reduced by arsenic(IV) which is a stronger reducing agent than arsenic(III). 

It is striking that reactions of arsenic(III) with 1-equivalent oxidizing agents take place fairly slowly, while reactions with 2-equivalent partners e.g. with iodine, are fast. To interpret this behaviour it was suggested by Waters that arsenious acid has the tautomeric forms... 

Guideline 10. Inverse orders arise from rapid equilibria prior to the rate determining step. In order to illustrate this point, consider the oxidation of arsenious acid in an aqueous solution. 

In iodimetry, quantitative oxidation of reducing agents, such as arsenious acid (H2As03) may be carried out by employing standard solutions of iodine as shown under ... 

Synonyms arsenic oxide arsenic sesquioxide white arsenic arsenic (III) oxide arsenious acid anhydride... 

The rate of a reaction is expressed as the increase in the amount of a particular product, or the decrease in the amount of a reactant, per second at a time t. For reactions in a fluid phase, it is usual to substitute concentrations, or for gases partial pressures, for amounts. (See Section 5.6 for treatment of the kinetics of some solid-state reactions.) Note that the numerical value of the rate may depend on how we choose to define it. For example, in the oxidation of aqueous iodide ion by arsenic acid to give triiodide ion and arsenious acid,... 

At ordinary temperature pure water, free from air, has no action upon arsenic, even after ten years contact in a sealed glass tube.6 In the presence of dissolved oxygen, absorbed by exposure to air, oxidation occurs, arsenious acid being formed, but no arsine.8 Under ordinary conditions oxidation is slight, and not more than 7 per cent, of arsenious acid is formed at 350° C.7 In the presence of alkali hydroxide, however, oxidation is more vigorous, arsenite, accompanied by a small amount of arsenate, being formed in quantity. Increase in the concentration of the alkali facilitates oxidation up to a point, beyond which further increase causes a marked decrease in the amount of arsenic oxidised. The reaction commences at about 200° C. and the rate increases rapidly up to 350° C., then decreases sharply. Complete oxidation does not occur and the reaction appears to be a balanced one, thus ... 

This quantitative relationship was observed by Dumas 7 and Soubeiran.8 With excess of oxygen the hydride explodes violently, but if the supply of oxygen is insufficient the hydrogen is first oxidised and the arsenic liberated, and this takes place also in the spontaneous oxidation of arsine by oxygen at ordinary temperatures. Exposure of the mixed gases to j3- or y-rays results in the formation of arsenious acid 9... 

The oxidation of arsine may be accomplished by means of the halogen oxyacids and their salts,8 although not so readily as with the halogens themselves. Hypochlorites and hypobromites cause complete oxidation to arsenic acid, but side reactions are liable to occur, especially if the gas is present in excess. Chloric add slowly oxidises arsine to arsenious acid a trace of silver nitrate catalyses the reaction. Chlorates are quite inactive. More complete oxidation results with solutions of bromic acid and bromates, iodic acid and iodates, especially in the presence of catalysts. The reactions are of the type represented by the equation8... 

Arsenious oxide is soluble in a number of organic liquids. Thus, 100 parts of absolute alcohol dissolve at 15° C. 0-025 part, and at boiling point 3-402 parts of the octahedral form. The solubility is increased by the addition of water. The vitreous modification dissolves to the extent of 1-060 parts per 100 at 15° C. and addition of water decreases the solubility. Esters of arsenious acid may be obtained by heating, with stirring, a mixture of the oxide and an alcohol in the presence of a hydrocarbon such as benzene, toluene or xylene.7... 

The acidity of arsenious acid in aqueous solution is increased by the addition of mannitol, sorbitol or a-mannitan, probably owing to the formation and superior ionisation of an acid of the type HAsD2 (D representing the diol residue).9 When sublimed arsenious oxide is heated with water on a water-bath for 5 hours, the dissolved acid has less than the normal acidity, but by boiling the solution for 7 hours under a reflux condenser it attains its original acidity. 

Oxidation is readily brought about by hypochlorites3 and by chlorates. In the latter case, in the presence of hydrochloric acid, the reaction is independent of the concentration of the arsenious acid and, according to Kubina,4 reduction of the chlorate first to a hypothetical intermediate product occurs, probably as follows ... 

Another interesting example of induced oxidation is the reaction between chromic acid and a manganous salt in the presence of arsenious acid.12 The chromic acid is reduced to a chromic salt, while the manganous salt is oxidised to the manganic state and the arsenious acid to... 

Tingle x observed that a solution of arsenious oxide in aqueous alcohol, after boiling for 26 hours, was oxidised to arsenic acid, but this was denied by Edgerton.2 Kessler3 observed that when sodium arsenite was undergoing oxidation by chromic acid, oxidation by atmospheric oxygen occurred simultaneously. 

By exposing an aqueous solution of arsenious acid to X-rays oxidation to arsenic acid occurs accompanied by the liberation of an equivalent amount of hydrogen.6... 

Tire speed of the reaction is thus proportional to the concentration of arsenious acid and to the square of the concentration of nitrous acid present. The effect of the presence of mercuric salts on the velocity of this oxidation is peculiar 8 at a concentration of 7-7 x 10 6 mol. per litre the reaction is completely inhibited, but with diminishing concentration the inhibiting effect becomes less marked until, at 7-7 x 10-9 mol. per litre, there is a definite positive catalytic effect which becomes more marked at the extremely low concentration of 7-7 x 10 u mol. per litre. The retarding effect of the mercuric salt may be overcome by the addition of a small amount of halogen acid to the nitric acid.9 The... 

Beryllium Arsenite.—Attempts to prepare definite compounds of beryllium with arsenious acid have not been successful.5 When beryllium hydroxide is precipitated from solutions containing arsenious oxide, much of the latter is adsorbed. 

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