Arsenic pentoxide, oxidant

Unlike phosphorus pentoxide, this oxide cannot be made directly. Arsenic(V) acid, H3ASO4 (strictly, tetraoxoarsenic acid), is first prepared by oxidising arsenic(III) oxide with concentrated nitric acid or some other strong oxidising agent ... 

Arsenic acid (arsenic pentoxide hydrate, arsenic V oxide hydrate, orthoarsenic acid) [12044 50-7] M 229.8 + XH2O, pKj 2.26, pK 6.76, pKj 11.29 (H3ASO4). Cryst from cone solns of... 

Since the corresponding nitro derivative is not always available, other oxidants have also found application—e.g. arsenic pentoxide. 

Arsenic pentoxide is prepared by dehydration of crystalline arsenic acid at 200°C or above. The former is made by treating arsenic metal or arsenious oxide with nitric acid. Also, the pentoxide can be prepared by the reaction of arsenic trioxide with oxygen under pressure. 

Oxidation in air at elevated temperatures form arsenic along with arsenic trioxide or arsenic pentoxide, the nature of the product depending on the arsine to oxygen ratio ... 

Arsenic is oxidised, mainly to arsenious oxide, when heated in nitrous oxide 8 the reaction becomes appreciable at 250° to 270° C. and ignition occurs at 400° to 450° C. This reaction takes place specifically between arsenic and the nitrous oxide and is not due to reaction with oxygen after thermal decomposition of the nitrous oxide, as such decomposition does not occur below 400° C. and is very slight at 460° C. Nor does the reaction resemble that which occurs in oxygen, except that, like the reaction in the dark with the latter gas (see p. 47), it is a surface reaction. No chemi-luminescence is observed, however, and there is no upper critical oxidation pressure. At 360° C. the product contains at least 99 per cent, of pure arsenious oxide, and at 420° C. it contains about 5-8 per cent, of arsenic pentoxide. 

By the action of chlorine and chlorine compounds on arsenious oxide. Chlorine reacts with the heated oxide to form arsenic trichloride and arsenic pentoxide.11 The gas also reacts with the aqueous solution or suspension. When chlorine is passed into a well-agitated 70 to 80 per cent, suspension of arsenious oxide at 60° to 70° C., about 80 per cent, of the latter is converted to the trichloride and the remainder to the pentoxide.15 A saturated solution of arsenious oxide in concentrated hydrochloric acid when heated with concentrated sulphuric acid yields... 

By the action of chlorine and chlorine compounds on arsenic pentoxide. Arsenic trichloride is formed when chlorine is passed over the heated oxide.16 Hydrogen chloride reacts at ordinary temperatures, but not at -20° C.17 Aqueous hydrochloric acid, or sulphuric acid and a metallic chloride, also reacts with the oxide or with alkali arsenates to produce arsenic trichloride. The reaction with hydrochloric acid is greatly influenced by catalysts such as ferrous sulphate18 or chloride,19 potassium bromide20 or iodide,21 hydrobromic acid,22 methyl alcohol23... 

Two well-defined oxides are known arsenious oxide, As2Oa, and arsenic pentoxide, As2Os. The former is the most important compound of arsenic, being the form in which the element is most used. A suboxide, As20, and a tetroxide, As204, have been described, but the existence of neither as a pure compound has been established. 

Arsenic pentoxide is a white amorphous powder of density 5 3-7 to 4-3. It is tasteless when first introduced into the mouth, but rapidly becomes sharp and bitter and exerts a toxic action 6 similar to that of arsenious oxide (see Chapter XI), probably owing to the formation of the latter by reduction. The heat of formation (2As, 50) is 218,300 calories,7 and (As203, O,) 64,710 calories 8 the heat of dissolution (As2Os, aq.) is 6000 calories. 

Arsenic pentoxide catalyses the reaction between sulphur dioxide and oxygen,9 the amount of sulphur trioxide formed reaching 54 per cent, at 660° C. The reaction consists in the alternate reduction of the pentoxide to arsenious oxide by the sulphur dioxide and reoxidation to the pentoxide, so that arsenious oxide acts similarly. The catalytic activity is less than that of ferric oxide, but the latter is activated by addition of arsenic pentoxide the maximum amount of conversion increases from 69-5 to 78-5 per cent, and occurs at a temperature 63° lower than is required in the absence of the promoter. Arsenic pentoxide does not activate catalysts which act rapidly, such as vanadium pentoxide. Platinum and silver catalysts are poisoned by arsenic pentoxide.10... 

By allowing solutions containing molecular proportions of alkali arsenate and chromic oxide, or of arsenic pentoxide and alkali chromate or dichromate, to crystallise, the following complex salts have been obtained 4... 

It may also be obtained by fusion of a mixture of arsenic and arsenious sulphide 8 in the calculated proportions. A fiery red product is formed by melting arsenious oxide or arsenic pentoxide with sodium thiosulphate.9 The red sulphide has also been found mixed with other sulphides in the flue dusts obtained during the roasting of arsenical ores.10... 

Note 1. If the residue on evaporation does not redissolve after warming it 10 minutes with 60 cc. of water, it contains arsenious oxide either from incomplete oxidation by nitric acid, or from a decomposition of arsenic pentoxide by overheating. Test 1 cc. of the suspension containing the undissolved substance by adding 10 cc. of water, then solid sodium bicarbonate until no more effervescence occurs, and then a considerable quantity in excess. Add to this a solution of iodine, drop by drop. The amount of the latter which is decolorized (if any) corresponds to the amount of arsenious acid (As203), which was in the sample. 

Arsenic-based antioxidants, such as arsenic pentoxide and arsenic thioarsenate, had been used extensively in the past to retard oxidation. In a polyimide adhesive formulation, for example, arsenic compounds were found to improve thermal resistance. At 315°C no loss in strength was exhibited after 1000 h and substantial strength (1300 psi) was retained after 2000-h exposure. Without the arsenic additive there was marked reduction after only 200 h at 315°C. 

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