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Arsenic Pentoxide, As2O5

Arsenic Pentoxide, As2O5, may be obtained in the dry way by the oxidation of arsenious oxide by heating with oxygen under pressure or with chlorine. In the former case the oxidation is only partial. It is accelerated by pressure or by the presence of a catalyst. If arsenious oxide is introduced into a mass of ferric oxide or alumina at about 218° C. in a current of air or oxygen, arsenic pentoxide is formed; activated charcoal containing cupric oxide also acts as an efficient catalyst. The pentoxide is more readily obtained by digesting arsenious oxide with nitric acid, preferably in the presence of oxygen, and dehydrating at 180° to 200° C. the " arsenic acid " produced.

Arsenic pentoxide is a white amorphous powder of density 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 similar to that of arsenious oxide, probably owing to the formation of the latter by reduction. The heat of formation (2As, 5O) is 218,300 calories, and (As2O3, O2) 64,710 calories; the heat of dissolution (As2O5, aq.) is 6000 calories.

From determinations of the heat capacity of the oxide at temperatures from about 57° to 296° Abs. the entropy at 25° C. has been calculated to be 25.2 calories per degree, and the free energy of formation is computed to be -185,400 calories per mole.

The pentoxide is hygroscopic and dissolves slowly in cold water, more readily in hot, to form an acid solution, apparently containing orthoarsenic acid, H3AsO4. The solubility at various temperatures is given on p. 184. It is soluble in alcohol, and in certain oils and other organic liquids; thus poppy oil at boiling temperature dissolves 2.7 parts per 100, while castor oil dissolves more than that amount; 95 per cent, formic acid dissolves 7.6 parts per 100 at 19° C.

When heated, arsenic pentoxide, unlike phosphorus pentoxide, loses oxygen:

As2O5As2O3 + O2

The reaction commences at a temperature above 400° C., before the melting temperature is reached, and the fused product therefore always contains some arsenious oxide. When heated in hydrogen, the pentoxide is reduced first to arsenious oxide and then to free arsenic. Similar reduction occurs when it is heated with carbon or phosphorus; with sulphur, arsenious sulphide is formed. Arsenic and metallic arsenides result when the pentoxide is heated with alkali metals, zinc, lead, iron or most other heavy metals; mercury and silver react only at high temperature; gold and platinum do not react.

Hydrogen chloride is absorbed by arsenic pentoxide with formation of arsenic trichloride:

As2O5 + 10HCl = 2AsCl3 + 2Cl2 + 5H2O

Phosphorus pentachloride reacts according to the equation

As2O5 + 5PCl5 = 2AsCl3 + 5POCl3 + 2Cl2

Iodine is liberated when the pentoxide is heated with an alkali iodide, thus

3As2O5 + 4KI = 4KNaO3 + As2O3 + 2I2

a similar reaction proceeding with alkali bromides but not with chlorides unless oxygen is present.

Hydrogen sulphide is absorbed with formation of the pentasulphide:

As2O5 + 5H2S = As2S5 + 5H2O

Fusion with sodium thiosulphate yields arsenic trisulphide.

With liquid ammonia in a sealed tube at the ordinary temperature, combination occurs to form the triammine, As2O5.3NH3.

Arsenic pentoxide catalyses the reaction between sulphur dioxide and oxygen, 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.

Hydrates of Arsenic Pentoxide

The existence of hydrates of arsenic pentoxide corresponding to the ortho-, meta- and pyro-phosphoric acids has not been established. Many such products have been described in the literature, the formation of which could not be confirmed by subsequent workers. It appears certain that a tetrahydrate, As2O5.4H2O, and a (3, 5)-hydrate, 3As2O5.5H2O, exist; there is also evidence of the formation of a heptahydrate, As2O5.7H2O, and of a dihydrate (pyroarsenic acid), As2O5.2H2O.

The tetrahydrate separates on cooling concentrated aqueous solutions of arsenic acid and is the product obtained in the industrial preparation of "arsenic acid". It may be obtained by concentrating the aqueous solution until it boils at 150° C.; a small portion is then inoculated with a crystal of the isomorphous tetrahydrate of phosphorus pejitoxide and the crystals which separate used to inoculate the main solution. The crystals are rhombic prisms or plates which melt at 3614 ± 005° C.

If the tetrahydrate is kept for some time in a superfused state, the (3, 5)-hydrate is formed as a hard white crystalline scale,5 and this hydrate may also be obtained by evaporating a solution of arsenic acid in an open vessel at temperatures from 50° to 100° C. or under increased pressure at 150° C.

The tetrahydrate loses water even at -10° C., and dehydration over sulphuric acid, phosphorus pentoxide or potassium hydroxide proceeds regularly at the ordinary temperature until the (3, 5)-hydrate remains; under these conditions there is no indication that pyroarsenic acid, As2O5.2H2O, is formed as an intermediate product. Complete dehydration occurs when either of the above hydrates is heated to 180° to 200° C., and the isobaric decomposition curve8 of the tetrahydrate gives no indication of the formation of any hydrate other than 3As2O5. 5H2O; neither solid solutions nor mixed crystals appear to be formed.

A heptahydrate, As2O5.7H2O, is stated to be obtained by evaporating at 130° C. a solution of arsenic acid and then cooling to -20° to -30° C.

The dihydrate, As2O5.2H2O, or pyroarsenic acid, H4As2O7, was described by Kopp as a hard mass formed when aqueous arsenic acid was evaporated at a temperature between 140° and 180° C., but the compound could not be obtained by Auger and other workers. Rosenheim and Antelmann, however, maintain that pyroarsenic acid does exist and is obtained in the form of hard microscopic prismatic crystals by evaporating a concentrated aqueous solution of pure arsenic acid in an open dish until a temperature of 170° to 180° C. is reached. The product remains at constant weight even when heated at 155° C. A number of pyroarsenates have been prepared, and Rosenheim and Antelmann conclude that pyroarsenic acid exists in solution in equilibrium with the ortho-acid, but is hydrolysed to the latter much more rapidly than is the case with the corresponding pyrophosphoric acid.

Manufacture of Arsenic Acid

The large-scale preparation of arsenic acid usually depends on the oxidation of arsenious oxide or sulphide with moderately concentrated nitric acid (sp. gr. not less than 1.35) or some other oxidising agent. The operation with nitric acid is conducted in chambers or steam-jacketed kettles made of acid-resisting material such as ferro-silicon alloy. The mixture of oxide and nitric acid is violently agitated and at the same time allowed to pass gradually into a second chamber or kettle. Considerable foaming accompanies the reaction and may be excessive with low-grade ore; this is met by causing the bulk of the reaction to take place outside the main batch kettle and within the pump or agitator below, the nitrous gases being released from a fountain discharge above the surface in the main kettle. The nitrous fumes may be recovered by passing over moist coke. The reaction is facilitated catalytically by the presence of a little hydrochloric acid or other halogen hydracid.

The oxidation is more satisfactorily carried out in the presence of air or oxygen. One method consists in treating arsenious oxide with nitric acid at a raised temperature and under an oxygen pressure of about 20 atmospheres. The arsenious oxide (or arsenious sulphide) is introduced into a closed vessel having a stirrer, and an equal weight of 60 per cent, nitric acid is added, together with a small amount of arsenic pentoxide or other catalyst. Oxygen at 20 atmospheres is then forced in and the mixture stirred for 12 to 18 hours at 70° to 90° C. The nitric acid, which remains almost unchanged and thus acts as a catalyst, can be distilled off and the arsenic acid remains as a syrupy liquid which crystallises (as the tetrahydrate) on cooling. This may be converted into the pentoxide by dehydration at 180° to 200° C. or may be converted into an arsenate.

If arsenious sulphide is used instead of the oxide, arsenic acid and sulphuric acid are formed and may be separated by precipitation of the latter by addition of lime. In the absence of nitric acid, oxygen at 20 atmospheres, even at 200° C., produces only arsenious acid from arsenious oxide, and arsenious and sulphuric acids from arsenious sulphide.

By passing arsenious oxide vapour and air into a tower containing nitric acid and water vapour, arsenic acid is produced and the nitrogen oxides formed are reoxidised by the air present.

An alkali chlorate is sometimes employed as the oxidising agent.

Thus a solution containing arsenious oxide (10 parts), sodium chlorate (4 parts) or potassium chlorate (3.8 parts) and water (20 parts) is heated to the boiling point, a little hydrochloric acid being added as catalyst.

The oxidation may also be accomplished by means of chlorine, bromine or iodine, hypochlorous acid, aqua regia, chromic acid or permanganic acid, and some metallic oxides. Arsenic acid is also formed by decomposition of arsenic trichloride by the action of chlorine water or chromic acid, or by the action of bromine water on arsenious sulphide.

Properties of Aqueous Solutions of Arsenic Acid

The Two-component System As2O5-H2O
The system As2O5-H2O has been investigated by determining the solubility curves of the (1, 4)- and the (3, 5)-hydrates, and also the curve for the depression of the freezing point of water. The data obtained are given in the following table and are graphically represented in fig.

Temperature, ° C.Solubility, g. H3AsO4 in 100 g. Solution.Solid Phase

The tetrahydrate readily forms supersaturated solutions and in the above investigation it was never observed to separate spontaneously from the solution, inoculation always being necessary. Above 29.5° C. the (3, 5)-hydrate is the more stable. The cryohydric point appears to lie close to -60° C. with about 69 per cent. of H3AsO4

The density of an aqueous solution containing p per cent, of H3AsO4 at 15° C. may be expressed as

D415 = 0.9992 + 0.0060p + 0.0000576p2

Gerlach gave the following data for densities at 15° C., the concentrations being expressed in parts of As2O5 in 100 parts of solution:


The relative viscosities of the following aqueous solutions at 25° C. have been determined: N-H3AsO4, 1.2707; N/2, 1.1291; N/4, 1.0595; N/8, 1.0309.

Although the acid itself has not been isolated, the solution behaves as though it contained a tribasic acid and reacts successively with three equivalents of sodium hydroxide. The aqueous solution is strongly acid to litmus. During neutralisation in the presence of methyl orange or lacmoid, the colour change occurs when one equivalent of alkali hydroxide has been added; with phenolphthalein the colour change occurs after two equivalents have been added. The successive heats of neutralisation with sodium hydroxide are: 1st mol. NaOH, 14,990 calories; 2nd mol., 12,590 calories; and 3rd mol., 8340 calories; total, 35,920 calories. The heat of dissolution is represented by Thomsen as (H3AsO4, aq.) = -400 calories.

The electrical conductivities (molecular) at 25° C. of aqueous solutions containing 1 mol. H3AsO4 in v litres are as follows:

v (litres).81632641282565121024
H (mhos).68.489.4117.4150.2188.4228.0264.2290.3

The three dissociation constants, k1, k2, and k3 of arsenic acid, determined by means of the glass electrode, are respectively 5.6×10-3, l.7×10-7 and 3.0×10-12. During electrolysis reduction occurs, arsenic being deposited and arsine liberated at the negative electrode and oxygen at the positive. The amount of arsine produced depends on the nature of the electrode, more being produced at a mercury cathode than at one of platinum; the quantity of arsine also increases with increasing concentration of arsenic acid. The reduction proceeds more readily with an alternating current than with direct current.

The oxidation-reduction potential of arsenious-arsenic acid solutions has been determined. A small quantity of iodide was added as catalyst, and it was found that true equilibrium values, varying normally with the concentration ratio, are obtained only if the solutions are acidified to an extent corresponding at least with N-HCl. For the cell

Pt | 0.01M-H3AsO4, 0.01M-H3NaO3, 0.001M-KI, M-HCl | NH4NO3 saturated solution | M-H2SO4, Hg2SO4 | Hg

E0 at 18° C. is + 0.574 volt, and at higher acidities the value increases. The buffering power of arsenic acid towards alkali is considerable, and much greater than that of phosphate or citrate buffers.

The absorption spectra of aqueous solutions have been examined.

Arsenic acid in solution is readily reduced by nascent hydrogen, arsine being evolved; in the presence of alkali, however, this reduction does not take place. When distilled with concentrated hydrochloric acid, arsenic trichloride is formed and passes into the distillate; the reaction is accelerated by the presence of organic matter or other reducing agents, such as ferrous or cuprous salts. The reaction

As2O5 + 10HCl ⇔ 2AsCl3 + 2Cl2 + 5H2O

is the basis of a method used for separating arsenic from other elements which do not yield volatile chlorides under the same conditions. The amount of trichloride produced increases with the concentration of the acid, and a high yield is obtained when fuming hydrochloric acid is used. Hydrobromic and hydriodic acids also reduce the pentoxide, but the products are arsenious acid and the free halogen; these reactions also are reversible. When heated with potassium chloride, some arsenic trichloride distils over; potassium bromide has very little reducing action, but with alkali iodides, iodine is liberated and arsenious acid formed. In the presence of dilute sulphuric acid the reaction with iodides is quantitative and proceeds according to the equation:

H3AsO4 + 2HI = H3NaO3 + I2 + H2O

This reaction may be employed for the determination of iodides; the iodine is expelled by heating and the equivalent amount of arsenious acid in the solution determined.

The reaction of arsenic acid solutions with hydrogen sulphide has been the subject of much investigation and it is found that arsenic pentasulphide or arsenic trisulphide may be precipitated as the main product, according to conditions. Thus, in an aqueous solution of the pentoxide at the ordinary temperature and in the presence of 8 to 14 per cent, of hydrochloric acid, the precipitate is the pentasulphide; as the concentration of hydrochloric acid is increased, the trisulphide and sulphur are also precipitated in increasing proportion, and at 32 to 33 per cent. HCl no pentasulphide is formed. On the other hand, a cold solution of an arsenate containing not less than 29 per cent, of hydrochloric acid yields only the pentasulphide. The formation of the latter is favoured by rapid passage of the hydrogen sulphide, whilst a rise in temperature above 50° C. favours reduction. A sufficiently rapid introduction of the gas into pure arsenic acid solutions always gives rise to arsenic pentasulphide, and the precipitation is progressively inhibited by hydrochloric acid in concentrations from N to 4N, but is promoted when the concentration exceeds 6N. If the addition of the hydrogen sulphide is interrupted before all the arsenic is precipitated, the solution is found to contain monothioarsenic acid, H3NaO3S, in amount corresponding with the incompleteness of the precipitation, and this acid is the primary product of the reaction, whether hydrochloric acid is present or not. The monothioarsenic acid under the influence of heat and mineral acids decomposes into free sulphur and arsenious acid, the latter with hydrogen sulphide then yielding arsenic trisulphide.

From solutions of pure arsenic acid the pentasulphide separates in a highly disperse form which adsorbs arsenic acid so strongly that the last traces of the latter react with great difficulty at the ordinary temperature, although this is not the case at 40° C. In the presence of salts of multivalent cations which by hydrolysis yield colloidal hydroxides, the arsenic pentasulphide is flocculated, but the completion of the reaction is greatly delayed owing to adsorption of the arsenic acid, especially at low temperatures.

If a solution of an alkali arsenate in moderately concentrated hydrochloric acid is saturated with hydrogen sulphide and heated in a sealed tube in the absence of air for one hour at 100° C., the arsenic is completely converted to the pentasulphide, no trisulphide or sulphur being formed. Solutions of arsenic acid are reduced to arsenious acid by sulphur dioxide, slowly in the cold but more rapidly when heated. The rate of reduction depends upon the degree of acidity of the solution and is complete only after prolonged heating or boiling unless the operation is carried out in a closed vessel. Under the latter conditions the reaction may be used for the preparation of arsenious oxide. The reduction is greatly retarded by the presence of vanadic acid in dilute sulphuric acid solution, but proceeds rapidly if the mixture is heated with a trace of potassium iodide present. Complete reduction of the mixture to arsenious acid and a vanadyl salt may then be brought about by heating in a sealed vessel for about one hour on a water-bath. At room temperatures solutions of sodium monohydrogen arsenate and sulphurous acid react very slowly; the reaction is greatly accelerated by mineral acids and reaches a maximum in the presence of 0.13N- hydrochloric acid or 0.20 to 0.26N-sulphuric acid.

Sodium thiosulphate also yields a precipitate of arsenic pentasulphide, slowly in the cold but more readily on heating and when hydrochloric acid is present; pentathionic acid is formed in the solution.

Hydrazine in the presence of sulphuric acid reduces arsenic acid to a slight extent to arsine, which is subsequently oxidised to arsenic; the reaction does not take place if a considerable concentration of hydrochloric acid is present. Arsenites, in the absence of arsenates, are not reduced by hydrazine. An aqueous solution of arsenic acid is reduced to the trioxide by hydrazine salts only after prolonged boiling. Phosphine yields a copper-coloured precipitate, possibly an arsenide of phosphorus. Phosphorus trichloride causes reduction to arsenic, as also do the tribromide and triiodide, though less readily. Hypophosphorous acid causes a similar reduction to arsenic; in hydrochloric acid solution the velocity of the reaction at various concentrations corresponds with that of a reaction of the second order and may be expressed by

where B and C0 are respectively the initial concentrations of hypo- phosphorous and arsenic acids and C is the concentration of the latter at time t. Formic and oxalic acids and their salts in the presence of mineral acid reduce arsenic acid and arsenates; the reaction is accelerated by boiling. Tartaric acid does not appear to form complexes with arsenic acid such as are formed with arsenious acid. Certain sugars, namely fructose and less rapidly sucrose, but not glucose, maltose or lactose, form labile esters of arsenic acid during fermentation in the presence of this acid; the reaction is purely a chemical one and not biochemical. Many metals, including magnesium, aluminium, zinc, tin and iron, precipitate arsenic and liberate arsine from aqueous arsenic acid. When copper is placed in such a solution containing mineral acid, copper arsenide is formed on the metal; this reaction is employed under the name of Reinseh's test in the qualitative detection of arsenic. If an aqueous solution of arsenic acid is heated with copper in a sealed tube for 18 hours at 180° to 200° C., arsenious oxide and copper arsenate are formed.

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