Chemical Properties of Arsenic

Arsenic does not combine directly with molecular hydrogen, and the element may be purified by sublimation in that gas. Hydrides, however, may be obtained by indirect methods. Arsenic may be displaced by the gas from solutions of its salts at high temperatures and pressures. Thus arsenic separates in large well-defined crystals when a solution of sodium arsenate is subjected to the action of hydrogen at 25 atm. pressure; the action commences at 300° C., 15 per cent, of the arsenic being precipitated at this temperature, but it increases rapidly with rising temperature and at 350° C. 77 per cent, of the arsenic is liberated. Arsine is not produced in the reaction.

The reduction in presence of alkali occurs in two stages:

Na₂HAsO₄ + 2H → Na₂HNaO₃ + H₂O
Na₂HNaO₃ + 3H → As + 2NaOH + H₂O

With hydrogen at 45 atm. pressure and at 200° C. only the first reaction takes place, but at 250° C. the arsenic begins to separate and at 350° C. the maximum separation, 96 per cent., is observed. The yield varies slightly with the amount of alkali present, but the maximum at this temperature (350° C.) can occur in the absence of alkali. Complete displacement has not been observed.

With pressures up to 150 atm. the quantity of arsenic displaced by hydrogen from solutions of arsenic trichloride in hydrochloric acid is proportional to the pressure. Between 15 and 250 atm., and with solutions not exceeding normal concentration, the reaction is one of the

first order; between 125° and 175° C. log K is a linear function of 1/T. The activating energy of the replacement is calculated to be 28,000 ± 2000 calories. It is estimated that the displacement of 1 per cent, of arsenic from a normal solution of arsenic trichloride at room temperature and 100 atm. of hydrogen would require 1140 years. Increase in the concentration of hydrochloric acid accelerates the reaction, which is inhibited by the presence of sodium chloride, and the reaction appears to be ionic rather than molecular.

Arsenic is not attacked by dry air, but in moist air the element, in the crystalline form, is superficially oxidised, acquiring a bronze tinge or even disintegrating to a black powder. The change is accelerated by exposure to light or by gently warming to 30° to 40° C. Amorphous arsenic is not attacked. At a higher temperature arsenic burns with a smouldering flame, emitting a reddish fume which has an odour of garlic and forming arsenious oxide; at a still higher temperature the flame is pale blue. Combustion is vigorous in oxygen, and the presence of moisture does not appear to be necessary for this reaction.

Arsenic, when gently heated in the presence of air or oxygen, exhibits phosphorescence which, as with phosphorus and sulphur, is accompanied by oxidation, arsenious oxide containing about 3 per cent, of arsenic oxide being produced. No ozone is formed, nor is there ionisation, as in the phosphorescence of the two elements mentioned. The arsenic oxide appears to be a primary product formed directly from the arsenic, as the lower oxide does not yield it under such conditions. Arsenious oxide is formed slowly below 200° C. without luminescence, but between 250° and 310° C. the glow appears suddenly so long as the pressure is between certain limits, outside of which no luminescence is observed. The lower limit, 4 to 10 mm. Hg, falls with increasing temperature, while the upper limit, 200 to 700 mm. Hg, rises with temperature. On appearance of the glow the temperature rises about 7° C. The glow appears at lower temperatures (220° to 245° C.) with flowing oxygen than with the stationary gas at the same pressure; also the glow temperature is 10° lower in air than in oxygen. The appearance of the glow is favoured by a rapid removal of the arsenious oxide formed. The introduction of a small quantity of carbon tetrachloride, nitrobenzene or sulphur dioxide into the oxygen does not affect the glow temperature, although the analogous glow of phosphorus is inhibited by this means. The arsenic glow may be completely extinguished at a given temperature by saturating the oxygen with the vapour of benzene, methyl or ethyl alcohol, hexane, acetone, chloroform, amyl or ethyl acetate, or chlorobenzene; the glow reappears on removal of the organic vapour or on raising the temperature by 12° to 30°. The spectrum of the phosphorescent flame consists of an apparently continuous band between 4300 and 4900 A., with a maximum intensity at about 4600 A.; there was no evidence of bands in the ultraviolet. The ordinary flame of arsenic burning in oxygen gives a similar spectrum. The non-luminescent reaction below 200° C. is a surface one, but the chemi-luminescent reaction occurs in the vapour phase. When arsenic is burned in oxygen at 15 to 40 atm. pressure, the ratio As₂O₅: As₂O₃ in the product increases with the concentration of the oxygen. Arsenic is oxidised to arsenic acid by ozone or by ozonised ether or turpentine.

At ordinary temperature pure water, free from air, has no action upon arsenic, even after ten years' contact in a sealed glass tube. In the presence of dissolved oxygen, absorbed by exposure to air, oxidation occurs, arsenious acid being formed, but no arsine. Under ordinary conditions oxidation is slight, and not more than 7 per cent, of arsenious acid is formed at 350° C. 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:

2As + 3H₂O ⇔ As₂O₃ + 3H₂

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.

Hydrogen peroxide reacts vigorously with arsenic to form arsenic pentoxide.

Fluorine and chlorine react vigorously with arsenic to form the trihalides and, in the case of fluorine, some pentafluoride. The reactions are accompanied by incandescence. Bromine and iodine also yield the trihalides, but the reactions occur much less readily and it is necessary to warm the powdered mixture of arsenic and iodine. The hydracids do not react readily; hydrogen fluoride and hydrogen chloride react in the presence of air, and hydrogen bromide and hydrogen iodide act the more rapidly as the halogen element is liberated by the ready dissociation of the gas. Arsenic is readily oxidised to arsenic acid by chloric or bromic acid, and to arsenate by boiling aqueous potassium chlorate. A mixture of powdered arsenic and potassium chlorate detonates on percussion. Iodine fluoride reacts energetically with arsenic.

Arsenic unites directly with sulphur when a mixture of the two elements is heated. When a solution of sulphur and arsenic in carbon disulphide is exposed to light, a powder, the colour of which varies from yellow to orange, is slowly deposited. After prolonged extraction with carbon disulphide, the powder contains the two elements in a proportion which depends on that in the original solution, and appears to consist of a mixture of sulphides, the composition of which, however, remains obscure. When arsenic is heated at 230° C. in hydrogen sulphide it gradually forms arsenic trisulphide; the reaction takes place more readily in the presence of aluminium chloride, hydrogen chloride also being formed. A precipitate of the sulphide is formed when hydrogen sulphide is passed into a solution of yellow arsenic in carbon disulphide. Sulphur dioxide deposits a small quantity of brown arsenic from such a solution. This gas does not react with solid arsenic, but with the vapour arsenious oxide and sulphide are formed. Aqueous sulphurous acid, heated with arsenic in a sealed tube at 200° C., produces a mixture of arsenious oxide, sulphuric acid and free sulphur. Sulphur trioxide and boiling concentrated sulphuric acid oxidise arsenic to arsenious oxide; in the latter case the reaction commences at about 110° C. and sulphur dioxide is evolved. Chlorsulphonic acid yields arsenic trichloride, sulphur dioxide and sulphuric acid. Aqueous solutions of alkali persulphates bring about the oxidation of arsenic to arsenic acid. Selenium and tellurium and their hydrides readily attack arsenic to form the selenide or telluride; the element is also dissolved by hot aqueous telluric acid.

Nitrogen does not react with arsenic. The latter dissolves in aqueous ammonia, apparently forming a complex compound. In anhydrous liquid ammonia it dissolves without reaction and from the solution the arsenic may be successfully electrodeposited. This is not the case with antimony or bismuth. The solution of arsenic in liquid ammonia does not react with metallic cyanides.

Arsenic is oxidised, mainly to arsenious oxide, when heated in nitrous oxide; 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, 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.

Aqueous nitric acid up to 50 per cent, concentration has little action on arsenic, but the concentrated acid or aqua regia causes rapid oxidation to arsenious and arsenic acids. When the acid is more dilute some ammonia may be formed. A mixture of arsenic and potassium nitrate detonates on ignition. Solutions of ammonium and barium nitrates slowly dissolve arsenic to form arsenite and arsenate. Hydrazoic acid dissolves the element with evolution of hydrogen, and the solution on evaporation deposits arsenious oxide. Nitrosyl chloride and potassium amide also react with arsenic.

Phosphorus, when heated to redness with arsenic, combines to form arsenic phosphide. When phosphorus pentoxide and arsenic are heated together at 290° C. the latter is oxidised to arsenious oxide and phosphorus is liberated. Phosphorus trichloride converts arsenic quantitatively into arsenic trichloride when the mixture is heated for 12 hours at 200° C.; phosphorus pentaehloride yields a mixture of the trichlorides.

Arsenic does not combine directly with carbon, silicon or boron. The reaction with metals to form definite arsenides or alloys. The presence of small quantities of arsenic or of its compounds in certain catalysts has a poisoning effect. The first traces added to the catalyst have the greatest effect; thus the activity of 0.35 g. of platinum was reduced linearly by the addition of arsenic up to 0.7 mg., this quantity reducing the catalytic activity to 45 per cent, of its original value; the addition of 10 mg. of arsenic, however, depressed the activity only to 26 per cent, of the original value. Vanadium catalysts are poisoned by the presence of arsenic, although the action is slow; arsenic pentoxide is formed.

Arsenic and many of its compounds exhibit catalytic properties, which may be positive or negative, especially in organic reactions. Thus the element itself in concentrations up to 1 per cent, inhibits the oxidation of acraldehyde for several hours and then accelerates it, while for l-pinene the order is reversed. The activity of arsenic and its oxides is relatively low on account of their insolubility, but the activities of the halogen derivatives are considerably higher.

The ability of arsenic to replace certain metals from solution. When finely divided arsenic is added to a 10 per cent, aqueous solution of silver nitrate, an immediate deposition of silver occurs, accompanied by the evolution of brown fumes of nitrogen dioxide.6 The main reaction may be represented by the equation:

3AgNO₃ + 4As = 3Ag + 2As₂O₃ + 3NO

If the arsenic is added as a piece the silver is deposited in the form of a dull, white, smooth plating. The reaction does not go to completion even after several months' contact. On the other hand, silver is completely displaced within a few hours from solutions saturated with silver nitrite or sulphate, and after a longer time from saturated aqueous solutions of silver acetate and tartrate. In each case arsenic goes into solution as the trioxide. With a solution of silver cyanide in aqueous potassium cyanide the reaction takes a different course, probably following the equation:

3KAg(CN)₂ + As = K₃As(CN)₆ + 3Ag

Only a small proportion of the silver is precipitated, however. Arsenic also replaces silver from some non-aqueous solutions, such as solutions of the nitrate or chloride in pyridine; in others, such as silver palmitate in ether or in acetone, there is no action.

The reaction with salts of mercury is similar, but with mercuric salts a precipitate of the mercurous salt first appears, which gradually disappears, leaving a deposit of mercury. The arsenic passes into solution as the trioxide. Similarly, copper is deposited from solutions of its common salts; with cupric chloride some cuprous chloride is precipitated. The reactions generally are incomplete.

Platinum and gold are slowly replaced from solutions of their chlorides, thus:

3PtCl₄ + 4As + 6H₂O = 3Pt + 2As₂O₃ + 12HCl
2AuCl₃ + 2As + 3H₂O = 2Au + As₂O₃ + 6HCl

No reaction occurs between arsenic and solutions of antimony and bismuth chlorides. Ferric chloride is reduced thus -

6FeCl₃ + 2As + 3H₂O = 6FeCl₂ + As₂O₃ + 6HCl

while cadmium sulphate is reduced to the yellow sulphide:

3CdSO₄ + 8As = 3CdS + 4As₂O₃

Many metallic arsenides are found in Nature. Arsenic combines directly with most metals to form stable compounds, those of the heavy metals being the most stable. The latter may be obtained by allowing an aqueous solution of a salt of the appropriate metal to drop into an atmosphere of arsine, air being completely absent, and the vessel continually shaken. Precipitation by passing arsine into the salt solution is not satisfactory as, in the case of copper, silver, gold, mercury and lead, a secondary reaction with the excess of metallic ions occurs:

M₃As + 3M⁺ + 3H₂O = 6M + 3H⁺ + As(OH)₃

With polyvalent metals, the lowest oxide of the metal is formed.

With iron, cobalt and nickel, alcoholic solutions of the salts should be used, since with aqueous solutions the resulting arsenide is contaminated with free arsenic. Zinc and manganese arsenides are readily hydrolysed and cannot be obtained by the above method, but are prepared by combination of the elements.

The arsenides of the heavy metals are usually black and readily oxidisable. On exposure to moist air they are converted into the metal and arsenious acid. In the dry finely divided state they may ignite spontaneously at ordinary temperatures. At higher temperatures, in absence of air, the arsenides of the noble metals lose nearly all their arsenic, while other heavy metal arsenides form lower arsenides. The arsenides are more stable than the corresponding phosphides.

The metallic arsenides generally melt between 800° and 1200° C., many with decomposition, volatilisation of arsenic occurring below the melting point. The majority of mineral sulpharsenides melt between 400° and 600° C. Arsenopyrite decomposes to arsenic, sulphur and ferrous sulphide at 675° to 685° C., while lollingite loses 23.83 per cent, of arsenic at 735° C.

Under ordinary conditions of temperature and pressure, arsenic does not combine directly with hydrogen unless the latter is in an activated or nascent condition. Three hydrides are known, however, two of which, hydrogen monarsenide, AsH or As₂H₂, and the di-arsenide, As₂H or As₄H₂, are solid at ordinary temperatures, the third being the gaseous hydride, arsine, AsH₃, which is the most stable.

Arsenic and Fluorine

Arsenic forms two fluorides, AsF₃ and AsF₅, both being produced by direct combination of the elements at ordinary temperature. The reaction renders the mass incandescent. The trifluoride is a colourless liquid, and the pentafluoride a colourless gas, both of which fume in contact with air.

Arsenic and Chlorine

One compound only of these two elements has been isolated, namely, arsenic trichloride, which at ordinary temperatures is a colourless oily liquid. Although a substance reported to be arsenic pentachloride has been described in the literature, evidence of the existence of such a compound is not forthcoming.

Arsenic and Bromine

Only one bromide of arsenic has been isolated; this is the tribromide, AsBr₃, and an investigation of the system AsBr₃-Br₂ gives no indication of the formation of a higher bromide. The freezing point curve shows a eutectic point at -34° C. and 81 atomic per cent, of bromine.

Arsenic and Iodine

The existence of two iodides has been established - arsenic diiodide, AsI₂ or As₂I₄, analogous to the corresponding phosphorus compound, and arsenic triiodide, AsI₃, which is the more important. In addition, a moniodide, AsI, and a pentiodide, AsI₅, have been described. The binary system As-I has been investigated, but the freezing point curve gives no indication of the formation of either of the two latter iodides. The oxyiodide of arsenic, AsOI, has not been isolated.

Two well-defined oxides are known arsenious oxide, As₂O₃, and arsenic pentoxide, As₂O₅. The former is the most important compound of arsenic, being the form in which the element is most used. A suboxide, As₂O, and a tetroxide, As₂O₄, have been described, but the existence of neither as a pure compound has been established.

Salts corresponding to the ortho-, meta- and pyro-acids are generally known, although the acids themselves do not appear to exist as stable compounds. The close analogy between arsenic and phosphoric acids and their derivatives is seen in the isomorphism which exists between corresponding salts, for example Na₂HAsO₄.12H₂O and Na₂HPO₄. 12H₂O or MgNH₄AsO₄.6H₂O and MgNH₄PO₄.6H₂O; and also in the formation of analogous heteropoly-acids and salts.

Intermediate compounds between the arsenates and thioarsenates are known, and are derived from the acids H₃NaO₃S, H₃AsO₂S₂ and H₃AsOS₃. These acids are very unstable and have not been isolated, but the first and second have been obtained in dilute aqueous solution.

Arsenious oxide dissolves in hot concentrated or fuming sulphuric acid to form compounds of composition As₂O₃.NSO₃ (n = 1 to 8), which separate as crystals on cooling. Two members of the series, As₂O₃.SO₃ and As₂O₃.3SO₃, have been detected in deposits from the flues of the pyrites burners of lead chamber plant. The compounds readily lose sulphur trioxide on heating, and in contact with water form sulphuric acid and arsenious oxide. This behaviour and the fact that they cannot be prepared by the agency of dilute sulphuric acid, suggests that they should not be considered as sulphates.

Arsenic is insoluble in hydrochloric acid, but readily dissolves in dilute nitric acid, yielding arsenious acid

As₄ + 4HNO₃ + 4H₂O = 4H₃NaO₃ + 4NO

It also dissolves in concentrated nitric acid or aqua regia, and in solutions of hypochlorites, to form arsenic acid

3As₄ + 20HNO₃ + 8H₂O = 12H₃AsO₄ + 20NO
As₄ + 10NaOCl + 6H₂O = 4H₃AsO₄ + 10NaCl

The element is generally met with in the form of its oxides or sulphides, or salts derived from these, but however it occurs it may readily be converted into a form which renders it easy of both detection and estimation. Owing to the necessity of detecting even the smallest traces of the element, the methods employed in many cases are of extreme sensitivity.

In routine analysis arsenic is precipitated from acid solution as sulphide and with antimony, molybdenum and tin is separated from the copper group by dissolution in yellow ammonium sulphide. The sulphides are reprecipitated from this solution by acidifying, and on treatment with concentrated hydrochloric acid all the sulphides, except arsenic, redissolve. The latter dissolves in aqua regia and arsenic may be identified in the solution by applying one of the tests described in the sequel.