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Arsenic Trichloride, AsCl3

Arsenic Trichloride, AsCl3, was discovered in 1648 by Glauber, who obtained it by heating in a retort a mixture of white arsenic, common salt and sulphuric acid; a thick oil, which he designated butter of arsenic, collected in the receiver. The compound is produced in many reactions and is usually formed when arsenic and its compounds are chlorinated. The more important methods of formation and preparation are as follows.
  1. By direct action of chlorine on arsenic. When powdered arsenic is sprinkled into chlorine gas, or into liquid chlorine near its boiling point, union occurs with incandescence and a white cloud of the trichloride is formed. If the gas is passed over arsenic in a tube connected with a receiver immersed in a freezing mixture, the powder which condenses is yellow, owing to excess of chlorine; the latter may be removed by distillation from powdered arsenic. The reaction is accelerated by the addition of a trace of bromine or of alkali halide. When chlorine is passed into a solution of yellow arsenic in carbon disulphide, brown arsenic is first precipitated, which is then converted to arsenic trichloride.

    In order to obtain specimens of arsenic trichloride sufficiently pure for atomic weight determinations, Krepelka fractionally distilled three times in nitrogen the product from the reaction of pure metallic arsenic and dry chlorine. The distillate was then fractioned in vacuo, the middle fraction being filled into bulbs.
  2. By the action of chlorine compounds on arsenic. The trichloride is produced by the action on arsenic of boiling hydrochloric acid in the presence of oxygen, or by gently heating arsenic with the chloride of ammonium, magnesium, aluminium or mercury. A convenient method of preparation is to heat a mixture containing arsenic (1 part) and mercuric chloride (6 parts). Other compounds which may be used are sulphur monochloride, sulphuryl chloride, chlorosulphonic acid, phosphorus pentachloride and phosphorus oxychloride.
  3. By the action of chlorine and chlorine compounds on arsenious oxide. Chlorine reacts with the heated oxide to form arsenic trichloride and arsenic pentoxide. 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 30 per cent, of the latter is converted to the trichloride and the remainder to the pentoxide. A saturated solution of arsenious oxide in concentrated hydrochloric acid when heated with concentrated sulphuric acid yields the trichloride, and the latter is also produced when a mixture of arsenious oxide and sodium chloride or lead chloride is heated with concentrated sulphuric acid. An optimum yield (85 per cent, of theoretical) is obtained when the reactants in the proportions As2O3 99 g., NaCl 261 g. and conc. H2SO4 213 c.c. are slowly heated (3 to 4 hours) to 180° C. Dry hydrogen chloride itself heated with arsenious oxide yields arsenic trichloride. The following may also be used as chlorinating agents: ammonium chloride, silicon tetrachloride, phosphorus trichloride, phosphorus pentachloride and phosphorus oxychloride. If chlorine is passed over a heated mixture of arsenious oxide and sulphur, or into a suspension of these two substances in arsenic trichloride at the boiling point of the latter, chlorination occurs and these are suitable methods of preparation. Sulphur monochloride itself may be used, but sulphur is deposited, and to avoid this Partington recommends passing chlorine into a mixture of arsenious oxide and sulphur monochloride, the reaction being

    4As2O3 + 3S2Cl2 + 9Cl2 = 8AsCl3 + 6SO2

    Two-thirds of the requisite amount of arsenious oxide are first added and the remainder after the reaction has progressed for a time, the gas being passed continuously. The arsenic trichloride may be distilled off and does not require purification. The reaction is somewhat violent, however. Arsenic trichloride may also be prepared conveniently by passing carbonyl chloride over a mixture containing 80 per cent, of arsenious oxide and 20 per cent, of carbon heated at 200° to 260° C.; the yield is almost quantitative.
  4. By the action of chlorine and chlorine compounds on arsenic pentoxide. Arsenic trichloride is formed when chlorine is passed over the heated oxide. Hydrogen chloride reacts at ordinary temperatures, but not at -20° C. 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 sulphate or chloride, potassium bromide or iodide, hydrobromic acid, methyl alcohol or pyrogallol, and by this means arsenic may be completely removed from solution as the volatile chloride. Arsenic acid and arsenates heated with phosphorus pentachloride or ammonium chloride also yield arsenic trichloride. When an arsenate is reduced by the action of a hydrazine salt in the presence of aqueous hydrochloric acid and potassium bromide, arsenic trichloride and elementary arsenic are formed. The latter may be reduced to a minimum by increasing the concentration of the acid to 5.5N, when the trichloride is practically the sole product.
  5. By the action of chlorine and chlorine compounds on the sulphides of arsenic. By passing chlorine over the dry sulphides, realgar or orpiment, at 130° to 140° C., theoretical yields of arsenic trichloride and of sulphur dichloride are obtained. The trisulphide reacts with hydrogen chloride in the cold, but chlorination by means of hydrochloric acid is difficult, only a small quantity of the chloride being volatilised. The reaction is facilitated by the presence of ferric chloride, cuprous chloride or potassium antimony 1 tartrate. Other chlorinating agents, effective with the sulphides, are sulphur mono- chloride, a mixture of ammonium chloride and nitrate, and mercuric chloride.

Physical Properties of Arsenic Trichloride

Arsenic trichloride is a colourless, transparent oily liquid at ordinary temperatures. The following values for the density, specific cohesion, surface tension and molecular surface energy were determined by Jager:

Temperature, ° C.-210°0.0°20.8°50.2°75.7°110.0°
Density2.2452.2052.1652.1052.0511.968
Spec. Cohesion (sq. mm.)3.983.833.713.543.403.21
Surf. Tension (dynes per cm.).43.841.439.436.634.231.0
Mol. Surf. Energy (ergs per sq. cm.)818.4782.0754.3713.9678.8632.4
The parachor is 212.0


At the boiling point, 129.6° C. at 760 mm., the density is 1-91813. The density at different temperatures referred to water at 4° C. may be represented by the equation

D = 2.20511 – 0.001856θ – 0.0000027θ2

The liquid may be used as a solvent for the ebullioscopic or cryoscopic determination of molecular weights, the ebullioscopic constant being 6.5 to 7.25. Beckmann used it in this way for determining the molecular weight of sulphur, but when this method is applied to solutions of the halides of phosphorus, antimony, titanium or tin in arsenic trichloride, or to solutions of any of these halides in one another, the molecular weights obtained are abnormally small, presumably owing to the halogen atoms being readily interchangeable so that, on mixing, reciprocal conversions occur with formation of mixed halides; the latter, however, cannot be isolated in solid or gaseous form. Alcohol and ether mix readily with arsenic trichloride, the liquid becoming warm; turpentine, olive oil and certain resins also dissolve in the liquid, as do many organic substances, often with formation of compounds.

The molecular volume of the liquid at the ordinary temperature is 92.4 c.c., and the ratio of this value to the sum of the atomic volumes of the constituent elements is 1.33. From measurements of density and coefficient of expansion at low temperature (-194° C.) the molecular volume at 0° Abs. has been calculated to be 67.7 c.c.

Vapour pressure determinations at various temperatures have been made, but the results obtained by different investigators are not in agreement. The following are probably the most reliable values:

Temperature, ° C.0253550100129.6
Vap. press., mm.2.4411.6519.5340.90301760


The specific heat of the liquid is 0.17604. The thermal expansion with rise in temperature may be represented by the equation:

v = 1 + 0.000991338θ + 0.0684914θ2 + 0.0827.551θ3

v being unity at 0° C. The heat of vaporisation according to Regnault is 69.741 calories, but Beckmann gave the value 44.51 calories. The molecular heat of vaporisation has been calculated to be 7420 cals. at 0° C., 7290 cals. at 50° C. and 6680 cals. at 100° C. The specific heat of the vapour is 0.11224. The critical temperature has been estimated to be 356° C. The heat of formation of the liquid is 71,390 calories.

The vapour density of arsenic trichloride was found by Dumas to be 6.301 (air = 1), which value agrees with that required by the formula AsCl3 ( = 6.27).

Arsenic trichloride may be solidified by cooling. It forms white, nacreous, acicular crystals of melting point -16° C. The solidification is accompanied by an appreciable decrease in volume.

The refractive index of the liquid for light of various wavelengths has been determined as follows:

λ, A.274029803940480058907680
n1.7811.730231.645401.619491.603951.59262


The specific refraction is 0.2732 and the refraction equivalent 49.50. The absorption of light by a 0.01N-solution of arsenic trichloride in N-hydrochloric acid has been examined, and it has been observed that the molecular extinction coefficient varies from 5 to 30 as the wavelength of the incident light decreases from 3570 to 2270 A.

The Raman spectra of arsenic trichloride and of its solutions in various organic solvents have been investigated; on admixture, the Raman frequencies of the solvent, for example benzene, carbon tetrachloride, methyl or ethyl alcohol, remain unchanged, but those of the arsenic trichloride are altered. The deviations are attributed to the polar character of the molecule and it is suggested that, for the frequencies of one component of a binary mixture to be altered by the other, the latter must have a considerable dipole moment, and in the former the linking between the parts of the molecule participating in active vibrations, in this case As-Cl, must be weak. The constants of the AsCl3 molecule calculated from the results agree with those derived from electron diffraction data. The free energies of formation of liquid and gaseous arsenic trichloride, at 25° C., are respectively -65,190 and -62,718 calories ±500 to 1400 calories. The value for the electron polarisation, PE = 30.24, has been derived from measurements of the refractive index, and it has been shown mathematically, both from energy considerations and the Raman and X-ray spectra, that the stable form for the molecule is a three-sided pyramid with the arsenic atom at the apex, the distance between the arsenic and chlorine atoms being 2.20 A. The atomic radius of the arsenic is 1.21 A.

Liquid arsenic trichloride when anhydrous is a very poor conductor of electricity, the specific conductivity at 0° C. being 11×10-7 mho. The solution of the chloride in liquid hydrogen sulphide conducts electricity, as also do solutions in certain organic solvents. Thus, in ethyl ether the conductivity is very small up to 40 per cent, of trichloride, but then increases to a maximum at 94 per cent, concentration; the temperature coefficient of the conductivity is negative at all concentrations. The decomposition potential of a 68.81 per cent, solution of arsenic trichloride in ether at 18° C. is 1.22 volts. On electrolysis, arsenic is quantitatively deposited at the cathode. From a systematic study of the viscosity and conductivity of the solutions of concentrations from 25 to 100 per cent. AsCl3 and over the temperature range 0° to 50° C., it has been shown that the compound AsCl3-(C2H5)2O is formed and acts as the electrolyte. Similar investigations of solutions in monochloromethyl ether, benzene, nitrobenzene and pyridine indicate the formation of the following compounds: AsCl3.CH2Cl.O.CH3; 2AsCl3.C6H6; AsCl3.2C6H5NO2; AsCl3.C5H5N. The decomposition potential of the solution in nitrobenzene is 0.78 volt. Viscosity measurements of solutions in anisole indicate the formation of the compound AsCl3.C6H5OCH3, but the solutions are non-conducting.

Arsenic trichloride appears to be a good ionising solvent for binary salts, although cobalt iodide is an exception. The dielectric constant is 12.8. From measurements of the dielectric constants and densities of solutions in benzene, the dipole moment of arsenic trichloride has been calculated to be 2.15×10-18 e.s.u.; the value previously obtained by Bergmann and Engel was 1.97×10-18 e.s.u.

The vapour is readily adsorbed by fibres such as hair and wool, and also by rubber.

Chemical Properties of Arsenic Trichloride

Arsenic trichloride vapour reacts with hydrogen when the mixture is subjected to an electric discharge, a brown deposit, thought to be a mixture of arsenic and a subchloride, being formed on the walls of the containing vessel. The vapour also reacts with oxygen when a mixture of the two is passed through a tube heated to redness, an oxychloride and chlorine being produced. Water in small quantity also produces the oxychloride; with more water arsenious oxide is precipitated, while with an excess of water arsenious and hydrochloric acids are formed, the heat of the reaction being as follows:

AsCl3 + Aq. = H3NaO3 aq. + 3HCl aq. + 17,600 calories

The solution is similar to that obtained by dissolving arsenious oxide in aqueous hydrochloric acid. When the liquid is distilled, all the arsenic passes over into the distillate and by this means the element may be separated from antimony and tin.

The trichloride reacts violently with fluorine to form the trifluoride. Chlorine dissolves readily at low temperatures without reaction, the gas being expelled on warming. Iodine also dissolves without reaction, the solubility increasing with temperature, thus:

Temperature, °C.01590
Grams Iodine per 100 grams AsCl3.8.4211.8836.89


Concentrated hydrochloric acid also dissolves the trichloride, about 100 g. of the latter dissolving in 1 litre of acid at 100° C. Dissolution in hydriodic acid is accompanied by evolution of heat and the triiodide is formed. Ethyl iodide reacts similarly. Double decomposition reactions occur when arsenic trichloride is heated with phosphorus triiodide, stannic iodide or germanium iodide, the reactions being complete. Similarly, potassium iodide heated with arsenic trichloride in a sealed tube at 240° C., and potassium bromide at 180° to 200° C., form respectively arsenic triiodide and tribromide. Stannous chloride, added to the solution in hydrochloric acid, causes reduction to arsenic. Arsenic trichloride may be completely separated from germanium chloride by extraction with concentrated hydrochloric acid. Ammonium, sodium and cobaltic chlorides react with arsenic trichloride to form additive compounds; with magnesium, zinc and chromic chlorides there is no reaction.

Ammonia is rapidly absorbed by liquid arsenic trichloride to yield a pale yellow solid which is usually described as arsenic tetramminotrichloride, AsCl3.4NH3, but which Hugot stated to be a mixture of ammonium chloride and arsenic triamide. The product is readily decomposed; on heating, the substance completely volatilises, ammonia being first evolved; on heating with water, ammonia, ammonium chloride and arsenious oxide are formed. Heating with concentrated sulphuric acid also causes decomposition. The addition of chloroplatinic acid to the aqueous solution does not cause precipitation of all the ammonia present. Liquid ammonia also reacts with arsenic trichloride. Hydroxylamine forms an addition compound. A boiling alcoholic solution of the trichloride dissolves hydrazine hydrochloride, apparently forming a complex salt. Nitrogen peroxide reacts to form nitrogen oxychloride and arsenic pentoxide. There is no reaction with nitrogen sulphide.

Phosphorus dissolves in arsenic trichloride on warming without reaction and is deposited on cooling. If the mixture is heated in the presence of aluminium chloride at 130° to 150° C. for 40 minutes, a brownish-red compound of composition AlAs3Cl3 results, from which, however, aluminium chloride can readily be removed by water or ammonia, leaving a residue of finely divided black arsenic. If the compound is heated to 190° C. in the absence of air, it turns black as arsenic trichloride and aluminium chloride distil off and a bright grey mixture of arsenic and arsenide remains. It has been suggested that the arsenic is co-ordinatively bound, and that the compound may be formulated analogous to the ammoniated compound Phosphine reacts to produce a brown substance of indefinite character, possibly arsenic phosphide, AsP, but more probably a mixture containing the free elements. Phosphorus trichloride, unlike the triiodide, causes reduction to arsenic, as also do hypophosphorous and phosphorous acids, while phosphorus pentachloride combines to form unstable double compounds of composition PCl5.AsCl3 and PCl5.AsCl5. A solution of arsenic trichloride in nitrobenzene is reduced to arsenic by the action of yellow phosphorus, antimony or bismuth.

Arsine reacts to yield arsenic, thus:

AsH3 + AsCl3 = 2As + 3HCl

Arsenious oxide dissolves in the boiling trichloride to form arsenic oxychloride, AsOCl. Arsenic trisulphide yields thiochlorides.

Sulphur is unattacked at room temperature, dissolves readily in hot arsenic trichloride and is deposited unchanged on cooling. The reaction with liquid hydrogen sulphide has been investigated at temperatures from -77° C. up to room temperature. Thiohydrolysis occurs and arsenious sulphide is precipitated immediately at all temperatures. The reaction differs from that with water, where the soluble acid is formed, the thio-acid not being produced. The specific conductivity of the saturated solution of arsenic trichloride in liquid hydrogen sulphide, after standing until equilibrium is attained, is 11.51×10-7 mho. Gaseous hydrogen sulphide reacts with arsenic trichloride to give a yellow precipitate of a thiochloride, As4S5Cl2, if the reactants are quite dry, but if water is present arsenious sulphide is precipitated.

Selenium and tellurium are attacked at room temperature, arsenic being liberated and selenium monochloride or tellurium tetrachloride formed.

Many carbon compounds, e.g. hydrocarbons, ketones, organic acids, bases and esters, dissolve in arsenic trichloride with formation of additive or complex compounds. Trialkyl arsines are formed by the addition of alkali to the double salts obtained by the interaction of zinc dialkyls and arsenic trichloride -

3ZnR2 + 2AsCl3 = 2R3As + 3ZnCl2

or by treating Grignard reagents with the trichloride:

3RMgCl + AsCl3 = R3As + 3MgCl2

Free arsenic is also produced owing to the reducing action of the arsine. If the tertiary arsine is heated with arsenic trichloride under pressure at a high temperature, the following reaction takes place -

R3As + 2AsCl3 = SRAsCl2

and similar products may be obtained by heating arsenic trichloride with mercury diaryls or with aryl mercuric chlorides:

R2Hg + 2AsCl3 = 2RAsCl2 + HgCl2
RHgCl + AsCl3 = RAsCl2 + HgCl2

Additive compounds are formed with aniline, piperazine, hexamethylenetetramine and quinoline; with pyridine, the two compounds, AsCl3. C5H5N (m.pt. 138.9° C.) and AsCl3.2C5H5N (m.pt. 64° C.), have been isolated. Addition of arsenic trichloride to dry 1:4-dioxan gives an oxonium compound, (C4H8O2)3.2AsCl3 (m.pt. 62° C.).

The trichloride dissolves in liquid cyanogen. It combines with cyanogen bromide when a mixture of the two is slowly heated in an autoclave to 180° C., followed by cooling and keeping at 120° C. for one hour; the product, of composition AsCl3.2BrCN, decomposes on heating above 190° C. Arsenic trichloride reacts with potassium thiocyanate solution, the latter being decomposed with liberation of ammonia, but no precipitate is produced, whereas both tin and antimony are precipitated as hydroxides under similar conditions; the reaction with excess of potassium thiocyanate may be used to effect a quantitative separation of these two metals from arsenic.

Boron bromide reacts with the trichloride thus:

AsCl3 + BBr3 = BCl3 + AsBr3 + 25 calories

Many metals when immersed in arsenic trichloride precipitate arsenic. With sodium, magnesium, zinc, aluminium, tin and lead, at the ordinary temperature, a coating of arsenic immediately forms on the metal and slows down the reaction; at 100° C. the reaction is vigorous. With copper, plating occurs very slowly but the reaction takes place more rapidly on heating. With iron, cobalt, nickel, antimony, bismuth, cadmium, mercury, silver and gold, the action is only slight at temperatures up to 100° C. Platinum and molybdenum are not acted upon. In acid solution magnesium precipitates arsenic and liberates arsine; cobalt and nickel yield arsenides. Aluminium in the presence of a little aluminium chloride behaves like phosphorus and when heated with the trichloride at 130° to 150° C. produces the brownish-red arsenochloride, AlAs3Cl3, which with water yields arsenic and aluminium chloride. Finely divided reduced silver heated with the trichloride in a sealed tube yields a product of composition 7Ag.2AsCl3, which however is probably not a chemical entity; copper yields a similar product. A boiling solution of the trichloride in toluene is reduced by potassium with separation of arsenic.

Rubidium and caesium salts of composition 3RbCl.2AsCl3 and 3CsCl.2AsCl3 have been obtained in the form of trigonal crystals by mixing saturated solutions of the alkali chloride and of arsenious oxide in 20 per cent, hydrochloric acid and then adding concentrated hydrochloric acid.

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