Arsenic Disulphide, As₂S₂

Arsenic Disulphide (Realgar), As₂S₂, the sandarach of Pliny, is also known as the red sulphide of arsenic and in the native form as ruby sulphur. Although it occurs as a mineral, the realgar of commerce is usually produced artificially. A mixture of mispickel and iron pyrites is distilled in a clay retort, the proportions in the mixture being such that the product will contain excess of arsenic. The condensers are operated at a temperature above the melting point of realgar so that most of the product is obtained in the fused state and any pulverulent material can be re-treated. The fused condensate is allowed to set, crushed to a powder, mixed with additional sulphur to give the composition which will produce the colour tint required, and again fused. To obtain a rich red-coloured product it is not necessary that the elements should be present in stoichiometrical proportions. In fact, although realgar may be produced by fusion of such mixtures of sulphur and arsenic, or sulphur and arsenious oxide, it is better to have the latter in each case in excess or some arsenious sulphide is formed; the equation for the reaction in the second case is

2As₂O₃ + 7S = 2As₂S₂ + 3SO₂

It may also be obtained by fusion of a mixture of arsenic and arsenious sulphide in the calculated proportions. A fiery red product is formed by melting arsenious oxide or arsenic pentoxide with sodium thiosulphate. The red sulphide has also been found mixed with other sulphides in the flue dusts obtained during the roasting of arsenical ores. Realgar may also be prepared in the laboratory by heating arsenious sulphide with an aqueous solution of sodium bicarbonate in a sealed tube at a temperature of 150° C.; the sulphide dissolves and, on cooling, realgar crystallises from the solution. The same result is obtained by dissolving arsenious sulphide in a boiling concentrated aqueous solution of sodium carbonate and cooling the liquid. The disulphide dissolves in hot carbon disulphide and can be recrystallised therefrom. Black needle-shaped crystals of realgar were obtained by Borodowski by heating in a sealed tube at 150° to 300° C. a mixture of arsenic and sulphur in atomic proportions with a 10 per cent, aqueous potassium carbonate solution, and cooling the resulting clear solution.

Realgar, both natural and artificial, varies considerably in colour according to the conditions of formation; it may be red, orange-red or orange-yellow, or even black. It forms short, transparent or translucent, prismatic crystals, with axial ratios 0.7201:1:0.4872 and β 66°11'. According to Beijerinck, the crystals are pleochroic, being red in the direction of the axis and yellow in the direction of the base. The faces in the prismatic zone are striated vertically. The (010)-cleavage is almost perfect; the (100)-, (001)-, (210)- and (110)- cleavages are somewhat less clear. The hardness is 1.5 to 2.0 and the density 3.2 to 3.5. Borodowski, whose investigation gave 3.506 for α-As₂S₂, 3.254 for β-As₂S₂ and 3.161 for the amorphous form. The optical character is negative. The optic axial angle 2H = 96°20' for red light and 92°58' for yellow light.

In powder form realgar is usually orange-red; when heated the colour darkens to brown, but on cooling returns to the original. The compound melts, according to Borgstrom, at 310° C. Below this temperature it may be sublimed in a vacuum, and under ordinary pressure out of contact with air it sublimes without change well below red heat. The vapour is yellowish-green and the sublimate reddish-yellow. The following values of the vapour density, determined by Szarvasy and Messinger, show that there is considerable association at lower temperatures:

Temperature, ° C	450	503	513	574	588	1000 (approx.)	2000 (approx.)
Vapour Density.	19.16	18.5	15.9	13.89	12.52	7.51	6.95

(Theory: As₂S₂ = 7.40)

The boiling point is 565° C. at 760 mm. According to Britzke and his co-workers, the vapour begins to dissociate into its elements between 781° and 830° C., and then contains As₂S₂, As₄, As₂ and S₂ molecules; the dissociation is complete at 1076° C.

The molar heat of formation from solid arsenic and rhombic sulphur has been calculated to be 28,900 calories. This value differs considerably from earlier determinations. From diatomic gaseous sulphur and solid arsenic the heat of formation is calculated to be 51,430 calories.

Under the influence of light, realgar disintegrates to a reddish-yellow powder which consists of arsenious sulphide and arsenious oxide, together with some tetrarsenic trisulphide. It was observed by Weigel that the incidence of light affects the electrical conductivity of the crystals, which is very small but which varies with the wavelength of the incident light, showing maxima at 5300 and 5500 A. Thus, fragments of realgar exposed to direct sunlight for 112 hours behind sheets of glass of various colours show the greatest change with a green glass of maximum transparency near 5200 A. Wiegel concluded that the photochemical disintegration is due to the separation of electrons and the loosening of the atomic linkings, thus facilitating oxidation of the arsenic.

Realgar is less diamagnetic than orpiment both in the solid form and in colloidal solution. The mineral is opaque to X-rays.

Arsenic disulphide is reduced to arsenic when heated in hydrogen; the reaction, which is reversible,

As₂S₂ + 2H₂ ⇔ 2H₂S + As₂

commences at about 300° C. In air, oxidation occurs slowly at ordinary temperatures and oxide usually occurs on the surface of natural sulphides, being produced by the reaction

6As₂S₂ + 3O₂ = 4As₂S₃ + 2As₂O₃

When heated to 215° C., realgar decreases in weight owing to oxidation to arsenious oxide and sulphur dioxide; the reaction becomes more rapid with rise in temperature, and the sulphide finally inflames and burns with a bluish flame. The heat of the reaction representing the roasting process has been given as

[As₂S₂] + 3.5(O₂) = [As₂O₃] + 2(SO₂) + 271,370 cal.

When oxygen under pressure acts upon an aqueous suspension of realgar, some sulphuric acid is produced but no arsenic acid can be detected.

Water does not sensibly attack realgar (at boiling temperature a little arsenious oxide and hydrogen sulphide are produced), but steam reacts at red heat to give a sublimate of arsenious oxide and arsenious sulphide.

The sulphide is decomposed by chlorine. In a rapid stream of the gas it inflames and yields a yellowish-brown liquid which, on fractionation, yields sulphur monochloride and arsenic trichloride. Bromine water oxidises realgar to arsenic acid. When fused with iodine, arsenious iodide and sulphur are formed:

As₂S₂ + 3I₂ = 2AsI₃ + 2S

The same reaction occurs if realgar is added to a solution of iodine in an organic solvent such as carbon disulphide, and the solution is decolorised.

Dilute aqueous ammonia has no action on realgar; more concentrated solutions cause a dulling of the surface. Liquid ammonia dissolves it. Heated with a mixture of ammonium chloride and ammonium nitrate, realgar yields arsenic trichloride. When boiled with aqueous alkali, realgar reacts to yield the trisulphide and a precipitate of arsenic, thus -

3As₂S₂ = 2As₂S₃ + 2As

No arsine is evolved, but probably a trace of hydrogen results from the decomposition of water by the finely divided arsenic. The latter gradually encrusts the remaining realgar and prevents the reaction from proceeding to completion. It was the residue from this reaction that Berzelius regarded as As₁₂S.

If finely powdered realgar is heated with aqueous sodium sulphide in a sealed tube at 100° C., a thioarsenate is formed and arsenic, which may be contaminated with sulphur, is precipitated. Heated with arsenious oxide, metallic arsenic sublimes and sulphur dioxide is evolved.

Strong oxidising agents convert realgar into sulphuric and arsenic acids; thus, in an atmosphere of oxygen under a pressure of 20 atm. and at 120° C., aqueous nitric acid (10 per cent.) completely oxidises half its weight of realgar in 30 minutes. With no oxygen present, a larger amount of nitric acid is required. Boiling with 5 per cent, nitric acid produces some hydrogen sulphide, but with more concentrated acid, oxides of nitrogen are evolved. Nitric oxide is evolved from a cold agitated mixture of realgar and 40 per cent, nitric acid. A mixture of nitre and realgar can be detonated, and the addition of sulphur to these two provides a mixture much used in pyrotechny for producing a white light of bluish tint; the so-called Indian fire and Bengal lights or "blue light" generally contain the ingredients in the proportions: realgar 2, sulphur 7, nitre 24. A mixture of these three substances was also employed by the Germans during the Great War in smoke candles; these on ignition gave off a heavy yellowish smoke.

Realgar is employed as a depilatory in tanning, its red colour being a desirable feature in the treatment of hides and skins. It has been used as a paint pigment under the name of arsenic orange, but it is not too permanent and is now seldom used. Its early use as a remedy for asthma, etc., has been mentioned. Unless carefully prepared, the commercial product is liable to contain white arsenic, and the poisonous nature of the latter tends to limit the applications of realgar.