Chemical elements
    Physical Properties
      Colloidal Arsenic
      Atomic Weight
    Chemical Properties
    Detection of Arsenic
    Estimation of Arsenic
    Physiological Properties
    PDB 1b92-1ihu
    PDB 1ii0-1tnd
    PDB 1tql-2hmh
    PDB 2hx2-2xnq
    PDB 2xod-3htw
    PDB 3hzf-3od5
    PDB 3ouu-9nse

Allotropy of Arsenic

That arsenic may exist in both crystalline and amorphous forms was observed by Berzelius, who designated them α- and β-arsenic, respectively. Two crystalline allotropes, metallic arsenic (the α-form) and yellow arsenic, are now recognised, and three amorphous forms, vitreous arsenic (the β-form), grey and brown amorphous arsenic, have been described. The majority of investigators, however, deny the existence of more than one amorphous form, and indeed, as will be seen, it is an open question whether any amorphous form is to be considered as a true allotrope.

Bettendorff showed that when arsenic is sublimed in a current of hydrogen the arsenic vapour condenses first as a crust of vitreous arsenic in the zone nearest the source of heat, beyond this as a crust of crystalline metallic arsenic, and in the coolest zone a yellow deposit forms which rapidly becomes grey and pulverulent. Bettendorff did not examine the unstable yellow deposit, but assumed it to be another allotropic variety; the grey powder, which microscopically resembles flowers of sulphur, he designated y-arsenic. If the arsenic is sublimed in a glass tube closed at one end, the sublimate contains some arsenious oxide, and between the vitreous and metallic deposits a brown transparent ring is formed which was thought to be a suboxide.

Metallic arsenic is the stable form. It consists of lustrous, steel-grey crystals belonging to the rhombohedral system and isomorphous with α-antimony, with bismuth and, according to Hittorf, probably with red phosphorus, but this is disputed. The crystals are brittle and of density 5.73. They are oxidised slowly in air, the oxidation being marked at 40° C., when the crystals become covered with a layer of arsenious oxide. The metallic arsenic may be purified by resub- limation in a vacuum. If resublimed in an open tube there appear in the following order: a mirror-like deposit of vitreous arsenic, dark brown specks, and most remote from the source of heat a grey crust. Erdmann and Reppert regarded the brown deposit, of which the density at 15° C. is 3.70, as amorphous arsenic, and the grey crust of γ-arsenic of density 4.64 as a definite crystalline variety. When heated to 360° C. the latter changed to α-arsenic. The light grey vitreous form, which Bettendorff showed to be always deposited when arsenic vapour is cooled to 210° to 220° C., has density 4.716 to 4.740, and is not oxidised in air even when finely powdered and heated to 80° C. It resembles the γ-form in changing to the a-form at 360° C.

In 1869 Bettendorff recorded the formation of a voluminous brown precipitate when stannous chloride was added to a solution of arsenious oxide, or of magnesium ammonium arsenate, in hydrochloric acid. The precipitate proved to be arsenic (96 to 99 per cent.) with traces of tin which were irremovable. The speed of precipitation depends upon the amount of arsenic present and the temperature. With solutions containing little arsenic, Bettendorff observed, on warming, a yellow colour before the precipitate appeared, but he was unable to prove that the colour was due to arsenic. The reaction involved may be represented thus -

2AsCl3 + 3SnCl2 = 3SnCl4 + 2As

and when employed for the detection of arsenic is known as Bettendorff s test.

When a mixture of phosphorus and arsenic trichlorides is treated with water, the arsenious acid first formed is rapidly reduced to arsenic, which is deposited as a brownish-black amorphous powder of density 3.7 at 15° C. Engel obtained this brown precipitate by reduction of arsenious acid with stannous chloride, hypophosphorous acid, copper, and by electrolysis. He gave the density as 4.6, however, and maintained that the vitreous γ-form, the grey y-form and the brown precipitate were all identical, the differences in appearance being due only to the state of subdivision. He suggested, therefore, that there were only two allotropic forms, crystalline and amorphous, and these he compared with white and red phosphorus, observing that the ratio between the densities of the two forms, 1.245, was the same as in the case of the two forms of phosphorus (1.244); moreover, the amorphous form sublimes in an inert gas at 280° to 310° C. and after some hours sublimation ceases, leaving a residue of crystalline arsenic which does not sublime below 360° C., the transformation temperature. In the same way ordinary phosphorus sublimes at a lower temperature than that at which it is converted to red phosphorus. The analogy may not be pushed too far, however. Attempts to produce a doubly refractive form of arsenic analogous to the doubly refractive form of white phosphorus, obtained by strongly cooling, have not been successful, even at -190° C.

Linck in 1899 established the existence of a yellow crystalline variety, obtaining it by gently heating ordinary arsenic in a current of carbon dioxide and rapidly cooling the vapour to below 0° C. in a receiver protected from light. The product dissolved readily in carbon disulphide and the solution yielded, on evaporation, microscopic rhombic dodecahedra belonging to the cubic system and strongly smelling of garlic. These change spontaneously to metallic arsenic, especially on heating or on exposure to light; in the latter case the transformation is complete in about three minutes and may be followed microscopically. The characteristic garlic odour usually associated with arsenic vapour appears to be evidence of the presence of the yellow variety. Erdmann and von Unruh obtained this allotrope by heating ordinary arsenic in an aluminium tube at a temperature above 360° C. As the hot vapour left the tube to enter a U-tube surrounded by ice-water, it met a current of cooled carbon dioxide and condensed as the yellow form. This was immediately dissolved by carbon disulphide contained in the U-tube. Stock and Siebert obtained a similar solution by an electrical method; a current of about 12 amperes was passed between a carbon anode and a cathode consisting of an alloy of equal parts of arsenic and antimony, both electrodes being immersed in carbon disulphide contained in a vessel cooled by ice-water. The arsenic dissolved, but the antimony, although disintegrated, did not dissolve. From the solution the yellow form of arsenic was obtained either by evaporation or by crystallisation at -70° C. The yellow allotrope was also obtained by subliming arsenic in vacuo, the vapour being cooled by liquid air in the dark.

Durrant in 1919 further investigated the reduction of arsenious acid by means of stannous chloride, using solutions of the two chlorides in hydrochloric acid. The anhydrous chlorides do not react, but the addition of a drop of water is sufficient to cause rapid liberation of arsenic. Durrant observed that in the hydrochloric acid solution the appearance of the arsenic precipitate is always preceded by a pale buff tint; the buff-brown precipitate then separates and the pale colour is best observed in mixtures of such dilution that the precipitation is very slow. If the deposit, after washing, is immediately shaken with carbon disulphide, arsenic dissolves, but the amount going into solution is greater if carbon disulphide is vigorously shaken with the solution of the two chlorides while the reaction is in progress. Evaporation of the carbon disulphide leaves a residue of grey arsenic, but during the process pale-coloured particles of arsenic may be seen to rise to the surface and then rapidly darken. Durrant therefore concluded that the yellow allotrope is first deposited but spontaneously changes to the grey amorphous form. Small quantities of arsenic soluble in carbon disulphide may be obtained by reduction of arsenious acid with zinc dust in presence of the solvent.

Yellow arsenic is extremely sensitive to light, especially ultraviolet, quickly darkening in colour as it changes to the grey form even at very low temperatures. It can be preserved for some time if kept away from light and at a temperature below - 60° C. According to Erdmann and Reppert, the formation of grey arsenic is an intermediate stage of the transformation of the yellow to metallic arsenic. In red light the formation of the metallic form is extremely slow. In solution in carbon disulphide, the yellow form shows no tendency to change to the metallic, but on standing the solution slowly deposits the brown modification, which is not sensitive to light.

The solubility of yellow arsenic in carbon disulphide is as follows:

Temperature, °C.-60-1501218-2046
Grams As in 100 c.c. CS2.0.8-1.02.0-2.53.8-4.05.5-6.07.5-8.011

The elevation of the boiling point of carbon disulphide resulting from the dissolution corresponds with the molecular formula As4.

The significance of the difference in density of the various forms of arsenic has been the subject of much speculation. The density of the yellow allotrope is, at 18° C., 2.026 and at -50° C. 2.35 and, like that of the metallic form, 5.73, is quite definite, whereas the amorphous forms vary considerably in density according to the conditions of formation, the following values being recorded: vitreous 4.71 to 4.74; grey or black, as obtained in arsenical mirrors, 4.60 to 4.74; brown, obtained by reduction of arsenic compounds in solution, 3.7 to 4.7. Watts, as early as 1850, suggested that this difference in the compactness of the constituent matter was the explanation of the supposed allotropic states, while Engel, said that there was only one amorphous allotrope, the different forms being due to the state of subdivision. Erdmanri, on the other hand, assumed that differences in molecular constitution explained the relation between the different varieties, thus -


these changes being brought about by light. Geuther had previously observed that the densities of the three last varieties were in the ratio 4:5:6 and suggested the formulae (As4)2, (As4 + As6) and (As6)2. But such molecular formulae were based on insufficient evidence. The contention that the compactness of the constituent matter was the governing factor was revived by Kohlschiitter and his co-workers, who considered that yellow arsenic, specific volume 0.492, passed into the metallic form, specific volume 0.175, merely by a process of condensation, a given amount of matter passing into a much smaller space; also that the grey and brown forms were the same as the metallic but in more diffuse states.

The grey form may be obtained from the yellow in carbon disulphide solution by treating with alcohol, or by cooling with carbon dioxide and ether or with liquid air. It is stable towards atmospheric oxygen, and is oxidised by nitric acid more slowly than the brown and metallic modifications.

Cooling Curves of Arsenic
Cooling Curves of (A) Metallic Arsenic, (B) Amorphous Arsenic. (C) Density Curve for Amorphous Arsenic.
The yellow, grey and brown forms do not conduct electricity, whereas metallic arsenic is a conductor. This difference has been made use of in determining the temperatures at which the metallic variety is produced from the other forms at a sensible rate. Bettendorff (1867), Engel (1883) and Linck (1899) stated that amorphous arsenic is transformed at 360° C., irreversibly and with considerable development of heat, into metallic arsenic; Erdmann and Reppert gave 303° C. as the transformation temperature, while Jolibois and Gaubeau determined the point of irreversible transformation both of the brown and grey varieties to be 270° to 280° C. Erdmann gave the transition point between the brown form and the grey form as 180° C., but such a critical point has not been substantiated. Jolibois asserted that his thermal observations admitted only two allotropes, the ordinary grey metallic form, stable up to its melting point, 850 ± 10° C., and an unstable yellow amorphous form which undergoes an irreversible transformation into the metallic form at 285° C. The yellow form he considered to be identical with the vitreous modification.

Thus, while it was generally accepted that the grey metallic and the yellow crystalline forms were true allotropic modifications, there was considerable confusion of thought as regards the so-called amorphous forms. Laschtschenko in 1922 therefore attempted to elucidate the nature of these forms. He measured the amount of heat evolved during cooling from a high temperature of samples of metallic arsenic, purified by sublimation in a vacuum, and of amorphous arsenic prepared by reduction with tin of arsenious oxide dissolved in hydrochloric acid. The operations were carried out in sealed quartz-glass tubes. The cooling curves, fig., showed for metallic arsenic, A, between 868° and 822° C. a sudden increase in the amount of heat evolved owing to solidification, the latter temperature being above the point of fusion of arsenic, and a break at 750° to 738° C., indicating the change to the yellow allotrope. The curve B for the amorphous form is typical of monotropic transformation. It will be seen that the transformation points given above, 270° to 280° C. and 360° C., correspond respectively with the maximum point of B and the point of coincidence of B and A. A series of determinations of density of amorphous arsenic which had been heated in sealed quartz-glass tubes and rapidly cooled to 15° C. showed continuous change (curve C); at 260° to 265° C. the density corresponds with that of the grey modification, and at 360° C., when the curve becomes almost horizontal, with that of the metallic form. The values obtained were:

t° C.15175235255275305365400

The density of the pure metallic form at 15° C. was 5.7301. These results suggested that the amorphous forms of arsenic are more probably solid solutions of the yellow and metallic forms rather than true allotropes, the densities depending on the proportions of the two forms present. Engel's experiments on the sublimation of arsenic support this contention. The sublimation temperature of amorphous arsenic in a vacuum is about 260° C. and 280° to 310° C. in an inert gas, while that of metallic arsenic is variously given as 450° in hydrogen and 554° and 616° C. at 760 mm. in air. At 360° C. sublimation of the amorphous form ceases, but by sufficiently prolonged sublimation at 310° C. complete transformation of the metastable amorphous phase into the stable metallic modification may be effected. This is in accordance with the distillation of the more volatile component of a solid solution, leaving the component which is non-volatile at the particular temperature. The objection to this conception is that such a change should be reversible, whereas the change of amorphous to metallic arsenic is definitely an irreversible exothermic transformation.

Laschtschenko therefore suggested that the amorphous forms may be of colloidal origin and may thus represent stages in the continuous passage from the colloidal to the crystalline state. The mode of preparation of the brown form by reduction of arsenic compounds in solution is favourable to sol formation, and the view that the various forms differed only in degree of dispersion of the particles has already been mentioned. X-ray investigations show that the precipitated brown arsenic, brown translucent arsenic obtained by sublimation, and the arsenic mirror, brown in thin layers by transmitted light, are all amorphous and apparently identical.

A more recent investigation has shown that the product obtained when arsenic vapour condenses below 100° C. in pure hydrogen is amorphous to the X-rays and is a powder of very small particle size. When the vapour is condensed between 100° C. and 130° C. a mixture of powder and coherent sheet is obtained, while between 130° and 250° C. only the coherent sheet or glass is formed. The metallic lustre of the latter increases with the temperature of condensation. It is shown to be amorphous by the X-rays. Above 250° C. the deposit is distinctly crystalline to the X-rays.

The transformation of amorphous arsenic to the crystalline form can be accelerated by the presence of certain catalysts, hydrogen iodide being one of the most active; the transformation temperature is thereby considerably lowered and the change has been induced at 180° C., 90° lower than the temperature previously observed.

Investigation by X-ray methods of the structure of samples of arsenolamprite from two different localities showed only partial agreement with that of metallic rhombohedral arsenic; the differences may be attributed to the presence of impurity in the minerals, but could also be explained by the presence of a second allotropic modification corresponding to black metallic phosphorus.

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