Arsenic Monohydride, AsH

Arsenic Monohydride (or Hydrogen Monarsenide), AsH or As₂H₂. - The formation of a solid product, which was apparently an arsenide of hydrogen, during the electrolysis of water using an arsenic cathode, was first observed by Davy. Other observers, however, could not obtain more than a trace of such a product by this means. Olszewsky obtained it by employing platinum electrodes in an aqueous solution of arsenious oxide. Weeks and Druce produced it in more satisfactory yield as a brown amorphous powder by electrolysis of a normal solution of sodium hydroxide, using a platinum anode and an arsenic cathode, the latter being suspended in a porous pot. The current density was 100 milliamperes per sq. cm. and the monarsenide was deposited around the cathode, arsine being simultaneously evolved.

Similar brown products may be obtained by the action of water on sodium, potassium or calcium arsenide, thus:

2Na₃As + 6H₂O = 6NaOH + As₂H₂ + 2H₂

Arsine is also evolved. When dilute mineral acids are used, the resulting solid appears to be mainly arsenic.

The hydride is frequently obtained by partial decomposition of arsine, as by the atmospheric oxidation of an aqueous solution, or by the influence on the gas of the silent electric discharge. Gaseous arsine may also undergo decomposition by the prolonged action of air, chlorine, nitric oxide, nitric or sulphuric acid, mercuric chloride or phosphorus pentachloride. In the last case the reaction may be represented by the equation:

2 AsH₃ + 2PCl₅ = 2PCl₃ + 4HCl 4- As₂H₂

This reaction does not proceed in carbon tetrachloride solution. Retgers stated that the solid hydride resulted when arsine was heated or when the flame of arsine impinged on a cold plate; but other observers maintain that arsenic alone is produced. A solid hydrogen-containing product has also been obtained by the action of zinc, in the presence of sulphuric and nitric acids, on arsenious oxide, and by the action of arsine on solid potassium hydroxide, followed by the addition of water. A good yield (up to 93 per cent.) of the hydride in a comparatively pure state may be obtained by mixing an ether solution of stannous chloride with a solution of arsenic trichloride in dilute hydrochloric acid. The reaction is:

2AsCl₃ + 4SnCl₂ + 2HCl = As₂H₂ + 4SnCl₄

The composition of the solid products obtained by the earlier investigators varied within wide limits, and various formulae, for example AsH₂, AsH and As₂H, were suggested. The products were undoubtedly indefinite mixtures containing arsenic, and later workers have found considerable difficulty in obtaining the hydride in a pure state. That the composition of the latter corresponds to the empirical formula AsH was established by Janowsky and has been confirmed. As₂H₂ is usually taken as the molecular formula in order to conform with valency considerations, the structure H.As:As.H being assumed.

The properties of hydrogen monarsenide are little known, many of those ascribed to it being really the properties of elementary arsenic. It is a brown, amorphous powder, stable in air at ordinary temperatures, but undergoing oxidation on heating at 70° to 80° C.; it cannot therefore be dried in an oven and a vacuum desiccator is generally used. It is only slightly soluble in water, 1 litre at 20° C. dissolving 0.35 gram. It is oxidised by boiling water. It is insoluble in alcohol and ether and also in hydrochloric and sulphuric acids. It dissolves in hot dilute nitric acid, in hot or cold concentrated nitric acid, or in aqua regia, arsenic acid being formed. When heated in vacuo it decomposes into arsenic and hydrogen. On boiling with ammoniacal silver nitrate, out of contact with air, silver is precipitated according to the equation:

As₂H₂ + 6Ag₂O = As₂O₅ + 12Ag + H₂O

This reduction is accomplished also by arsine, arsenious oxide and arsenic itself. The monarsenide also reduces Fehling's solution. It reacts with the halogens, sulphur and phosphorus. With phosphorus trichloride it yields arsenic trichloride and phosphide, thus -

3As₂H₂ + 4PCl₃ = 6HCl + 2AsCl₃ + 4AsP

and with sodium, hydrogen is liberated:

As₂H₂ + 6Na = 2Na₃As + H₂