Chemical elements
  Arsenic
      Occurrence
      Ubiquity
      History
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    Physical Properties
    Chemical Properties
      Aluminium Arsenide
      Antimony Arsenides
      Barium Arsenide
      Bismuth Arsenides
      Cadmium Arsenides
      Calcium Arsenide
      Cerium Arsenide
      Chromium Arsenides
      Cobalt Arsenides
      Copper Arsenides
      Gold Arsenides
      Iridium Arsenide
      Iron Arsenides
      Lead Arsenides
      Lithium Arsenide
      Magnesium Arsenide
      Manganese Arsenides
      Mercury Arsenides
      Molybdenum Arsenide
      Nickel Arsenides
      Niobium Arsenide
      Palladium Di-arsenide
      Platinum Arsenides
      Potassium Arsenides
      Rhodium Arsenide
      Ruthenium Arsenide
      Silver Arsenides
      Sodium Arsenide
      Strontium Arsenide
      Thallium Arsenide
      Tin Arsenides
      Tungsten Arsenide
      Uranium Arsenide
      Zinc Arsenides
      Arsenic Subhydride
      Arsenic Monohydride
      Arsenic Trihydride
      Arsenic Trifluoride
      Arsenic Pentafluoride
      Arsenic Nitrosyl Hexafluoride
      Arsenic Trichloride
      Arsenic Oxychloride
      Arsenic Pentachloride
      Arsenic Tribromide
      Arsenic Oxybromide
      Arsenic Moniodide
      Arsenic Diiodide
      Arsenic Triiodide
      Arsenic Pentiodide
      Arsenic Suboxide
      Arsenious Oxide
      Aluminium Arsenite
      Ammonium Arsenites
      Antimony Arsenite
      Barium Arsenites
      Beryllium Arsenite
      Bismuth Arsenite
      Cadmium Arsenites
      Calcium Arsenites
      Chromic Arsenite
      Cobalt Arsenites
      Copper Arsenites
      Gold Arsenites
      Iron Arsenites
      Lead Arsenites
      Lithium Arsenite
      Magnesium Arsenites
      Manganese Arsenites
      Mercury Arsenites
      Nickel Arsenites
      Palladium Pyroarsenite
      Platinum Arsenites
      Potassium Arsenites
      Arsenites of Rare Earth Metals
      Rubidium Metarsenite
      Silver Arsenites
      Sodium Arsenites
      Strontium Arsenites
      Thallous Orthoarsenite
      Tin Arsenites
      Titanyl Tetrarsenite
      Tungsto-arsenites
      Uranyl Metarsenite
      Zinc Arsenites
      Zirconium Arsenite
      Arsenic Tetroxide
      Arsenic Pentoxide
      Aluminium Arsenates
      Ammonium Arsenates
      Barium Arsenates
      Beryllium Arsenates
      Bismuth Arsenates
      Cadmium Arsenates
      Caesium Arsenate
      Calcium Arsenates
      Chromium Arsenates
      Cobalt Arsenates
      Copper Arsenates
      Hydroxylamine Orthoarsenate
      Iron Arsenates
      Lead Arsenates
      Lithium Arsenates
      Magnesium Arsenates
      Manganese Arsenates
      Mercury Arsenates
      Molybdenum Arsenates
      Nickel Arsenates
      Palladium Arsenate
      Platinic Arsenate
      Potassium Arsenates
      Rare Earth Metals Arsenates
      Rhodium Arsenate
      Rubidium Arsenates
      Silver Arsenates
      Sodium Arsenates
      Strontium Arsenates
      Thallium Arsenates
      Thorium Arsenates
      Tin Arsenates
      Titanyl Arsenate
      Tungsto-arsenic Acids
      Uranium Arsenates
      Vanado-arsenates
      Zinc Arsenates
      Zirconium Arsenates
      Perarsenates
      Arsenic and Sulphur
      Arsenic Subsulphide
      Tetrarsenic Trisulphide
      Arsenic Disulphide
      Arsenic Trisulphide
      Arsenic Pentasulphide
      Thioarsenates
      Ammonium Thioarsenates
      Antimony Thioarsenate
      Barium Thioarsenates
      Beryllium Thioarsenate
      Bismuth Thioarsenate
      Cadmium Thioarsenates
      Calcium Thioarsenates
      Cerium Thioarsenates
      Chromium Thioarsenate
      Cobalt Thioarsenate
      Copper Thioarsenates
      Gold Thioarsenates
      Iron Thioarsenates
      Lead Thioarsenates
      Lithium Thioarsenates
      Magnesium Thioarsenates
      Manganese Thioarsenates
      Mercury Thioarsenates
      Molybdenum Thioarsenates
      Nickel Thioarsenates
      Platinic Thioarsenate
      Potassium Thioarsenates
      Silver Thioarsenates
      Sodium Thioarsenates
      Strontium Thioarsenates
      Thallium Orthothioarsenate
      Tin Thioarsenates
      Uranyl Thioarsenate
      Yttrium Thioarsenate
      Zinc Thioarsenates
      Zirconium Thioarsenate
      Trioxythioarsenic Acid
      Dioxydithioarsenic Acid
      Oxytrithioarsenic Acid
      Arsenic Monosulphatotrioxide
      Arsenic Disulphatotrioxide
      Arsenic Trisulphatotrioxide
      Arsenic Tetrasulphatotrioxide
      Arsenic Hexasulphatotrioxide
      Arsenic Octasulphatotrioxide
      Complex salts of Sulphato-compounds of Arsenic
      Arsenic Nitride
      Arsenic Imide
      Arsenic Amide
      Arsenic Phosphides
      Arsenic oxyphosphides
      Arsenic Phosphate
      Arsenic Thiophosphate
      Arsenic Tricarbide
      Arsenic Pentasilicide
      Boron Arsenate
    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

Arsenic Pentasulphide, As2S5






When arsenic is fused with an excess of sulphur the product contains arsenic, sulphur and arsenic pentasulphide; the last-named may be extracted with liquid ammonia or, by careful fractionation, the arsenic and sulphur may be removed, leaving the sulphide. If the elements are fused together in stoichiometric proportions, a greenish-yellow plastic mass is obtained which gradually hardens and becomes lemon-yellow; if this product is powdered and digested with aqueous ammonia, a yellow solution results and insoluble sulphur remains. After filtering, the addition of an acid to the yellow solution precipitates the pentasulphide.

Borodowski failed to obtain evidence of the formation of this sulphide in his study of the freezing point curves because, with mixtures containing 20 to 60 molar per cent, of arsenic, the products are viscous and do not give definite freezing points.

Arsenic pentasulphide may also be prepared from aqueous arsenic acid or a solution of an arsenate by the action of hydrogen sulphide, but the nature of the product depends upon conditions. Berzelius reported the formation of the pentasulphide when the gas acted on a moderately concentrated solution of arsenic acid, but W^ckenroder stated that the arsenic acid was first reduced by hydrogen sulphide to arsenious acid, even in the presence of hydrochloric acid, and that a mixture of arsenic trisulphide and sulphur was then precipitated. Rose, after passing hydrogen sulphide into a solution of arsenic acid, heated the solution and filtered off the precipitate, and then by the addition of silver nitrate showed that both arsenious and arsenic acids were present in the filtrate. It was therefore accepted that reduction takes place and the reaction was represented thus:

2H3ASO4 + 5H2S = As2S3 + S2 + 8H2O

Bunsen showed that the passage of a rapid stream of hydrogen sulphide through a hot solution of an alkali arsenate acidified with hydrochloric acid produced a precipitate of arsenic pentasulphide, and that this was a satisfactory method of determining arsenic quantitatively. These results were confirmed by McCay, and led to more systematic investigation of the subject, the result of which showed that the conditions favourable for the formation of arsenic pentasulphide, when hydrogen sulphide acts on aqueous arsenic acid or acid solutions of arsenates, are (a) a considerable excess of hydrochloric acid present, (b) a rapid passage of the gas, and (c) a comparatively low temperature - the liquid should be warm, as precipitation is extremely slow in the cold. Under these conditions arsenic pentasulphide alone is formed:

2H3ASO4 + 5H2S = As2S5 + 8H2O

When these conditions are not fulfilled, in addition to the above, a secondary reaction occurs which proceeds in two stages, thus:

H3AsO4 + H2S = H3NaO3 + S + H2O
2H3NaO3 + 3H2S = As2S3 + 6H2O

Thus, in the absence of hydrochloric acid, a solution containing 0.6 per cent, of arsenic pentoxide gave a precipitate which, after removing the free sulphur, contained 85 per cent, of the pentasulphide. In the presence of 8 per cent, of hydrochloric acid the precipitate consisted of the pure pentasulphide. Solutions containing 0.3664 per cent, of As2O5 and varying quantities of hydrochloric acid, after treatment with hydrogen sulphide for 12 hours at 15° C., gave precipitates which, after removal of free sulphur, had the following compositions:

HCl, per cent.1.87.910.7614.3425.132.27
As2O5, per cent.91100100100580
As2O3, per cent.900042100


A boiling dilute aqueous solution of an alkali thioarsenate on addition of hydrochloric acid yields a precipitate which contains arsenic pentasulphide and may contain sulphur. Under ordinary conditions the precipitation is not quantitative and the excess of sulphur is difficult to remove; for complete precipitation the liquid should be kept overnight before filtration.

A convenient method of preparing arsenic pentasulphide is to boil arsenic pentoxide with piperazine, when a solution is obtained which, by the prolonged action of hydrogen sulphide, yields the compound As2S5.3C4H10N2.3H2S; if this is treated with cold dilute hydrochloric or acetic acid, crystals of arsenic pentasulphide are formed.


Properties

Arsenic pentasulphide is usually obtained as an amorphous lemon-yellow powder. It is stable in air up to a temperature of about 95° C., but above this temperature a surface film of the trisulphide is formed; nevertheless, it maybe dried at about 110° C. without sensible decomposition. At a higher temperature it melts, the melting point being somewhat higher than that of sulphur. The liquid is rather deeper in colour than the solid. On distillation, the vapour contains arsenic trisulphide and sulphur and the residue becomes continually richer in arsenic trisulphide. The pentasulphide is very slightly soluble in pure water, 1 litre of which at 0° C. dissolves 1.36 mg. of As2S5. It is much less soluble in the presence of hydrogen sulphide, and 1 litre of water containing 0.002 per cent, of the gas dissolves only 0.27 mg. As2S5. It is insoluble in alcohol or carbon disulphide, but is soluble in aqueous ammonia, alkali hydroxides or alkali sulphides, forming thioarsenates and oxythioarsenates. It also dissolves in aqueous solutions of citric acid and alkali citrates. When the pentasulphide is boiled with water, a solution of arsenic trioxide is obtained, with separation of sulphur. The pentasulphide is insoluble in hydrochloric acid and also in dilute nitric acid; it is attacked by more concentrated nitric acid with evolution of nitric oxide. With acid of 20 to 40 per cent, concentration the action is observed only on boiling or vigorous agitation; with 60 per cent, acid the reaction proceeds in the cold. Ammonia is absorbed by the pentasulphide, but is again liberated on exposure to air. Arsenic pentasulphide is readily reduced by heating in hydrogen or with carbon, or when fused with a mixture of alkali cyanide and carbonate.
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