Page:EB1911 - Volume 02.djvu/694

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652
ARSENIC


the heated portion it is decomposed and a black deposit formed. Instead of heating the tube, the gas may be ignited at the mouth of the tube and a cold surface of porcelain or platinum placed in the flame, when a black deposit is formed on the surface. This may be distinguished from the similar antimony deposit by its ready solubility in a solution of sodium hypochlorite. A blank experiment should always be carried out in testing for small quantities of arsenic, to ensure that the materials used are quite free from traces of arsenic. It is to be noted that the presence of nitric acid interferes with the Marsh test; and also that if the arsenic is present as an arsenic compound it must be reduced to the arsenious condition by the action of sulphurous acid. Arsenic compounds can be detected in the dry way by heating in a tube with a mixture of sodium carbonate and charcoal when a deposit of black amorphous arsenic is produced on the cool part of the tube, or by conversion of the compound into the trioxide and heating with dry sodium acetate when the offensive odour of the extremely poisonous cacodyl oxide is produced. In the wet way, arsenious oxide and arsenites, acidified with hydrochloric acid, give a yellow precipitate of arsenic trisulphide on the addition of sulphuretted hydrogen; this precipitate is soluble in solutions of the alkaline hydroxides, ammonium carbonate and yellow ammonium sulphide. Under like conditions arsenates only give a precipitate on long-continued boiling.

Arsenic is usually estimated either in the form of magnesium pyroarsenate or as arsenic sulphide. For the pyroarsenate method it is necessary that the arsenic should be in the arsenic condition, if necessary this can be effected by heating with nitric acid; the acid solution is then mixed with “magnesia mixture” and made strongly alkaline by the addition of ammonia. It is then allowed to stand twenty-four hours, filtered, washed with dilute ammonia, dried, ignited to constant weight and weighed, the filter paper being incinerated separately after moistening with nitric acid. From the weight of magnesium pyroarsenate obtained the weight of arsenic can be calculated.

In the sulphide method, the arsenic should be in the arsenious form. Sulphuretted hydrogen is passed through the liquid until it is thoroughly saturated, the excess of sulphuretted hydrogen is expelled from the solution by a brisk stream of carbon dioxide, and the precipitate is filtered on a Gooch crucible and washed with water containing a little sulphuretted hydrogen and dried at 100° C.; it is then well washed with small quantities of pure carbon disulphide to remove any free sulphur, again dried and weighed. Arsenic can also be estimated by volumetric methods; for this purpose it must be in the arsenious condition, and the method of estimation consists in converting it into the arsenic condition by means of a standard solution of iodine, in the presence of a cold saturated solution of sodium bicarbonate.

The atomic weight of arsenic has been determined by many different chemists. J. Berzelius, in 1818, by heating arsenious oxide with excess of sulphur obtained the value 74.3; J. Pelouze (Comptes rendus, 1845, 20, p. 1047) titrated arsenic chloride with silver solution and obtained 75.0; and F. Kessler (Pogg. Ann. 1861, 113, p. 134) by converting arsenic trisulphide in hydrochloric acid solution into arsenic pentasulphide also obtained 75.0.

Compounds.—Arsenic forms two hydrides:—The dihydride, As2H2, is a brown velvety powder formed when sodium or potassium arsenide is decomposed by water. It is a somewhat unstable substance, decomposing on being heated, with liberation of hydrogen. Arsenic trihydride (arsine or arseniuretted hydrogen), AsH3, is formed by decomposing zinc arsenide with dilute sulphuric acid; by the action of nascent hydrogen on arsenious compounds, and by the electrolysis of solutions of arsenious and arsenic acids; it is also a product of the action of organic matter on many arsenic compounds. It is a colourless gas of unpleasant smell, excessively poisonous, very slightly soluble in water. It easily burns, forming arsenious oxide if the combustion proceeds in an excess of air, or arsenic if the supply of air is limited; it is also decomposed into its constituent elements when heated. It liquefies at −40° C. and becomes solid at −118.9° C. (K. Olszewski). Metals such as tin, potassium and sodium, when heated in the gas, form arsenides, with liberation of hydrogen; and solutions of gold and silver salts are reduced by the gas with precipitation of metallic gold and silver. Chlorine, bromine and iodine decompose arsine readily, the action being most violent in the case of chlorine.

Arsenic tribromide, AsBr3, is formed by the direct union of arsenic and bromine, and subsequent distillation from the excess of arsenic; it forms colourless deliquescent prisms which melt at 20°-25° C., and boil at 220° C. Water decomposes it, a small quantity of water leading to the formation of the oxybromide, AsOBr, whilst a large excess of water gives arsenious oxide, As4O6.

Arsenic certainly forms two, or possibly three iodides. The di-iodide, As2I4 or AsI2, which is prepared by heating one part of arsenic with two parts of iodine, in a sealed tube to 230° C., forms dark cherry-red prisms, which are easily oxidized, and are readily decomposed by water. The tri-iodide, AsI3, prepared by subliming arsenic and iodine together in a retort, by leading arsine into an alcoholic iodine solution, or by boiling powdered arsenic and iodine with water, filtering and evaporating, forms brick-red hexagonal tables, of specific gravity 4.39, soluble in alcohol, ether and benzene, and in a large excess of water; in the presence of a small quantity of water, it is decomposed with formation of hydriodic acid and an insoluble basic salt of the composition 4AsOI·3As4O6·24H2O. It combines with alkaline iodides to form very unstable compounds. The pentaiodide, AsI5, appears to be formed when a mixture of one part of arsenic and seven parts of iodine is heated to 190° C., but on dissolving the resulting product in carbon bisulphide and crystallizing from this solvent, only the tri-iodide is obtained.

Arsenic trichloride, AsCl3, is prepared by distilling white arsenic with concentrated sulphuric acid and common salt, or by the direct union of arsenic with chlorine, or from the action of phosphorus pentachloride on white arsenic. It is a colourless oily heavy liquid of specific gravity 2.205 (0° C.), which, when pure and free from chlorine, solidifies at −18° C., and boils at 132° C. It is very poisonous and decomposes in moist air with evolution of white fumes. With a little water it forms arsenic oxychloride, AsOCl, and with excess of water it is completely decomposed into hydrochloric acid and white arsenic. It combines directly with ammonia to form a solid compound variously given as AsCl3·3NH3, or 2AsCl3·7NH3, or AsCl3·4NH3.

Arsenic trifiuoride, AsF3, is prepared by distilling white arsenic with fluorspar and sulphuric acid, or by heating arsenic tribromide with ammonium fluoride; it is a colourless liquid of specific gravity 2.73, boiling at 63° C.; it fumes in air, and in contact with the skin produces painful wounds. It is decomposed by water into arsenious and hydrofluoric acids, and absorbs ammonia forming the compound 2AsF3·5NH3. By the action of gaseous ammonia on arsenious halides at −30° C. to −40° C., arsenamide, As(NH2)3, is formed. Water decomposes it into arsenious oxide and ammonia, and when heated to 60° it loses ammonia and forms arsenimide, As2(NH)3 (C. Hugot, Compt. rend. 1904, 139, p. 54). For AsF5, see Ber., 1906, 39, p. 67.

Two oxides of arsenic are definitely known to exist, namely the trioxide (white arsenic), As4O6, and the pentoxide, As2O5, while the existence of a suboxide, As2O(?), has also been mooted. Arsenic trioxide has been known from the earliest times, and was called Hüttenrauch (furnace-smoke) by Basil Valentine. It occurs naturally in the mineral claudetite, and can be artificially prepared by burning arsenic in air or oxygen. It is obtained commercially by roasting arsenical pyrites in either a Brunton’s or Oxland’s rotatory calciner, the crude product being collected in suitable condensing chambers, and afterwards refined by resublimation, usually in reverberatory furnaces, the foreign matter being deposited in a long flue leading to the condensing chambers. White arsenic exists in two crystalline forms (octahedral and prismatic) and one amorphous form; the octahedral form is produced by the rapid cooling of arsenic vapour, or by cooling a warm saturated solution in water, or by crystallization from hydrochloric acid, and also by the gradual transition of the amorphous variety, this last phenomenon being attended by the evolution of heat. Its specific gravity is 3.7; it is only slightly soluble in cold water, but is more soluble in hot water, the solution reacting faintly acid. The prismatic variety of the oxide can be obtained by crystallization from a saturated boiling solution in potassium hydroxide, or by the crystallization of a solution of silver arsenite in nitric acid. Its specific gravity is 4.15. In the amorphous condition it can be obtained by condensing the vapour of the oxide at as high a temperature as possible, when a vitreous mass is produced, which melts at 200° C., has a specific gravity of 3.68-3.798, and is more soluble in water than the crystalline variety.

Arsenious oxide is very poisonous. It acts as a reducing agent; it is not convertible into the pentoxide by the direct action of oxygen; and its solution is reduced by many metals (e.g. zinc, tin and cadmium) with precipitation of arsenic and formation of arseniuretted hydrogen. The solution of arsenious oxide in water reacts acid towards litmus and contains tribasic arsenious acid, although on evaporation of the solution the trioxide is obtained and not the free acid. The salts of the acid are, however, very stable, and are known as arsenites. Of these salts several series are known, namely the ortho-arsenites, which are derivatives of the acid H3AsO3, the meta-arsenites, derivatives of HAsO2, and the pyro-arsenites, derivatives of H4As2O5. The arsenites of the alkali metals are soluble in water, those of the other metals are insoluble in water, but are readily soluble in acids. A neutral solution of an arsenite gives a yellow precipitate of silver arsenite, Ag3AsO3, with silver nitrate solution, and a yellowish-green precipitate (Scheele’s green) of cupric hydrogen arsenite, CuHAsO3, with copper sulphate solution. By the action of oxidizing agents such as nitric acid, iodine solution, &c., arsenious acid is readily converted into arsenic acid, in the latter case the reaction proceeding according to the equation H3AsO3 + I2 + H2O = H3AsO4 + 2HI. Arsenic pentoxide, As2O5, is most easily obtained by oxidation of a solution of arsenious acid with nitric acid; the solution on concentration deposits the compound 2H3AsO4·H2O (below 15° C.), which on being heated to a dark red heat loses its water of crystallization and leaves a white vitreous mass of the pentoxide. This substance dissolves slowly in water, forming arsenic acid; by heating to redness it decomposes into arsenic and oxygen. It deliquesces in moist air, and is easily reduced to arsenic by heating with carbon.

Arsenic acid, H3AsO4, is prepared as shown above, the compound 2H3AsO4·H2O on being heated to 100° C. parting with its water of crystallization and leaving a residue of the acid, which crystallizes in needles. On heating to 180° C. it loses water and yields pyroarsenic acid, H4As2O7, which at 200° C. loses more water and leaves