ARSENIC, a hard, steel-grey, metallic element occurring widely in various ores. The oxide known as white arsenic is men tioned by the Greek alchemist, Olympiodorus, who obtained it by roasting arsenic sulphide. These substances were all known to the later alchemists who used minerals containing arsenic to give a white colour to copper. Albertus Magnus was the first to state that arsenic contained a metal-like substance, although later writers considered it to be a bastard or semi-metal, and frequently called it arsenicum rex. In 1733 G. Brandt showed that white arsenic was the calx (oxide) of this element which has the symbol As, atomic number 33, and atomic weight Arsenic occurs in the uncombined condition in various localities, but more generally in combination with other metals and sulphur, in the form of more or less complex sulphides. Native arsenic is usually of a dull grey colour, owing to surface tarnish; only on fresh fractures is the colour tin-white with metallic lustre. The hardness is 3.5 and the specific gravity 5.63-5.73. Crystals of arsenic belong to the rhombohedral system ; natural crystals are, however, of rare occurrence, and are usually acicular in habit. Na tive arsenic occurs usually in metalliferous veins in association with ores of antimony, silver, etc., the silver mines of Freiberg in Saxony, St. Andreasberg in the Harz, Norway, Borneo, United States, Chile and the province Echizen in Japan being well-known localities.
Arsenic is a constituent of the minerals realgar, orpi ment, arsenical pyrites or mispickel, FeAsS ; cobaltite, CoAsS ; smaltite,CoAs2j cobalt bloom, pharmac olite and mimetisite (Pb[Pb3(As0,),]3)F2, whilst it is also met with in small quantities in nearly all speci mens of iron pyrites. The ordinary commercial arsenic is either the naturally occurring form, which is, however, more or less contaminated with other metals, or is the product obtained by heating arsenical pyrites, out of contact with air, in earthenware retorts fitted with a roll of sheet iron at the mouth, and an earthen ware receiver. By this method of distillation the arsenic sublimes into the receiver, leaving a residue of iron sulphide in the retort. For further purification, it may be sublimed, after having been previously mixed with a little powdered charcoal, or it may be mixed with a small quantity of iodine and heated. It can also be obtained by the reduction of white arsenic (arsenious oxide) with carbon. • Arsenic exists in more than one form and in this display of allotropy (q.v.) it resembles phosphorus, with which it is allied. (See PERIODIC LAW.) In its stable form arsenic possesses a steel-grey colour, and a decided metallic lustre ; it crystallizes in rhombohedra, isomor phous with those of antimony and tellurium. It is very brittle. It is volatile at temperatures above Ioo°C, and rapidly vaporizes at a dull red heat. When heated to 450°C, under atmospheric pres sure solid arsenic volatilizes without passing through the liquid phase, but it liquefies when heated under increased pressure, and its melting point lies between 830° and 86o°C. By sublimation an amorphous variety is obtained in addition to rhombohedral ar senic. The vapour of arsenic is of a golden yellow colour, and has a garlic odour. The vapour density is io.6 (air= I) at 564°C, corresponding to a tetratomic molecule As,; at a white heat the vapour density shows a considerable lowering in value, due to the dissociation of the complex molecule.
When arsenic vapour is cooled suddenly below o°C, yellow ar senic is produced which is soluble in carbon disulphide and crystal lizes therefrom in cubic crystals. Even at 6o°C it begins to change into the metallic form. Arsenic heated in a current of oxygen burns with a pale lavender-coloured flame, forming arsenious oxide. It is easily oxidized to arsenic acid by heating with con centrated nitric acid. It burns in an atmosphere of chlorine to form the trichloride; it also combines directly with bromine and sulphur on being heated, while on fusion with alkalis it forms arsenites.
Arsenic alloys with most metals and is frequently present as an impurity in commercial alloys. A small proportion (o.3 to o.6%) added to molten lead facilitates the formation of truly spherical pellets when the liquid metal is poured down a shot-tower. Elec tro-deposition of arsenic is employed to give a dark oxidized finish to brass; the depositing bath contains potassium cyanide, sodium phosphate and arsenious oxide, to which a nickel salt is some times added to produce a darker effect.
Detection.—Arsenic and most of its soluble compounds are very poisonous, and consequently the methods used for the de tection of arsenic are important. The usual methods are : (a) Reinsch's test. A piece of clean copper is dipped in a solution of an arsenious compound, which has been previously acidified with pure hydrochloric acid. A grey film is produced on the surface of the copper, probably due to the formation of a copper arsenide. The reaction proceeds better if the solution is heated. On remov ing, washing and gently drying the metal and heating it in a glass tube, a white crystalline sublimate is formed on the cool part of the tube ; under the same conditions antimony does not produce a crystalline sublimate.
(b) Fleitmann's test depends on the fact that arsenic and its compounds, when present in a solution in which hydrogen is being generated, are converted into arseniuretted hydrogen, or arsine which can be readily detected either by its action on silver nitrate solution or by its decomposition on heating. The solution containing the arsenious compound is mixed with pure potassium hydroxide solution and a piece of pure zinc or aluminium foil dropped in and the whole then heated. A piece of absorbent paper, moistened with silver nitrate, is held over the mouth of the tube, and if arsenic be present a grey or black deposit is seen on the paper, due to the silver nitrate being reduced by the arseniuretted hydrogen. Antimony gives no reaction under these conditions, so that the method can be used to detect arsenic in the presence of antimony, but the test is not so delicate as either Reinsch's or Marsh's method.
(c) Marsh's Test.--The solution containing the arsenious com pounds is mixed with pure hydrochloric acid and placed in an apparatus in which hydrogen is generated from pure zinc and pure sulphuric acid. The arseniuretted hydrogen produced is passed through a tube containing lead acetate paper and soda-lime, and finally through a narrow glass tube, constricted at various points, and heated by a very small flame. As the arseniuretted hydrogen passes over 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 anti mony deposit by its ready solubility in a solution of sodium hypo chlorite. A blank experiment should always be carried out in test ing for small quantities of arsenic, to ensure that the materials used are quite free from traces of arsenic. The presence of nitric acid interferes with Marsh's test, and if the arsenic is present as an arsenic compound it must be reduced to the arsenious con dition by the action of sulphurous acid.
(d) Gutzeit'sTest.—The acidified solution to be tested is treated with zinc, in the presence of stannous and ferrous salts, and the gas, passed through lead acetate solution to remove hydrogen sul phide, and then passed over paper soaked in mercuric chloride; a dark stain is due to arsenic. The stain becomes much more dis tinct when viewed in ultra-violet light.
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 am monium sulphide. Under like conditions arsenates only give a precipitate on long-continued boiling.
In the sulphide method, the arsenic should be in the tervalent 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 I oo ° 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 tervalent condition, and the method of estimation consists in converting it into the quinquevalent condi tion by means of a standard solution of iodine, in the presence of a cold saturated solution of sodium bicarbonate.