ARSENIC
Occurrence. Arsenic occurs in considerable quantities in nature as the native element, as the sulphides realgar (As2S2) and orpiment (As2S3), as oxide (As2O3), and as a constituent of many metallic sulphides, such as arsenopyrite (FeAsS).
Preparation. The element is prepared by purifying the native arsenic, or by heating the arsenopyrite in iron tubes, out of contact with air, when the reaction expressed by the following equation occurs:
FeAsS = FeS + As.
The arsenic, being volatile, condenses in chambers connected with the heated tubes. It is also made from the oxide by reduction with carbon:
2As2O3 + 3C = 4As + 3CO2.
Properties. Arsenic is a steel-gray, metallic-looking substance of density 5.73. Though resembling metals in appearance, it is quite brittle, being easily powdered in a mortar. When strongly heated it sublimes, that is, it passes into a vapor without melting, and condenses again to a crystalline solid when the vapor is cooled. Like phosphorus it can be obtained in several allotropic forms. It alloys readily with some of the metals, and finds its chief use as an alloy with lead, which is used for making shot, the alloy being harder than pure lead. When heated on charcoal with the blowpipe it is converted into an oxide which volatilizes, leaving the charcoal unstained by any oxide coating. It burns readily in chlorine gas, forming arsenic trichloride,—
As + 3Cl = AsCl3.
Unlike most of its compounds, the element itself is not poisonous.
Arsine (AsH3). When any compound containing arsenic is brought into the presence of nascent hydrogen, arsine (AsH3), corresponding to phosphine and ammonia, is formed. The reaction when oxide of arsenic is so treated is
As2O3 + 12H = 2AsH3 + 3H2O.
Arsine is a gas with a peculiar garlic-like odor, and is intensely poisonous. A single bubble of pure gas has been known to prove fatal. It is an unstable compound, decomposing into its elements when heated to a moderate temperature. It is combustible, burning with a pale bluish-white flame to form arsenic trioxide and water when air is in excess:
2AsH3 + 6O = As2O3 + 3H2O.
When the supply of air is deficient water and metallic arsenic are formed:
2AsH3 + 3O = 3H2O + 2As.
These reactions make the detection of even minute quantities of arsenic a very easy problem.
Fig. 72
Marsh's test for arsenic. The method devised by Marsh for detecting arsenic is most frequently used, the apparatus being shown in Fig. 72. Hydrogen is generated in the flask A by the action of dilute sulphuric acid on zinc, is dried by passing over calcium chloride in the tube B, and after passing through the hard-glass tube C is ignited at the jet D. If a substance containing arsenic is now introduced into the generator A, the arsenic is converted into arsine by the action of the nascent hydrogen, and passes to the jet along with the hydrogen. If the tube C is strongly heated at some point near the middle, the arsine is decomposed while passing this point and the arsenic is deposited just beyond the heated point in the form of a shining, brownish-black mirror. If the tube is not heated, the arsine burns along with the hydrogen at the jet. Under these conditions a small porcelain dish crowded down into the flame is blackened by a spot of metallic arsenic, for the arsine is decomposed by the heat of the flame, and the arsenic, cooled below its kindling temperature by the cold porcelain, deposits upon it as a black spot. Antimony conducts itself in the same way as arsenic, but the antimony deposit is more sooty in appearance. The two can also be distinguished by the fact that sodium hypochlorite (NaClO) dissolves the arsenic deposit, but not that formed by antimony.
Oxides of arsenic. Arsenic forms two oxides, As2O3 and As2O5, corresponding to those of phosphorus. Of these arsenious oxide, or arsenic trioxide (As2O3), is much better known, and is the substance usually called white arsenic, or merely arsenic. It is found as a mineral, but is usually obtained as a by-product in burning pyrite in the sulphuric-acid industry. The pyrite has a small amount of arsenopyrite in it, and when this is burned arsenious oxide is formed as a vapor together with sulphur dioxide:
2FeAsS + 10O = Fe2O3 + As2O3 + 2SO2.
The arsenious oxide is condensed in appropriate chambers. It is a rather heavy substance, obtained either as a crystalline powder or as large, vitreous lumps, resembling lumps of porcelain in appearance. It is very poisonous, from 0.2 to 0.3 g. being a fatal dose. It is frequently given as a poison, since it is nearly tasteless and does not act very rapidly. This slow action is due to the fact that it is not very soluble, and hence is absorbed slowly by the system. Arsenious oxide is also used as a chemical reagent in glass making and in the dye industry.
Acids of arsenic. Like the corresponding oxides of phosphorus, the oxides of arsenic are acid anhydrides. In solution they combine with bases to form salts, corresponding to the salts of the acids of phosphorus. Thus we have salts of the following acids:
| H3AsO3 | arsenious acid. |
| H3AsO4 | orthoarsenic acid. |
| H4As2O3 | pyroarsenic acid. |
| HAsO3 | metarsenic acid. |
Several other acids of arsenic are also known. Not all of these can be obtained as free acids, since they tend to lose water and form the oxides. Thus, instead of obtaining arsenious acid (H3AsO3), the oxide As2O3 is obtained:
2H3AsO3 = As2O3 + 3H2O.
Salts of all the acids are known, however, and some of them have commercial value. Most of them are insoluble, and some of the copper salts, which are green, are used as pigments. Paris green, which has a complicated formula, is a well-known insecticide.
Antidote for arsenical poisoning. The most efficient antidote for arsenic poisoning is ferric hydroxide. It is prepared as needed, according to the equation
Fe2(SO4)3 + 3Mg(OH)2 = 2Fe(OH)3 + 3MgSO4.
Sulphides of arsenic. When hydrogen sulphide is passed into an acidified solution containing an arsenic compound the arsenic is precipitated as a bright yellow sulphide, thus:
2H3AsO3 + 3H2S = As2S3 + 6H2O,
2H3AsO4 + 5H2S = As2S5 + 8H2O.
In this respect arsenic resembles the metallic elements, many of which produce sulphides under similar conditions. The sulphides of arsenic, both those produced artificially and those found in nature, are used as yellow pigments.