Arsenic - chemistry.

Arsenic - chemistry. I INTRODUCTION Arsenic, symbol As, extremely poisonous semimetallic element. The atomic number of arsenic is 33. Arsenic is in group 15 (or Va) of the periodic table (see Periodic Law). II PROPERTIES AND OCCURRENCE Chemically, arsenic is intermediate between metals and nonmetals. Its properties lie, in general, in the middle of the series formed by the family of the elements nitrogen, phosphorus, arsenic, antimony, and bismuth. Arsenic ranks about 52nd in natural abundance among the elements in crustal rocks. Arsenic occurs in several different forms, or allotropes. Gray arsenic is the most common. It is metallic in appearance and has a specific gravity of 5.7. When gray arsenic is heated under standard pressure it sublimes, passing directly from solid to gaseous form at 613°C (1135°F). A yellow, nonmetallic form also exists and has a specific gravity of 2.0. The atomic weight of arsenic is 74.9216. Arsenic has been known since ancient times. The pure element can be easily prepared by heating a common ore called arsenopyrite (FeAsS). Other common minerals are realgar (As2S2); orpiment (As2S3); and arsenic trioxide (As2O 3); occasionally the pure element is found in nature. Arsenic occurs frequently in place of some of the sulfur in the sulfides that are the principal ores of many of the heavy metals. When these ores are roasted, the arsenic sublimes and can be collected from the dust in the flues as a by-product. III USES Arsenic is used in large quantities in the manufacture of glass to eliminate a green color caused by impurities of iron compounds. A typical charge in a glass furnace contains 0.5 percent of arsenic trioxide. Arsenic is sometimes added to lead to harden it and is also used in the manufacture of such military poison gases as lewisite and adamsite. Until the introduction of penicillin, arsenic was of great importance in the treatment of syphilis. In other medicinal uses, it has been displaced by sulfa drugs or antibiotics. Lead arsenate, calcium arsenate, and Paris green are used extensively as insecticides. Certain arsenic compounds, such as gallium arsenide (GaAs), are used as semiconductors. GaAs is also used as a laser material. Arsenic disulfide (As2S2), also known as red orpiment and ruby arsenic, is used as a pigment in the manufacture of fireworks and paints. Arsenic is poisonous in doses significantly larger than 65 mg (1 grain), and the poisoning can arise from a single large dose or from repeated small doses, as, for example, inhalation of arsenical gases or dust. On the other hand, some persons, notably the so-called "arsenic eaters" of the mountains of southern Austria, have found that arsenic has a tonic effect and have built up a tolerance for it, so that they can ingest each day an amount that would normally be a fatal dose. This tolerance, however, does not protect them against the same amount of arsenic administered hypodermically. A reliable test that can detect the presence of minute amounts of arsenic is often important, because arsenic is a violent poison, yet it is widely used and therefore is a frequent contaminant. The Marsh test, named for its inventor, the English chemist James Marsh, supplies a simple method for detecting traces of arsenic so minute that they would escape discovery in ordinary analysis. The substance to be tested is placed in a hydrogen generator and any arsenic present is converted to arsine, (AsH3), which mixes with the evolved hydrogen. If the stream of hydrogen is heated as it passes through a glass tube, the arsine decomposes, and metallic arsenic is deposited in the tube. Minute amounts cause an appreciable stain; as little as 0.1 mg (0.000003 oz) of arsenic or antimony can be detected by using the Marsh test. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.