Arsine

In its standard state arsine is a colorless, denser-than-air gas that is slightly soluble in water (2% at 20 °C) Arsine itself is odorless, but it oxidizes in air and this creates a slight garlic or fish-like scent when the compound is present above 0.5ppm. This compound is kinetically stable: at room temperature it decomposes only slowly. At temperatures of ca. 230 °C, decomposition to arsenic and hydrogen is sufficiently rapid to be the basis of the Marsh test for arsenic presence. Similar to stibine, the decomposition of arsine is autocatalytic, as the arsenic freed during the reaction acts as a catalyst for the same reaction. Several other factors, such as humidity, presence of light and certain catalysts (namely alumina) facilitate the rate of decomposition. AsH3 is a trigonal pyramidal molecule with H–As–H angles of 91.8° and three equivalent As–H bonds, each of 1.519 Å length. ==Discovery and synthesis==

AsH3 is generally prepared by the reaction of As3+ sources with H− equivalents. ::4 AsCl3 + 3 NaBH4 → 4 AsH3 + 3 NaCl + 3 BCl3 As reported in 1775, Carl Scheele reduced arsenic(III) oxide with zinc in the presence of acid. This reaction is a prelude to the Marsh test. Alternatively, sources of As3− react with protonic reagents to also produce this gas. Zinc arsenide and sodium arsenide are suitable precursors: ::Zn3As2 + 6 H+ → 2 AsH3 + 3 Zn2+ ::Na3As + 3 HBr → AsH3 + 3 NaBr ==Reactions==

The understanding of the chemical properties of AsH3 is well developed and can be anticipated based on an average of the behavior of pnictogen counterparts, such as PH3 and SbH3. Thermal decomposition Typical for a heavy hydride (e.g., stibine|, , ), is unstable with respect to its elements. In other words, it is stable kinetically but not thermodynamically. :: This decomposition reaction is the basis of the Marsh test, which detects elemental As. Oxidation Continuing the analogy to SbH3, AsH3 is readily oxidized by concentrated O2 or the dilute O2 concentration in air: ::2 AsH3 + 3 O2 → As2O3 + 3 H2O Arsine will react violently in presence of strong oxidizing agents, such as potassium permanganate, sodium hypochlorite, or nitric acid. Gutzeit test A characteristic test for arsenic involves the reaction of AsH3 with Ag+, called the Gutzeit test for arsenic (see Sanger–Black apparatus for more). Although this test has become obsolete in analytical chemistry, the underlying reactions further illustrate the affinity of AsH3 for "soft" metal cations. In the Gutzeit test, AsH3 is generated by reduction of aqueous arsenic compounds, typically arsenites, with Zn in the presence of H2SO4. The evolved gaseous AsH3 is then exposed to AgNO3 either as powder or as a solution. With solid AgNO3, AsH3 reacts to produce yellow Ag4AsNO3, whereas AsH3 reacts with a solution of AgNO3 to give black Ag3As. Acid-base reactions The acidic properties of the As–H bond are often exploited. Thus, AsH3 can be deprotonated: ::AsH3 + NaNH2 → NaAsH2 + NH3 Upon reaction with the aluminium trialkyls, AsH3 gives the trimeric [R2AlAsH2]3, where R = (CH3)3C. This reaction is relevant to the mechanism by which GaAs forms from AsH3 (see below). AsH3 is generally considered non-basic, but it can be protonated by superacids to give isolable salts of the tetrahedral species [AsH4]+. Reaction with halogen compounds Reactions of arsine with the halogens (fluorine and chlorine) or some of their compounds, such as nitrogen trichloride, are extremely dangerous and can result in explosions. Catenation In contrast to the behavior of PH3, AsH3 does not form stable chains, although diarsine (or diarsane) H2As–AsH2, and even triarsane H2As–As(H)–AsH2 have been detected. The diarsine is unstable above −100 °C. ==Applications==

Microelectronics applications AsH3 is used in the synthesis of semiconducting materials related to microelectronics and solid-state lasers. Related to phosphorus, arsenic is an n-dopant for silicon and germanium. ==Forensic science and the Marsh test==

AsH3 is well known in forensic science because it is a chemical intermediate in the detection of arsenic poisoning. The old (but extremely sensitive) Marsh test generates AsH3 in the presence of arsenic. This procedure, published in 1836 by James Marsh, is based upon treating an As-containing sample of a victim's body (typically the stomach contents) with As-free zinc and dilute sulfuric acid: if the sample contains arsenic, gaseous arsine will form. The gas is swept into a glass tube and decomposed by means of heating around 250–300 °C. The presence of As is indicated by formation of a deposit in the heated part of the equipment. On the other hand, the appearance of a black mirror deposit in the cool part of the equipment indicates the presence of antimony (the highly unstable SbH3 decomposes even at low temperatures). The Marsh test was widely used by the end of the 19th century and the start of the 20th; nowadays more sophisticated techniques such as atomic spectroscopy, inductively coupled plasma, and x-ray fluorescence analysis are employed in the forensic field. Though neutron activation analysis was used to detect trace levels of arsenic in the mid 20th century, it has since fallen out of use in modern forensics. ==Toxicology==

The toxicity of arsine is distinct from that of other arsenic compounds. The main route of exposure is by inhalation, although poisoning after skin contact has also been described. Arsine attacks hemoglobin in the red blood cells, causing them to be destroyed by the body. The first signs of exposure, which can take several hours to become apparent, are headaches, vertigo, and nausea, followed by the symptoms of haemolytic anaemia (high levels of unconjugated bilirubin), haemoglobinuria and nephropathy. In severe cases, the damage to the kidneys can be long-lasting. Symptoms of poisoning appear after exposure to concentrations of 0.5 ppm. There is little information on the chronic toxicity of arsine, although it is reasonable to assume that, in common with other arsenic compounds, a long-term exposure could lead to arsenicosis. Arsine is a potent hemolytic agent, which is in line with its pathophysiological course of action. However, inhaling high concentrations of this agent is highly capable of inducing severe and direct pulmonary trauma. From a strictly clinical perspective, this typically manifests as chemical pneumonitis as well as non-cardiogenic pulmonary edema. Generally, pathological examinations of the acute fatal cases of arsine poisoning have positively identified two primary mechanisms of pulmonary injury: • Diffuse Alveolar Damage: This is characterized by extensive pulmonary edema, where the alveoli become diffusely infiltrated with leukocytes and fluid. This results in a cellular exudate that fills the air spaces, and therefore severely compromising and constraining the process of physiological gaseous exchange. • Necrotizing Bronchitis: This involves direct damage to the primary pulmonary conducting airways. In such instances, the smaller bronchi and bronchioles could potentially become surrounded by inflammatory cells and necrotic tissue. This damage to the epithelium can lead to focal lesions resembling bronchopneumonia. It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities. Occupational exposure limits ==See also==