Everything about Arsine totally explained
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Arsine is the chemical
compound with the
formula AsH3. This flammable,
pyrophoric, and highly toxic gas is the simplest compound of arsenic. Aside from its lethality, it finds applications in the semiconductor industry and for the synthesis of
organoarsenic compounds.
General properties
At its standard state, arsine is a colorless, denser-than-air gas that's soluble in
water (200 mL/L) and in many
organic solvents as well. Whereas arsine itself is odorless, owing to its oxidation by air it's possible to smell a slight,
garlic-like scent when the compound is present at about 0.5
ppm. This compound is generally regarded as stable, since at room temperature it decomposes only slowly. At temperatures of ca. 230 °C decomposition to arsenic and hydrogen is rapid. Several factors, such as
humidity, presence of light and certain
catalysts (namely
aluminium) facilitate the rate of decomposition.
» :4 AsCl
3 + 3 NaBH
4 → 4 AsH
3 + 3 NaCl + 3 BCl
3
This type of reaction was employed in the discovery of arsine as reported in
1775 by
Carl Scheele who reduced
arsenic(III) oxide with zinc in the presence of acid. This reaction is a prelude to the
Marsh test, described below.
Alternatively, sources of As
3− react with protonic reagents to also produce this gas:
» :Zn
3As
2 + 6 H
+ → 2 AsH
3 + 3 Zn
2+
Reactions
The chemical properties of AsH
3 are well developed and can be anticipated based on an average of the behavior of
PH3 and
SbH3.
Thermal decomposition
Typical for a heavy hydride (for example,
SbH3, H
2Te, SnH
4), AsH
3 is unstable with respect to its elements. In other words, AsH
3 is stable kinetically but not thermodynamically.
» :2 AsH
3 → 3 H
2 + 2 As
This decomposition reaction is the basis of the Marsh Test described below, which detects the metallic As.
Oxidation
Continuing the analogy to SbH
3, AsH
3 is readily
oxidized by O
2 or even air:
» :2 AsH
3 + 3 O
2 → As
2O
3 + 3 H
2O
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 AsH
3 with Ag
+, called the Gutzeit test for arsenic. Although this test has become obsolete in
analytical chemistry, the underlying reactions further illustrate the affinity of AsH
3 for "soft" metal cations. In the Gutzeit test, AsH
3 is generated by reduction of aqueous arsenic compounds, typically
arsenites, with Zn in the presence of H
2SO
4. The evolved gaseous AsH
3 is then exposed to AgNO
3 either as powder or as a solution. With "solid" AgNO
3, AsH
3 reacts to produce yellow Ag
4AsNO
3, whereas AsH
3 reacts with a "solution" of AgNO
3 to give black Ag
3As.
Acid-base reactions
The acidic properties of the As–H bond are often exploited. Thus, AsH
3 can be deprotonated:
» :AsH
3 + NaNH
2 → NaAsH
2 + NH
3
Upon reaction with the aluminium trialkyls, AsH
3 gives the trimeric [R
2AlAsH
2]
3, where R = (CH
3)
3C. This reaction is relevant to the mechanism by which GaAs forms from AsH
3 (see below).
AsH
3 is generally considered non-basic, but it can be protonated by "super acids" to give isolable salts of the tetrahedral species [AsH
4]
+.
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.
Forensic science and the Marsh test
AsH
3 is also well known in
forensic science because it's a chemical intermediate in the detection of arsenic poisoning. The old (but extremely sensitive)
Marsh test generates AsH
3 in the presence of arsenic.
This procedure, developed around
1836 by
James Marsh, is based upon treating a As-containing sample of a victim's body (typically the stomach) 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. The formation of a black mirror deposit in the cool part of the equipment indicates the presence of Sb.
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 fallen out of use in modern forensics.
Toxicology
For the toxicology of other arsenic compounds, see Arsenic, Arsenic trioxide, and Arsenicosis.
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
haemoglobin 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.
Exposure to arsine concentrations of 250 ppm is rapidly fatal: concentrations of 25–30 ppm are fatal for 30 min exposure, and concentrations of 10 ppm can be fatal at longer exposure times. Symptoms of poisoning appear after exposure to concentrations of 0.5 ppm. There is little information on the chronic toxicity of arsine, although it's reasonable to assume that, in common with other arsenic compounds, a long-term exposure could lead to
arsenicosis.
Further Information
Get more info on 'Arsine'.
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