Editing Marsh test (section)

==Circumstances and methodology==
Though precursor tests existed, they had sometimes proven not to be sensitive enough. In 1832, a certain John Bodle was brought to trial for poisoning his grandfather by putting arsenic in his coffee. [[James Marsh (chemist)|James Marsh]], a chemist working at the [[Royal Arsenal]] in [[Woolwich]], was called by the prosecution to try to detect its presence. He performed the standard test by passing hydrogen sulfide through the suspect fluid. While Marsh was able to detect arsenic, the yellow precipitate did not keep very well, and, by the time it was presented to the jury, it had deteriorated. The jury was not convinced, and John Bodle was acquitted.

Angered and frustrated by this, especially when John Bodle confessed later that he indeed killed his grandfather, Marsh decided to devise a better test to demonstrate the presence of arsenic. Taking Scheele's work as a basis, he constructed a simple glass apparatus capable of not only detecting minute traces of arsenic but also measuring its quantity. Adding a sample of tissue or body fluid to a glass vessel with zinc and acid would produce arsine gas if arsenic was present, in addition to the hydrogen that would be produced regardless by the zinc reacting with the acid. Igniting this gas mixture would oxidize any arsine present into arsenic and water vapor. This would cause a cold ceramic bowl held in the jet of the flame to be stained with a silvery-black deposit of arsenic, physically similar to the result of Metzger's reaction. The intensity of the stain could then be compared to films produced using known amounts of arsenic.<ref>{{Cite web|url=http://www.chm.bris.ac.uk/motm/arsine/arsineh.htm|title=Arsine - Molecule of the Month - January 2005 - HTML version|access-date=2008-10-25|archive-date=2008-10-07|archive-url=https://web.archive.org/web/20081007085917/http://www.chm.bris.ac.uk/motm/arsine/arsineh.htm|url-status=live}}</ref> Not only could minute amounts of arsenic be detected (as little as 0.02&nbsp;mg), the test was very specific for arsenic. Although [[antimony]] (Sb) could give a false-positive test by forming [[stibine]] ({{chem2|SbH3}}) gas, which decomposes on heating to form a similar black deposit, it would not dissolve in a solution of [[sodium hypochlorite]] (NaOCl), while arsenic would. [[Bismuth]] (Bi), which also gives a false positive by forming [[bismuthine]] ({{chem2|BiH3}}), similarly can be distinguished by how it resists attack by both NaOCl and [[polysulfide|ammonium polysulfide]] (the former attacks As, and the latter attacks Sb).<ref name=Holl>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001.{{ISBN|0-12-352651-5}}.</ref>