Editing Marsh test (section)

==Precursor methods==
The first breakthrough in the detection of arsenic poisoning was in 1775 when [[Carl Wilhelm Scheele]] discovered a way to change arsenic trioxide to garlic-smelling [[arsine]] gas ({{chem2|AsH3}}),  by treating it with [[nitric acid]] ({{chem2|HNO3}}) and combining it with [[zinc]]:<ref>Scheele, Carl Wilhelm (1775) [http://babel.hathitrust.org/cgi/pt?id=mdp.39015039452928;view=1up;seq=293 "Om Arsenik och dess syra"] {{Webarchive|url=https://web.archive.org/web/20160105084518/http://babel.hathitrust.org/cgi/pt?id=mdp.39015039452928;view=1up;seq=293 |date=2016-01-05 }} (On arsenic and its acid), ''Kongliga Vetenskaps Academiens Handlingar'' (Proceedings of the Royal Scientific Academy [of Sweden]), '''36''' :  263-294.  From p. 290:  ''"Med Zinck.  30. (a)  Denna år den endaste af alla så hela som halfva Metaller, som i digestion met Arsenik-syra effervescerar."''  (With zinc.  30. (a)  This is the only [metal] of all whole- as well as semi-metals that effervesces on digestion with arsenic acid.)  Scheele collected the arsine and put a mixture of arsine and air into a cylinder.  From p. 291:  ''"3:0, Då et tåndt ljus kom når o̊pningen, tåndes luften i kolfven med en småll, lågan for mot handen, denna blef o̊fvedragen med brun fårg, … "''  (3:0, Then as [the] lit candle came near the opening [of the cylinder], the gases in [the] cylinder ignited with a bang; [the] flame [rushed] towards my hand, which became coated with [a] brown color, … )</ref>
:{{chem2|As2O3 + 6 Zn + 12 HNO3 -> 2 AsH3 + 6 Zn(NO3)2 + 3 H2O}}

In 1787, German physician {{ill|Johann Metzger|de|Johann Daniel Metzger}} (1739-1805) discovered that if arsenic trioxide were heated in the presence of carbon, the arsenic would sublime.<ref>Metzger, Johann Daniel, ''Kurzgefasstes System der gerichtlichen Arzneiwissenschaft'' (Concise system of forensic medicine), 2nd ed. (Königsberg and Leipzig, (Germany):  Goebbels und Unzer, 1805), [https://archive.org/details/b21918314/page/238 pp. 238–239.]  In a footnote on p. 238, Metzger mentions that if a sample that's suspected of containing arsenic trioxide (''Arsenik'') is heated on a copper plate (''Kupferblech''), then, when arsenic vapor lands on the plate, it will condense to form a shiny silver-white (''weisse Silberglanz'') patch.  He also mentions that if a sample containing arsenic trioxide is large enough, metallic arsenic can be produced from it.  From the footnote on p. 239:  ''"b)  Am besten geschieht sie, wenn mann den Arsenik mit einem fetten Oel zum Brey macht und in einer Retorte so lange distillirt, bis keine ölichte Dämpfe mehr übergehen, dann aber das Feuer verstärkt, wodurch der Arsenik - König sich sublimirt."''  ( b)  It's best of all when one makes a paste of the arsenic trioxide with a fatty oil and distills it in a retort long enough until no more oily vapors pass over [and] then one intensifies the fire, whereby [metallic] arsenic is sublimated.)</ref> This is the reduction of {{chem2|As2O3}} by [[carbon]]:
:{{chem2|2 As2O3 + 3 C -> 3 CO2 + 4 As}}

In 1806, [[Valentin Rose (pharmacologist)|Valentin Rose]] took the stomach of a victim suspected of being poisoned and treated it with [[potassium carbonate]] ({{chem2|K2CO3}}), [[calcium oxide]] (CaO) and nitric acid.<ref>Valentin Rose (1806) [http://babel.hathitrust.org/cgi/pt?id=uc1.$b618714;view=1up;seq=679 "Ueber das zweckmäßigste Verfahren, um bei Vergiftungen mit Arsenik letzern aufzufinden und darzustellen"] {{Webarchive|url=https://web.archive.org/web/20191216034437/https://babel.hathitrust.org/cgi/pt?id=uc1.$b618714;view=1up;seq=679 |date=2019-12-16 }} (On the most effective method, in cases of poisoning with arsenic, to discover and show the latter), ''Journal für Chemie und Physik'', '''2''' :  665-671.</ref> Any arsenic present would appear as arsenic trioxide and then could be subjected to Metzger's test.

The most common test (and used even today in water test kits) was discovered by [[Samuel Hahnemann]]. It would involve combining a sample fluid with [[hydrogen sulfide]] ({{H2S}}) in the presence of [[hydrochloric acid]] (HCl). A yellow precipitate, [[arsenic trisulfide]] ({{chem2|As2S3}}) would be formed if arsenic was present.<ref>{{cite book|last1=Hahnemann|first1=Samuel|title=Ueber die Arsenikvergiftung, ihre Hülfe und gerichtliche Ausmittelung|trans-title=On poisoning by arsenic: its treatment and forensic detection|date=1786|publisher=Siegfried Lebrecht Crusius|location=Leipzig, (Germany)|language=de|url=https://archive.org/details/ueberdiearsenikv00hahn}}  On p. 15, §34, and pp. 25–26, §67, Hahnemann noted that when hydrogen sulfide — ''Schwefelleberluft'' = gas (''Luft'') of liver (''Leber'') of sulfur (''Schwefel''); "[[liver of sulfur]]" is a mixture of sulfides of potassium; hydrogen sulfide was prepared by adding acid to liver of sulfur — dissolved in water was added to an acidified solution containing arsenic trioxide, a yellow precipitate — arsenic trisulfide, {{chem2|As2S3}}, which he called ''Operment'' (English:  orpiment, yellow arsenic; German:  ''Rauschgelb'') — was produced.  From pp. 25-26:  ''"§67.  Noch müssen wir der Schwefelleberluft erwähnen, die in Wasser aufgelöst, sich am innigsten mit dem Arsenikwasser verbindet, und als Operment mit ihm zu Boden fält."'' (We still must mention hydrogen sulfide, which [when  it's] dissolved in water, binds most closely with arsenic [trioxide in] water, and falls to the bottom with it as arsenic trisulfide.)  In Chapter 11 (''Elftes Kapitel.  Chemische Kennzeichen des Thatbestands (corporis delicti) einer Arsenikvergiftung'' [Ch. 11.  Chemical indications of evidence of an arsenic poisoning]), Hahemann explains how to identify arsenic in autopsy samples (e.g., stomach contents).  On p. 239, §429, he explains how to distinguish mercury poisoning from arsenic poisoning.  And on p. 246, §440, he describes the course of the reaction:  ''"§440.  Mit Schwefelleberluft gesättigtes Wasser bildet in einer wenig gesättigten Arsenikauflösung zuerst eine durchsichtige Gilbe, nach einigen Minuten begint die Flüssigkeit erst trübe zu werden und nach mehrern Stunden erscheint dann nach und nach der lokere pomeranzengelbe Niederschlag, den man mit einigen zugetröpfelten Tropfen Weinessig beschleunigen kan."'' (§440.  With water saturated with hydrogen sulfide, [there] forms, in a little saturated solution of arsenic, at first a transparent yellow; after some minutes the fluid begins first to become cloudy, and after several hours [there] then appears bit by bit a fluffy orange-yellow precipitate, [the formation of] which one can accelerate with some drops of acetic acid added dropwise.)</ref>