The Marsh test is a highly sensitive method in the detection of arsenic, especially useful in the field of forensic toxicology when arsenic was used as a poison. It was developed by the chemist James Marsh and first published in 1836.[1] The method continued to be used, with improvements, in forensic toxicology until the 1970s.[2]
Arsenic, in the form of white arsenic trioxideAs 2O 3, was a highly favored poison, being odourless, easily incorporated into food and drink, and before the advent of the Marsh test, untraceable in the body. In France, it came to be known as poudre de succession ("inheritance powder"). For the untrained, arsenic poisoning will have symptoms similar to cholera.[citation needed]
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 (AsH3), by treating it with nitric acid (HNO3) and combining it with zinc.[3]
In 1787, German physician Johann Metzger[de] (1739-1805) discovered that if arsenic trioxide were heated in the presence of carbon, the arsenic would sublime.[4] This is the reduction of As2O3 by carbon:
2 As2O3 + 3 C → 3 CO2 + 4 As
In 1806, Valentin Rose took the stomach of a victim suspected of being poisoned and treated it with potassium carbonate (K2CO3), calcium oxide (CaO) and nitric acid.[5] 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 (As2S3) would be formed if arsenic was present.[6]
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, 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.[7] Not only could minute amounts of arsenic be detected (as little as 0.02mg), the test was very specific for arsenic. Although antimony (Sb) could give a false-positive test by forming stibine (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 (BiH3), similarly can be distinguished by how it resists attack by both NaOCl and ammonium polysulfide (the former attacks As, and the latter attacks Sb).[8]
Specific reactions involved
The Marsh test treats the sample with sulfuric acid and arsenic-free zinc. Even if there are minute amounts of arsenic present, the zinc reduces the trivalent arsenic (As3+). Here are the two half-reactions:
Oxidation: Zn → Zn2+ + 2 e−
Reduction: As2O3 + 12 e− + 6 H+ → 2 As3− + 3 H2O
Overall, we have this reaction:
As2O3 + 6 Zn + 6 H+ → 2 As3− + 6 Zn2+ + 3 H2O
In an acidic medium, As3− is protonated to form arsine gas (AsH3), so adding sulphuric acid (H2SO4) to each side of the equation we get:
Although the Marsh test was efficacious, its first publicly documented use—in fact, the first time evidence from forensic toxicology was ever introduced—was in Tulle, France in 1840 with the celebrated Lafarge poisoning case. Charles Lafarge, a foundry owner, was suspected of being poisoned with arsenic by his wife Marie. The circumstantial evidence was great: it was shown that she bought arsenic trioxide from a local chemist, supposedly to kill rats that infested their home. In addition, their maid swore that she had mixed a white powder into his drink. Although the food was found to be positive for the poison using the old methods as well as the Marsh test, when the husband's body was exhumed and tested, the chemists assigned to the case were not able to detect arsenic. Mathieu Orfila, the renowned toxicologist retained by the defense and an acknowledged authority of the Marsh test, examined the results. He performed the test again, and demonstrated that the Marsh test was not at fault for the misleading results, but, rather, that those who performed it did it incorrectly. Orfila thus proved the presence of arsenic in Lafarge's body using the test. As a result of this, Marie was found guilty and sentenced to life imprisonment.
Effects
The Lafarge case proved to be controversial, for it divided the country into factions who were convinced or otherwise of Mme. Lafarge's guilt; nevertheless, the impact of the Marsh test was great. The French press covered the trial and gave the test the publicity it needed to give the field of forensic toxicology the legitimacy it deserved, although in some ways it trivialized it: actual Marsh test assays were conducted in salons, public lectures and even in some plays that recreated the Lafarge case. [citation needed]
The existence of the Marsh test also served a deterrent effect: deliberate arsenic poisonings became rarer because the fear of discovery became more prevalent. [citation needed]
In Alan Bradley's As Chimney Sweepers Come To Dust, 12-year old sleuth and chemistry genius Flavia de Luce uses the Marsh test to determine that arsenic was the murderer's weapon.[11]
In the first episode of the 2017 BBC television series Taboo a mirror test, referencing the Marsh test, is used to verify the protagonist's father was killed via arsenic poisoning. As the setting of the series is between 1814-1820, however, the test's appearance is anachronistic.[12]
In the episode "The King Came Calling" of the first season of Ripper Street, police surgeon Homer Jackson (Matthew Rothenberg) performs Marsh's test on the contents of a poisoning victim and determines that the fatal poison was antimony, not arsenic, since the chemical residue deposited by the flames does not dissolve in sodium hypochlorite.[13]
Arsine (IUPAC name: arsane) is an inorganic compound with the formula AsH3. This flammable, pyrophoric, and highly toxic pnictogen hydride gas is one of the simplest compounds of arsenic. Despite its lethality, it finds some applications in the semiconductor industry and for the synthesis of organoarsenic compounds. The term arsine is commonly used to describe a class of organoarsenic compounds of the formula AsH3−xRx, where R = aryl or alkyl. For example, As(C6H5)3, called triphenylarsine, is referred to as "an arsine".
Nitrous acid is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite salts. It was discovered by Carl Wilhelm Scheele, who called it "phlogisticated acid of niter". Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.
Zinc sulfate describes a family of inorganic compounds with the formula ZnSO4(H2O)x. All are colorless solids. The most common form includes water of crystallization as the heptahydrate, with the formula ZnSO4·7H2O. As early as the 16th century it was prepared on the large scale, and was historically known as "white vitriol" (the name was used, for example, in 1620s by the collective writing under the pseudonym of Basil Valentine). Zinc sulfate and its hydrates are colourless solids.
Mathieu Joseph Bonaventure Orfila was a Spanish toxicologist and chemist, the founder of the science of toxicology.
Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find the elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, therefore materials in other forms may need to be brought to this state before using standard methods. The solution is then treated with various reagents to test for reactions characteristic of certain ions, which may cause color change, precipitation and other visible changes.
Devarda's alloy is an alloy of aluminium (44% – 46%), copper (49% – 51%) and zinc (4% – 6%).
Stibine (IUPAC name: stibane) is a chemical compound with the formula SbH3. A pnictogen hydride, this colourless, highly toxic gas is the principal covalent hydride of antimony, and a heavy analogue of ammonia. The molecule is pyramidal with H–Sb–H angles of 91.7° and Sb–H distances of 170.7 pm (1.707 Å). The smell of this compound from usual sources (like from reduction of antimony compounds) is reminiscent of arsine, id est garlic-like.
In chemistry, an arsenide is a compound of arsenic with a less electronegative element or elements. Many metals form binary compounds containing arsenic, and these are called arsenides. They exist with many stoichiometries, and in this respect arsenides are similar to phosphides.
Arsenic trioxide is an inorganic compound with the formula As 2O 3. As an industrial chemical, its major uses include the manufacture of wood preservatives, pesticides, and glass. It is sold under the brand name Trisenox among others when used as a medication to treat a type of cancer known as acute promyelocytic leukemia. For this use it is given by injection into a vein.
Iodometry, known as iodometric titration, is a method of volumetric chemical analysis, a redox titration where the appearance or disappearance of elementary iodine indicates the end point.
Arsenic acid or arsoric acid is the chemical compound with the formula H3AsO4. More descriptively written as AsO(OH)3, this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but is only found in solution, where it is largely ionized. Its hemihydrate form (2H3AsO4·H2O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.
Marie-Fortunée Lafarge was a French woman who was convicted of murdering her husband by arsenic poisoning in 1840. Her case became notable because it was one of the early trials to be followed by the public through daily newspaper reports, and because she was the first person convicted largely on direct forensic toxicological evidence. Nonetheless, questions about Lafarge's guilt divided French society to the extent that it is often compared to the better-known Dreyfus affair.
Cacodylic acid is an organoarsenic compound with the formula (CH3)2AsO2H. With the formula R2As(O)OH, it is the simplest of the arsinic acids. It is a colorless solid that is soluble in water.
Arsenic pentoxide is the inorganic compound with the formula As2O5. This glassy, white, deliquescent solid is relatively unstable, consistent with the rarity of the As(V) oxidation state. More common, and far more important commercially, is arsenic(III) oxide (As2O3). All inorganic arsenic compounds are highly toxic and thus find only limited commercial applications.
Scheele's green, also called Schloss green, is chemically a cupric hydrogen arsenite, CuHAsO 3. It is chemically related to Paris green. Scheele's green was invented in 1775 by Carl Wilhelm Scheele. By the end of the 19th century, it had virtually replaced the older green pigments based on copper carbonate. It is a yellowish-green pigment commonly used during the early to mid-19th century in paints as well as being directly incorporated into a variety of products as a colorant. It began to fall out of favor after the 1860s because of its toxicity and the instability of its color in the presence of sulfides and various chemical pollutants. The acutely toxic nature of Scheele's green as well as other arsenic-containing green pigments such as Paris green may have contributed to the sharp decline in the popularity of the color green in late Victorian society. By the dawn of the 20th century, Scheele's green had completely fallen out of use as a pigment but was still in use as an insecticide into the 1930s. At least two modern reproductions of Scheele's green hue with modern non-toxic pigments have been made, with similar but non-identical color coordinates: one with hex#3c7a18 and another with hex#478800. The latter is the more typically reported color coordinate for Scheele's green.
Organoarsenic chemistry is the chemistry of compounds containing a chemical bond between arsenic and carbon. A few organoarsenic compounds, also called "organoarsenicals," are produced industrially with uses as insecticides, herbicides, and fungicides. In general these applications are declining in step with growing concerns about their impact on the environment and human health. The parent compounds are arsane and arsenic acid. Despite their toxicity, organoarsenic biomolecules are well known.
Pnictogen hydrides or hydrogen pnictides are binary compounds of hydrogen with pnictogen atoms covalently bonded to hydrogen.
Compounds of arsenic resemble in some respects those of phosphorus which occupies the same group (column) of the periodic table. The most common oxidation states for arsenic are: −3 in the arsenides, which are alloy-like intermetallic compounds, +3 in the arsenites, and +5 in the arsenates and most organoarsenic compounds. Arsenic also bonds readily to itself as seen in the square As3− 4 ions in the mineral skutterudite. In the +3 oxidation state, arsenic is typically pyramidal owing to the influence of the lone pair of electrons.
↑ Scheele, Carl Wilhelm (1775) "Om Arsenik och dess syra"Archived 2016-01-05 at the Wayback Machine (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, … )
↑ Metzger, Johann Daniel, Kurzgefasstes System der gerichtlichen Arzneiwissenschaft (Concise system of forensic medicine), 2nd ed. (Königsberg and Leipzig, (Germany): Goebbels und Unzer, 1805), 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.)
↑ Hahnemann, Samuel (1786). Ueber die Arsenikvergiftung, ihre Hülfe und gerichtliche Ausmittelung[On poisoning by arsenic: its treatment and forensic detection] (in German). Leipzig, (Germany): Siegfried Lebrecht Crusius. 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, 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.)
Mohr, C. F. (1837). "Zusätze zu der von Marsh angegebenen Methode, den Arsenik unmittelbar im regulinischen Zustande aus jeder Flüssigkeit auszuscheiden" [Addenda to the method given by Marsh for separating arsenic immediately in the metallic state from any liquid]. Annalen der Chemie und Pharmacie. 23 (2): 217–225. doi:10.1002/jlac.18370230218.
Campbell, W. A. (1965). "Some landmarks in the history of arsenic testing". Chemistry in Britain. 1: 198–202.
Bertomeu-Sánchez, José Ramón; Nieto-Galan, Agustí, eds. (2006). Chemistry, Medicine, and Crime: Mateu J. B. Orfila (1787–1853) and his times. Sagamore Beach (MA): Science History Publications. ISBN0-88135-275-6.
Wanklyn, James Alfred (1901). Arsenic. London: Kegan Paul, Trench, Trübner & Co. Ltd. pp.39–57. Retrieved 2007-12-16. James Marsh Test.
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