Marsh Test (1836)
Definition
Confirmatory test for arsenic devised by James Marsh after the Bodle case (1832), in which Marsh was unable to convince a jury despite detecting arsenic. The test later gained international fame at the Marie Lafarge trial in 1840. Zn + dilute H2SO4 + sample generates nascent hydrogen; any arsenic present reduces to arsine gas (AsH3), which decomposes in a heated glass tube to a shiny black mirror of metallic arsenic. The arsenic mirror is soluble in NaOCl; the antimony mirror is not (the key distinction).
- Devised by
- James Marsh, following the Bodle case (1832)
- Famous trial
- Marie Lafarge, 1840
- Chemistry
- Zn + dilute H2SO4 reduces arsenic to arsine gas
- Key distinguishing test
- Arsenic mirror dissolves in NaOCl; antimony mirror does not
Common questions
Why did the Bodle case push Marsh to refine his test in the first place?+
Marsh had detected arsenic in the victim using an earlier, less visually persuasive method, but the jury acquitted the defendant because the evidence presented, a simple precipitate, was not compelling enough, which motivated him to develop the more dramatic and interpretable metallic mirror as courtroom-ready evidence.
Why did the sodium hypochlorite solubility check matter so much for the test's credibility?+
Antimony forms a mirror in the same apparatus and can be mistaken for arsenic by eye, so without a way to tell them apart a false identification was possible; the differential solubility in NaOCl, dissolving the arsenic mirror but leaving the antimony mirror intact, gave examiners a reliable confirmatory step.
Related terms
- Arsenic (As)
- Metalloid; trivalent arsenite (As III) is 60 times more toxic than pentavalent arsenate (As V). Thiol-group blocker that poisons mitochondrial enzymes (pyruvate...
- Dry Ashing
- Viscera or hair is dried and incinerated in a muffle furnace at 450 to 550 degrees Celsius. Inorganic ash is dissolved in...
- Hydride Generation (HG-AAS)
- Sensitivity booster for As, Sb and Se. Sample plus NaBH4 in acid liberates AsH3 / SbH3 / SeH2 gas, which is swept...
- Lead (Pb)
- Cumulative neurotoxin from chronic occupational exposure (battery recycling, smelter work), leaded paint, contaminated cosmetics (low-grade sindoor, surma, kohl), and adulterated Ayurvedic preparations....
- Mercury (Hg)
- Three toxic forms: elemental (vapour from broken thermometers, dental amalgam), inorganic salts (mercuric chloride, corrosive sublimate), and organic (methylmercury from biomagnification, ethylmercury...
- Reinsch Test
- Classical screen for arsenic, antimony, mercury and bismuth. A clean copper strip is immersed in the acidified sample (concentrated HCl, near-boiling) for...
- Thallium (Tl)
- The 'poisoner's poison' because thallium sulphate is tasteless, odourless and colourless. Substitutes for potassium in cellular transport. Alopecia at 2 to 3...
- Wet Digestion
- Viscera is digested with HNO3 + H2SO4 + HClO4 (or HNO3 + H2O2) at controlled temperature. Closed-vessel microwave-assisted digestion in Teflon bombs...