A forensic investigator is testing a suspected arsenic-containing beverage using the Marsh test. The glass apparatus produces a silver mirror deposit in the reduction tube when the sample is acidified and heated with zinc. The deposit dissolves in sodium hypochlorite but NOT in ammonium sulphide. Which substance has been deposited?
- A Antimony (Sb)
- B Bismuth (Bi)
- C Tin (Sn)
- D Arsenic (As) ✓
Explanation
In the Marsh test, arsenic and antimony both produce metallic mirror deposits in the heated glass reduction tube. The critical differentiation: arsenic deposit dissolves readily in sodium hypochlorite (bleach) and ammonium sulphide, while antimony dissolves in ammonium sulphide but NOT in sodium hypochlorite. Conversely, arsenic dissolves in hypochlorite but NOT antimony — making hypochlorite solubility specific for arsenic. If the deposit dissolves in NaOCl → arsenic; if it does not → antimony. This differential solubility was classically used to distinguish the two metals before atomic absorption spectrophotometry became the standard.
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
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Written and medically reviewed by the StethoPrep medical team.