Marsh Test
Definition
Sensitive 1836 method for arsenic in which the acidified sample is added to zinc and dilute sulphuric acid. The evolved arsine gas (AsH3) is decomposed in a heated glass tube and deposits a mirror of elemental arsenic on a cool zone of the tube. Sensitivity down to a few micrograms. Superseded by GFAAS and HG-AAS but still taught and still examined.
- Introduced
- 1836
- Reagents
- Zinc and dilute sulphuric acid
- Product detected
- Arsine gas (AsH3) decomposed to an arsenic mirror
- Sensitivity
- Down to a few micrograms
- Current status
- Superseded by GFAAS and HG-AAS
Common questions
Why is the Marsh test still taught if it has been superseded by instrumental methods?+
It established the underlying chemistry, arsenic reduction to arsine gas, that modern hydride-generation methods still rely on, and understanding the classical test helps explain both the history of toxicology casework and why interferences like antimony must be distinguished in the modern instrumental versions.
What historically limited the reliability of the Marsh test in casework?+
Reagents themselves could contain trace arsenic contamination, producing a false positive unless a reagent blank was run alongside the sample, and antimony present in a sample can also form a similar-looking mirror, so early practitioners needed the sodium hypochlorite solubility test to confirm the mirror was actually arsenic.
Related terms
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- Speciation
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