Marsh test is used to identify traces of the element,

Correct option is A

​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. Arsenic, in the form of white arsenic trioxide As₂O₃, 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.

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:

$$\text{Oxidation: } \text{Zn} \rightarrow \text{Zn}^{2+} + 2e^{-}\text{Reduction: } \text{As}_2\text{O}_3 + 12e^{-} + 6\text{H}^{+} \rightarrow 2 \text{As}^{3-} + 3\text{H}_2\text{O}$$

Overall, we have this reaction

$$\text{As}_2\text{O}_3 + 6 \text{Zn} + 6 \text{H}^{+} \rightarrow 2 \text{As}^{3-} + 6 \text{Zn}^{2+} + 3 \text{H}_2\text{O}$$

​In an acidic medium, As^3- is protonated to form arsine gas (AsH₃), so adding sulphuric acid (H₂SO₄) to each side of the equation we get:

$$\text{As}_2\text{O}_3 + 6 \text{Zn} + 6 \text{H}^{+} + 6 \text{H}_2\text{SO}_4 \rightarrow 2 \text{As}^{3-} + 6 \text{H}_2\text{SO}_4 + 6 \text{Zn}^{2+} + 3 \text{H}_2\text{O}$$

​As the As^3- combines with the H⁺ to form arsine:

$$\text{As}_2\text{O}_3 + 6 \text{Zn} + 6 \text{H}^{+} + 6 \text{H}_2\text{SO}_4 \rightarrow 2 \text{AsH}_3 + 6 \text{ZnSO}_4 + 3 \text{H}_2\text{O} + 6 \text{H}^{+}$$

By eliminating the common ions

$$\text{As}_2\text{O}_3 + 6 \text{Zn} + 6 \text{H}_2\text{SO}_4 \rightarrow 2 \text{AsH}_3 + 6 \text{ZnSO}_4 + 3 \text{H}_2\text{O}$$

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