Editing Marsh test (section)

==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 ({{chem2|As(3+)}}). Here are the two half-reactions:
:Oxidation: {{chem2|Zn -> Zn(2+) + 2 e−}}
:Reduction: {{chem2|As2O3 + 12 e− + 6 H+ -> 2 As(3−) + 3 H2O}}
They combine into this reaction:
:{{chem2|As2O3 + 6 Zn + 6 H+ -> 2 As(3−) + 6 Zn(2+) + 3 H2O}}

In an acidic medium, {{chem2|As(3-)}} is protonated to form [[arsine]] gas ({{chem2|AsH3}}), so with adding sulphuric acid ({{chem2|H2SO4}}) to each side of the equation:
:{{chem2|As2O3 + 6 Zn + 6 H+ + 6 H2SO4 -> 2 As(3−) + 6 H2SO4 + 6 Zn(2+) + 3 H2O}}

As the {{chem2|As3−}} combines with the {{H+}} to form arsine:
:{{chem2|As2O3 + 6 Zn + 6 H+ + 6 H2SO4 -> 2 AsH3 + 6 ZnSO4 + 3 H2O + 6 H+}}

By eliminating the common ions:
:{{chem2|As2O3 + 6 Zn + 6 H2SO4 -> 2 AsH3 + 6 ZnSO4 + 3 H2O}}