Mercury is identified (in group 2) as mercury (II). It belongs to copper subgroup, and is precipitated as mercuric sulfide, HgS, one of the most insoluble salts absolute.
Let's see how to work on HgS (for more details check the full article of group 2 analysis)
- Treatment with aqua regia
3HgS + 2NO3- + 12Cl- + 8H3O+ 3HgCl42- + 2NO + 12H2O + 3S
Aqua regia is a mixture of conc. HCl and conc HNO3 in ratio 3:1 . The solubilization turns Hg in HgCl42- , a complex called chloromercurate stable in water.
It is necessary to use aqua regia because HNO3 is not able to solubilize HgS, or ,at least, not easily and not quickly.
The reaction has to be performed in a capsule, with 2-3 mL of aqua regia, heating (bunsen burner). The solution is heated almost to dryness (about 1 mL left).
The prolonged heating, is necessary to eliminate the excess of nitric and hydrochloric acid, that may otherwise interfere with specific confirmatory tests.
The resulting solution is divided into two or more aliquots to perform the tests.
1) Confirmatory test with stannous chloride (SnCl2)
Sn2+ act as reducing agent (E° Sn+4/Sn+2: = 0.15 V) in respect to Hg2+ (E° Hg 2+/Hg22+ = 0.92 V, E°Hg22+/Hg ° = 0.79 V).
The nature of the reagent also explains why previously we evaporated in capsule; nitric acid would easily oxidize tin (II) to tin (IV).
The addition of stannous chloride (SnCl2) provokes the following reaction:
for further oxidation (of stannous chloride) we get further reduction of Hg2Cl2 to Hg°:
Why do we need the oxidation to go further? Mercurous chloride, could be confused with other substances, such as cuprous chloride (could come from the reduction of Cu2+ to Cu+1). As an underlying rule we have to set up the most specific confirmatory test we're able to. There must be no uncertainty.
So we have to make sure to move the reaction up to the reduction to metallic mercury, black. It is therefore necessary an excess of reagent, SnCl2.
Otherwise an unreacted part of mercurous chloride would remain.
2) Confirmatory test with KI
Once again, starting from the solution obtained after the treatment with aqua regia, we add dropwise KI, potassium iodide. We get a red precipitate of HgI2,mercuric iodide. If we keep on adding an excess of potassium iodide the mercuric iodide solubilizes as iodo mercurate that will reprecipitate as mercuric iodide by subsequent dilution.
