3) You have a solution that contains 0.1 M Pb2+, 0.1 M Hg2+ and 0.1 M Fe3+. You

Question

3) You have a solution that contains 0.1 M Pb2+, 0.1 M Hg2+ and 0.1 M Fe3+. You would like to separate these ions by precipitation. For quality purposes, the precipitation of each ion must be complete to >99.9% and there must not be more than 0.1% (by weight) of one of the other ions in the precipitate, i.e., the relative error must be <0.1%. In other words, at the end of the experiment, you want to have three different piles, one that contains >99.9 of the lead, one that contains >99.9 of the mercury, and one that contains >99.9 of the iron. Explain your answer: nature of precipitate, pH, concentrations etc. Use the textbook for any constants you need. Be quantitative in your answer.

Explanation / Answer

METHOD OF SEPARATION:

The solution containing the three ions is acidified with dil HCl and H2 S gas is passed.

In acid medium (pH less than 7) Pb++and Hg++ alone are precipitated as their sulfides. Fe+++ ions are not precipitated in acid medium.

In order to ensure complete precipitation of Pb++and Hg++ as their sulfides, the solution must be 0.2 to 0.3 M in HCl. If the solution is too strongly acidic lead may be incompletely precipitated.

The precipitate is filtered off. The filtrate is used for the separation of Fe+++ions

SEPARATION OF PbS and HgS

From the precipitate which is a mixture of PbS and HgS, the two sulfides are separated by using the following procedure.

The precipitate is heated with 2M HNO3. PbS alone goes into solution as Pb(NO3)2

SEPARATION OF Hg++ AS Hg2Cl2

The HgS is filtered off and heated with a mixture of dil HCl and HNO3. HgS dissolves to produce Hg++ ions in solution. To this solution, stannous chloride SnCl2 solution is added. The SnCl2 reduces Hg++ ions into insoluble mercurous chloride Hg2Cl2 which is filtered and dried (pile 1)

SEPARATION OF Pb++ AS PbCrO4

The filtrate from the above procedure is treated with conc H2SO4 and evaporated to SO3 fumes.It is then cooled and diluted with H2O. Pb++ is precipitated as PbSO4. This precipitate is heated with a mixture of acetic acid and ammonium acetate to get a clear solution of Lead acetate. This solution on treatment with a solution of Potassium chromate K2CrO4 precipitates Pb++ as insoluble Lead chromate PbCrO4.The PbCrO4 is filtered and dried (pile2)

SEPARATION OF Fe+++ AS Fe(OH)3

The filtrate obtained after the precipitation of Pb++ and Hg++ as their sulfide is first boiled to expel the H2S completely and then treated with excess of ammonium hydroxide NH4OH in the presence of ammonium chloride NH4Cl (pH greater than 7). The Fe+++ ions are precipitated completely as Ferric hydroxide Fe(OH)3 which is filtered and dried (pile3)

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