Separations with Thioacetamide

Behavior of Ions with Sulfide

Ions not listed here either do not react with sulfide (\(\ce{S2^{-}}\)­), or they should have been removed by precipitation as chlorides or sulfates before sulfide is added to the metal ion mixture. Ions that will precipitate in acidic solutions of sulfide:

\(\ce{As^{3+}}\), \(\ce{Bi^{3+}}\), \(\ce{Hg^{2+}}\), \(\ce{Cd^{2+}}\), \(\ce{Sn^{2+}}\), \(\ce{Sn^{4+}}\), \(\ce{Sb^{3+}}\), \(\ce{Pb^{2+}}\), \(\ce{Cu^{2+}}\)

Of these, \(\ce{Hg^{2+}}\), \(\ce{Sn^{2+}}\), and \(\ce{Sb^{3+}}\) dissolve in basic solutions containing excess \(\ce{S2^{-}}\)­ due to complex ion formation. The others remain insoluble in basic solution.

Ions that will precipitate in moderately basic (pH=9) solutions of sulfide:

\(\ce{Fe^{2+}}\), \(\ce{Fe^{3+}}\), \(\ce{Al^{3+}}\), \(\ce{Zn^{2+}}\), \(\ce{Mn^{2+}}\), \(\ce{Cr^{3+}}\), \(\ce{Co^{2+}}\), \(\ce{Ni^{2+}}\)

Of these ions, \(\ce{Al^{3+}}\), \(\ce{Fe^{3+}}\), and \(\ce{Cr^{3+}}\) precipitate as the hydroxide rather than the sulfide. Of those that form insoluble sulfides, all except \(\ce{CoS}\) and \(\ce{NiS}\) are soluble in dilute aqueous hydrochloric acid.

The concentration of \(\ce{S2^{-}}\)­­ is the controlling factor in determining whether an ion will precipitate. Consider the dissociation equilibrium for hydrosulfuric acid:

\[\ce{H2S (aq) <=> 2H^{+}(aq) + S2^{-}­(aq)} \nonumber \]

The more strongly acidic the solution, the lower the concentration of \(\ce{S2^{-}}\). The sulfides that precipitate in base will not precipitate in acid, because the concentration of \(\ce{S2^{-}}\) is too low. Those sulfides that precipitate in acid will also precipitate in base because the concentration of \(\ce{S2^{-}}\)­ is higher than necessary for precipitation.

Procedure for Sulfide Separations

To separate the acidic sulfide group ions from the basic sulfide group ions, follow this procedure:

To 5 mL of solution containing ions from both sulfide groups, add 10 drops (0.5 mL) of 6 M \(\ce{HNO3}\) and 20 drops (1 mL) of 6 M \(\ce{HCl}\). Evaporate the mixture to dryness slowly in an evaporating dish in a hood. The last few drops should be evaporated with steam by placing the evaporating dish on top of a beaker of boiling water. Heating to complete dryness with a flame could evaporate the chloride salts \(\ce{PbCl2}\), \(\ce{HgCl2}\), or \(\ce{SnCl4}\), if they are present. It is necessary to use steam to get the salt mixture completely dry so there will be no excess acid present after evaporation.

Add 2 mL of \(\ce{H2O}\) to the cool sulfide salt mixture. Swirl and stir to dissolve as much salt as possible. Transfer the solution and the residue to a test tube for precipitation. Rinse the evaporating dish with 1 mL of \(\ce{H2O}\) and 4 drops of 6 M \(\ce{HCl}\) and add to the same test tube. The test tube should now contain all your salts in 3 mL of solution. If a precipitate is still present, it is probably some oxychloride salts that may not be completely dissolved in the 0.38 M \(\ce{H^{+}(aq)}\) solution.

Precipitate the acidic sulfide group ions by adding 1 mL of 1 M thioacetamide. Stir and heat the mixture in a boiling water bath for 7 minutes. Then add 1.5 mL \(\ce{H2O}\) and 0.5 mL thioacetamide and heat for another 5 minutes. Prepare the following wash solution while heating your sample if you need your precipitate for additional separations or tests.

Wash solution: Add 2 drops of 1 M thioacetamide and 1 mL of 1 M \(\ce{NH4Cl}\) to 1 mL of \(\ce{H2O}\) and heat in a water bath. Remove any pale yellow elemental sulfur present from decomposition of thioacetamide by centrifugation and decanting.

The solution should contain any basic sulfide group ions, so centrifuge and save the solution for analysis, if it might contain any basic sulfide group ions. Wash the precipitate, which contains the acidic sulfide group ions as sulfide (or hydroxide) salts, with 1 mL of the wash solution. Centrifuge and add the decanted wash liquid to the basic sulfide group solution. Wash the precipitate again with the remaining 1 mL of wash solution. Centrifuge and discard the decanted wash solution.

Sulfide precipitates can be dissolved by adding 2­5 mL of 6 M \(\ce{HNO3}\). If necessary to dissolve all the solid, add more nitric acid. Heat the mixture in a boiling water bath for a few minutes. Centrifuge and remove the solution. Nitric acid will dissolve some precipitates by shifting the solubility equilibrium; for example:

\[\ce{CuS + 2H^{+} \rightarrow Cu^{2+} + H2S} \nonumber \]

Hot nitric acid will also oxidize sulfide ion to sulfur:

\[\ce{3S_2^{-}­ + 2NO3^{-}(aq)­ + 8H^{+} \rightarrow 3S + 2NO (g) + 4H2O (l)} \nonumber \]

\(\ce{HgS}\) does not dissolve unless heated for a long time with more concentrated \(\ce{HNO3}\) because it is so insoluble. Be cautious however, since prolonged heating might oxidize sulfur to sulfate ion, which could precipitate \(\ce{PbSO4}\) if lead ion is present.

To get rid of excess sulfide ion in a solution, acidify the solution with \(\ce{HNO3}\) and heat. Centrifuge off any sulfur formed. The solution can be tested for sulfide ion with lead acetate paper, which will turn black due to formation of lead sulfide if sulfide ion is present in the solution.