Copper subgroup analysis
We have seen that the elements of copper arsenic subgroup are soluble in an excess of base, in particular (NH4)2S, while not surprisingly the sulfides of the copper subgroup are soluble in acid.
The precipitate is treated therefore with 2-3 mL of HNO3 2N, heating and stirring with the glass rod. The nitric acid is an oxidizing acid, and oxidizes the sulfide ions, S2-, to elemental sulfur, S. The only sulfide that is not solubilized after the treatment is, if present, the mercury sulfide, HgS. This compound is known to be very slightly soluble. Anyway, its solubility increases proportionally to the acid concentration and the temperature. For this reason it preferable to use diluted HNO3 (2N).
Here are the oxidation reactions that sulphides of copper subgroup face:
3CuS + 2NO3- + 8H3O+ 3Cu2+ + 3S° + 2NO + 12H2O
3CdS+ 2NO3- + 8H 3O+ 3Cd2+ + 3S° + 2NO + 12H2O
3PbS + 2NO3- + 8H3O+ 3Pb2+ + 3S° + 2NO + 12H2O
Bi2S3 + 2NO3- + 8H3O+ 2BI3+ + 3S° + 2NO + 12H2O
HgS however, is not attached (if present) and constitutes an undissolved residue. This residue should appear black, but may also appear reddish because of the presence of Hg(NO3)2.
Elemental sulfur forms a colloidal yellowish precipitate but more probably dark, almost black, very light, which lies at the level of the surface of the solution and may be removed easily with the glass rod. Obviously, the precipitate of HgS must first be centrifuged.
Here is the copper subgroup analysis pattern:
The analysis of this subgroup is relatively simple. We have already seen what are the effects of treatment with HNO3 2N. Now let's see the treatments with NH3 and NaOH.
Copper-cadmium separation from lead-bismuth
All the sulfides or this subgroup except mercury sulfide (then sulfides of Pb 2+, Bi 3+, Cu 2+, Cd 2+) have been brought back in solution by treatment with 2N nitric acid. Now, we break into two parts this little group by treatment with 6N NH4OH. The solution is acid because of HNO3, a strong acid, so it is preferable to work with NH4OH concentrated, in order to save time and work with smaller volumes (it is always a good practice). If until now we have done well, thoroughly washing the precipitate, we should not worry too much about bringing the solution to low values of pH.
This treatment has a dual effect, and it is a beautiful demonstration of knowledge of the chemistry of the elements.
The raising of the pH, indeed, causes the precipitation of lead and bismuth hydroxides (if present), leaving in solution Cu2+ and Cd2+ (if present), as amino complexes (evidently, lead and bismuth do not form complexes with ammonia) .Moreover, copper amino complexes (if they were present), would color in blue the solution, and this represents itself a solid proof of the presence of Cu2+ in unknown substance.
Although lead has amphoteric properties, ammonia does not have sufficient strength as base to bring it into solution as plumbite ion.
Cu2+ + 4NH3 Cu(NH3)42+ → amino complexes blue staining
Cd2+ + 4NH3 Cd(NH3)42+
The ammonia leads to an excess of OH- in solution:
NH3 + H2O NH4+ + OH -
Increasing [OH-] the Ksp value is exceeded for lead and bismuth hydroxides and as a consequence:
Pb2+ + 2OH- Pb(OH)2 → white precipitate
Bi3+ + 3OH- Bi(OH)3 → white precipitate
Separation of Lead and Bismuth is carried out playing on the amphoteric features of lead. Ineed, the lead, when treated with a strong base, behaves as a Lewis acid coordinating 4 molecules of hydroxyl ion OH- and then dissolving as plumbite ion:
Pb(OH)2 + 2OH- Pb(OH)42- PbO22- + 2H2O
Bi(OH)3 remain undissolved
The bismuth hydroxide (or bismuth more generally) do not have this feature. Right for this reason, treating the precipitate which contains the two hydroxides with 2-3 mL of 2N NaOH, stirring and heating, we continue to have a white insoluble residue, consisting of bismuth hydroxide, while in solution we will have the plumbite ion.
Now that we have seen the most important events of the copper subgroup analysis, we can proceed to the recognition of his elements through specific assays.