The analysis of group 4 cations regards those cations who precipitate as sulphides around pH 9. If you are new to qualitative chemical analysis, don't worry, you can have a look at the previous articles.
The precipitating reagent of this analysis is again Na2S (do you remember group 2?).
Which are group 4 cations? Zn2+, Co2+, Ni2+, Mn2+. The corresponding sulphides are way more soluble than the sulphides of group 2; in fact, the acidic pH of group 2 analysis did not allow the precipitation (n.b. precipitation mechanism of sulphides)
Don't forget that our analysis has to be performed on a solution that passed through group 3 analysis! This means we're working on a basic solution (ammonia-ammonium chloride buffer → pH 9). What happened to group 4 cations during group 3 analysis? They were solubilized as amino complexes (except manganese), with 4-6 molecules NH3 coordinated each.
As soon as we add Na2S the precipitation reaction will be then (similarly for zinc (II) and nickel (II):
Co(NH3)62+ + S2- CoS↓ + 6NH3
For manganese (II), it will be simply:
Mn2+ + S2- MnS ↓
During and before the precipitation, pay attention to the color of the solution; there may be some colored complexes, such as Co(NH3)62+ brown-reddish solution or Ni(NH3)62+ blue solution.
Some theory: precipitation of group 4 sulphides
We're now going to show a little bit of theory about sulfides precipitation. If you're not interested, just skip to the following page and continue with the analysis! The two fundamental relationships are summarized here:
Taking in account this two equations, we can make a comparison between group 2 and group 4, since they are both based on sulfides precipitation. Consider that solution of group 2 had the pH buffered at 2, while the one of group 4 has it buffered at 9.
If you try with some calculation you will find out that for group 2 the [S2-] was 10-19M while for group 4 is 10-5M. At pH 9 the concentration of free sulfide ions is then vastly higher and this is the reason why we are able to quantitatively precipitate the group 4 cations as sulfides.