Have you ever heard of analysis of group 2 cations? If not, don't be scared, we prepared this summary for you!
Group 2 consists of those cations who precipitate as sulphides around pH 0-2. The precipitating reagent is sodium sulphide Na2S. The solution is acidic because of hydrochloric acid; it corresponds to the supernatant coming from the analysis of group 1 cations.
Here is a table showing the sulfides precipitated in the analysis of group 2 cations:
Specifically, the precipitating reactive is S2- released in wide quantitatives by Na2S (extremely soluble), but its concentration in solution is necessarily bound to pH. If you want to know more about this realtionship you need to do some calculations that you can find at the following link (precipitation mechanims of sulfides).
For what concern the aim of this article, you just need to consider the following table:
-pH needed to precipitate sulfides of group 2
Let's assume now that we have a solution with pH = 0, like the one coming from the analysis of group 1 cations. What will it happen when we add Na2S ? The concentration of sulfide ion is related to the pH. According to the following equilibrium:
If we had, for example, As3+ in solution, would this concentration of sulfide be enough to precipitate As2S3?
we place as average concentration of a semimicro analysis:
The concentration of sulfide ion requested to start the precipitation of S2- is therefore 10-38 M. At pH = 0 the concentration of free sulfide ions is 10-23 M. The concentration is more than sufficient for the precipitation of the sulfide.
With the same approach, you could also find out wether the calculation will be quantitative or not.