Treatment with CH3COOH
We had solubilized the elements of arsenic subgroup (once separated from copper subgroup), by adding an excess of sulphide ion (NH4)2S. Now, our purpose is to eliminate the excess of sulfide ion by acidic treatment. In some cases it is prefered to acidify with 2N HCl, but it is advisable to acidify with acetic acid for two reasons. First, using HCl, it is likely to lead antimony and tin in solution as chloro complexes. Furthermore the pH is lowered to much, with the risk of solubilization of the more soluble sulphides (always because fo the usual relationship between [S2-] and [H+]). With acetic acid both of these problems are avoided. The process may anyway take a few minutes more.
Acidification leads to the elimination of sulfide ions as hydrogen sulfide, H2S, a very toxic acid (gas at room temperature). The operation must be carried out safely under the hood.
2As3-3 + 6H+ As2S3 + 3H2S↑
The other sulfides (of arsenic subgroup) undergo the same reaction. Since the reaction proceeds with development of gas, it will be favored (it will be faster) by heating. The heater should not be excessive, because the gas can escape violently (forming bubbles) causing us to lose part of the solution.
It occurs to identify the end of development of H2S using a small piece of paper moistened with lead acetate. As long as H2S forms the paper will turn black because of the formation of PbS.
Treatment with concentrated hydrochloric acid
Once separated and washed the precipitate of arsenic subgroup sulfides, we perform the treatment with concentrated HCl. Are added 2mL of conc. HCl (37%) and 1mL of H2O, then heating (bain-marie) and stirring. The treatment is performed twice. The two supernatants are then assembled. The treatment performed in two stages is a useful precaution. By treating with HCl just once and for long, also the arsenic sulfides might, in the end, be solubilized.
The precipitate, if present, should be yellowish, consisting of the sulphides of As (III) and As (V). This precipitate is washed firstly with HCl, to make sure any remaining Sb and Sn are definitely solubilized and removed. This is important, because in the washing with H2O executed thereafter these cations may precipitate as basic salts. The subsequent washing with H2O must be performed in order to remove the chloride ions, which may interfere in subsequent essays.
Here's an example of the complexing action of concentrated hydrochloric acid on antimony and tin sulfides:
Sb2S3 + 6H+ + 8Cl - 2SbCl4- + 3H2S
SnS2 + 4H+ + 6Cl- SnCl6- + 2H2S
Once we've separated the precipitate of arsenic sulfide from the supernatant possibly containing Sb and Sn we are ready to perorm specific recognition essays.