Antimony is identified as one of group 2 cations. It is part of the arsenic subgroup.
If you need a small resume, you can quicly check the summary of arsenic subgroup analysis with this scheme.
Let's see how is it possible to spot antimony in an acqueous solution.
During group 2 analysis, antimony was separated from a precipitate made of arsenic sulfides. In particular, it had been solubilized as a chlorine-complex (SbCl6- ,SbCl4-) after the treatment with concentrated HCl. It is therefore on this solution that we are going to perform the tests in order to identify antimony.
1) Addition of oxalic acid and Na2S
You might already know that tin can be very annoying when you're trying to identify the antimony, it is the main interferent. For this reason we need to find the best way to get rid of it. Coincidentally , oxalic acid forms a very stable complex with tin so that this will not precipitate as soon as we add the real reagent to the solution, sodium sulfide (Na2S).
Oxalic acid acts as a maskerating agent:
Sn4+ + 3C2O42- Sn(C2O4)32-
Afterwards we can add Na2S to the solution, and, if Sb is present, we get a peculiar orange precipitate of antimony (III) or antimony (V).
2) Test with Sn
Elementary tin act as a reductant towards various elements, including antimony. The reaction produces elementary antimony, which is colored in black.
2Sb3+ + 3Sn 2Sb + 3Sn2+
2Sb 5+ + 5Sn 2Sb + 5Sn2+
This, as mentioned, is confirmed by their respective standard reduction potentials:
- E°Sn2+/Sn = - 0,14 V
- E°Sb3+/Sb = 0,16 V
- E°Sb5+/Sb3+ = 0.6 V