Arsenic identification

Arsenic is identified as one of group 2 cations. Is part (obviousy) 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 discuss about arsenic. After having separated the precipitate consisting of As2S3 and/or As2S5, we treat it with concentrated nitric acid to solubilize As3+ and/or As5+ . Nitric acid oxidizes the sulfide ion to elemental sulfur, and arsenic (both As3+ and As5+) is solubilized as arsenic acid (or orthoarsenic acid) which is obviously weaker than nitric acid. Here it is the reaction:

3As2S3 + 10NO3- + 10H+ + 4H2O 6H3AsO4 + 9S° + 10NO

That is similarly given by As5+.

For what concerns the lab practice, we add ten drops of concentrated nitric acid (about 1 mL) to the precipitate. We heat the test tube (water-bath). The precipitate is solubilized and the test tube developes red vapors of nitric oxide, NO, highly toxic. The just formed elemental sulfur has to be eliminated with the rod and the solution obtained is put into a capsule and heated almost to dryiness. We add then few mL of H2O. We are finally ready to perform specific confirmatory test for As(V).

1) Confirmartory test with silver nitrate (AgNO3)

The solution (acidic) is neutralized with calcium carbonate (up to pH 7). That's why, since the reagent is silver-based, you cannot use ammonia (silver would form amino complexes) or soda (would precipitate as Ag2O) to neutralize the nitric solution.

2) Confirmatory test with magnesium mixture (MgCl2 - NH4Cl - NH4OH)

NH4OH and NH4Cl are used to buffer the solution in order not to precipitate magnesium hydroxide, considering that we are adding MgCl2In alkaline solution arsenic acid is completely dissociated (H3AsO4 →  AsO43-)

3) Confirmatory test with molybdic reagent (NH4)2MoO4 in HNO3

The precipitation conditions of this compound are a highly acidic environment and a consistent excess of reactive, all tips that there are also provided by the stoichiometry of the reaction. The precipitate has an intense yellow color, and can not be confused with the yellowish-white precipitate that may eventually form of molibdic dioxide. To improve the yield of the reaction we can also take advantage of the common ion effect of a salt such as NH4NO3.