Qualitative Inorganic Analysis - Detecting Cations - 2nd Analytical Group of Cations

The 2nd analytical group of cations consists of ions that form acid-insoluble sulfides. Cations in the 2nd group include: Cd2+, Bi3+, Cu2+, As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+ and Hg2+. Pb2+ is usually also included here in addition to the first group. Although these methods refer to solutions that contain sulfide (S2-), these solutions actually only contain H₂S and bisulfide (SH-). Sulfide (S2-) does not exist in appreciable concentrations in water.

The reagent used can be any substance that gives S2− ions in such solutions; most commonly used are H₂S (at 0.2-0.3 M), AKT (at 0.3-0.6 M). The test with the sulfide ion must be conducted in the presence of dilute HCl. Its purpose is to keep the sulfide ion concentration at a required minimum, so as to allow the precipitation of 2nd group cations alone. If dilute acid is not used, the early precipitation of 4th group cations (if present in solution) may occur, thus leading to misleading results. Acids beside HCl are rarely used. Sulfuric acid may lead to the precipitation of the 4th group cations, whereas nitric acid oxidises the sulfide ion in the reagent, forming colloidal sulfur.

The precipitates of these cations are almost indistinguishable, except for CdS, which is yellow. All the precipitates, except for HgS, are soluble in dilute nitric acid. HgS is soluble only in aqua regia, which can be used to separate it from the rest. The action of ammonia is also useful in differentiating the cations. CuS dissolves in ammonia forming an intense blue solution, whereas CdS dissolves forming a colourless solution. The sulfides of As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+ are soluble in yellow ammonium sulfide, where they form polysulfide complexes.

This group is determined by adding the salt in water and then adding dilute hydrochloric acid followed by hydrogen sulfide. Usually it is done by passing hydrogen sulfide over the test tube for detection of 1st group cations. If it forms a reddish brown or black precipitate then Bi3+, Cu2+, Hg2+ or Pb2+ is present. Otherwise, if it forms a yellow precipitate, then Cd2+ or Sn4+ is present; or if it forms a brown precipitate, then Sn2+ must be present; or if a red orange precipitate is formed, then Sb3+ is present.

To distinguish between ions in the black or reddish brown precipitate, it is first boiled in diluted HNO₃. If it is insoluble, then Hg2+ is present. If it is soluble, then Cu2+, Bi3+ or Pb2+ may be present; sulfuric acid is then added to the resulting solution. If a white precipitate forms, then Pb2+ may be present; if no precipitate is formed, then a new solution is made by adding an excess of ammonium hydroxide in the original salt solution. A resulting blue color indicates the presence of Cu2+, and a white precipitate indicates bismuth.

To distinguish between ions in the yellow precipitate, an excess of NaOH is added to the original salt solution to form a white precipitate. The test tube is then shaken, and if the white precipitate dissolves, then Sn4+ is present; otherwise, Cd2+ is present.

Confirmation test for lead:

Pb2+ + 2 KI → PbI₂ + 2 K+

Pb2+ + K₂CrO₄ → PbCrO₄ + 2 K+

Confirmation test for copper:

2 Cu2+ + K₄ + CH₃COOH → Cu₂ + 4 K+

Cu2+ + 2 NaOH → Cu(OH)₂ + 2 Na+

Cu(OH)₂ → CuO + H₂O (endothermic)

Confirmation test for bismuth:

Bi3+ + 3 KI (in excess) → BiI₃ + 3 K+

BiI₃ + KI → K

Bi3+ + H₂O (in excess) → BiO+
+ 2 H+

Confirmation test for mercury:

Hg2+ + 2 KI (in excess) → HgI₂ + 2 K+

HgI₂ + 2 KI → K₂ (red precipitate dissolves)

2 Hg2+ + SnCl₂ → 2 Hg + SnCl₄ (white precipitate turns gray)