Editing Qualitative inorganic analysis (section)

=== 2nd analytical group of cations ===

The ''2nd analytical group of cations'' consists of ions which form acid-insoluble [[sulfide]]s. Cations in the 2nd group include: Cd<sup>2+</sup>, [[bismuth|Bi<sup>3+</sup>]], [[copper|Cu<sup>2+</sup>]], [[arsenic|As<sup>3+</sup>]], As<sup>5+</sup>, [[antimony|Sb<sup>3+</sup>]], Sb<sup>5+</sup>, Sn<sup>2+</sup>, Sn<sup>4+</sup> and Hg<sup>2+</sup>. Pb<sup>2+</sup> is usually also included here in addition to the first group. Although these methods refer to solutions that contain sulfide (S<sup>2−</sup>), these solutions actually only contain H<sub>2</sub>S and [[bisulfide]] (HS<sup>−</sup>).  Sulfide (S<sup>2−</sup>) does not exist in appreciable concentrations in water.

The reagent used can be any substance that gives S<sup>2&minus;</sup> ions in such solutions; most commonly used are [[hydrogen sulfide]] (at 0.2-0.3 M), [[thioacetamide]] (at 0.3-0.6 M), addition of hydrogen sulfide can often prove to be a lumbersome process and therefore sodium sulfide can also serve the purpose.  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 5th 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 [[Cadmium sulfide|CdS]], which is yellow. All the precipitates, except for [[Cinnabar|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 As<sup>3+</sup>, As<sup>5+</sup>, Sb<sup>3+</sup>, Sb<sup>5+</sup>, Sn<sup>2+</sup>, Sn<sup>4+</sup> 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 (to make the medium acidic) followed by hydrogen sulfide gas. 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 Bi<sup>3+</sup>, Cu<sup>2+</sup>, Hg<sup>2+</sup> or Pb<sup>2+</sup> is present. Otherwise, if it forms a yellow precipitate, then Cd<sup>2+</sup> or Sn<sup>4+</sup> is present; or if it forms a brown precipitate, then Sn<sup>2+</sup> must be present; or if a red orange precipitate is formed, then Sb<sup>3+</sup> is present.

:Pb<sup>2+</sup> + K<sub>2</sub>CrO<sub>4</sub> → PbCrO<sub>4</sub> + 2 K<sup>+</sup>

Confirmation test for copper:

:2 Cu<sup>2+</sup> + K<sub>4</sub>[Fe(CN)<sub>6</sub>] + CH<sub>3</sub>COOH → Cu<sub>2</sub>[Fe(CN)<sub>6</sub>] + 4 K<sup>+</sup>
:Cu<sup>2+</sup> + 2 NaOH → Cu(OH)<sub>2</sub> + 2 Na<sup>+</sup>
:Cu(OH)<sub>2</sub> → CuO + H<sub>2</sub>O (endothermic)
:(Another very sensitive test for copper utilizes the fact that Cu<sup>2+</sup> can serve as a catalyst for the oxidation of [[thiosulfate]] ions by [[Ferric|Fe<sup>3+</sup>]] ions. In the absence of Cu<sup>2+</sup>, Fe<sup>3+</sup> can form the purple complex Fe(S<sub>2</sub>O<sub>3</sub>)<sub>2</sub><sup>−</sup> without undergoing [[redox]]. If the added sample contains Cu<sup>2+</sup>, the solution will rapidly discolor.)<ref name=":0" />

Confirmation test for bismuth:

:Bi<sup>3+</sup> + 3 KI (in excess) → BiI<sub>3</sub> + 3 K<sup>+</sup>
:BiI<sub>3</sub> + KI → K[BiI<sub>4</sub>]
:Bi<sup>3+</sup> + H<sub>2</sub>O (in excess) → BiO{{su|p=+}} + 2 H<sup>+</sup>
:(Bismuth ions can form th bright yellow complex Bi([[Thiourea|tu]])<sub>3</sub><sup>3+</sup> in the presence of [[thiourea]] under acidic conditions, which can be precipitated as the orange-red Bi(tu)<sub>3</sub>I<sub>3</sub>•Cu(tu)<sub>3</sub>I in the presence of Cu<sup>2+</sup> and I<sup>−</sup>, and this can also act as a test for bismuth.)<ref name=":0" />

Confirmation test for mercury:

:Hg<sup>2+</sup> + 2 KI (in excess) → HgI<sub>2</sub> + 2 K<sup>+</sup>
:HgI<sub>2</sub> + 2 KI → K<sub>2</sub>[HgI<sub>4</sub>] (red precipitate dissolves)
:2 Hg<sup>2+</sup> + SnCl<sub>2</sub> →  2 Hg + SnCl<sub>4</sub> (white precipitate turns gray)
:(Hg<sup>2+</sup> may otherwise be detected via Cu<sub>2</sub>HgI<sub>4</sub> formation, see Hg<sub>2</sub><sup>2+</sup> in 1st group cations.)<ref name=":0" />