Group 1: Insoluble Chlorides

Most metal chloride salts are soluble in water; only Ag⁺, Pb²⁺, and Hg₂²⁺ form chlorides that precipitate from water. Thus the first step in a qualitative analysis is to add about 6 M HCl, thereby causing AgCl, PbCl₂, and/or Hg₂Cl₂ to precipitate. If no precipitate forms, then these cations are not present in significant amounts. The precipitate can be collected by filtration or centrifugation.

Group 2: Acid-Insoluble Sulfides

Next, the acidic solution is saturated with H₂S gas. Only those metal ions that form very insoluble sulfides, such as As³⁺, Bi³⁺, Cd²⁺, Cu²⁺, Hg²⁺, Sb³⁺, and Sn²⁺, precipitate as their sulfide salts under these acidic conditions. All others, such as Fe²⁺ and Zn²⁺, remain in solution. Once again, the precipitates are collected by filtration or centrifugation.

Group 3: Base-Insoluble Sulfides (and Hydroxides)

Ammonia or NaOH is now added to the solution until it is basic, and then (NH₄)₂S is added. This treatment removes any remaining cations that form insoluble hydroxides or sulfides. The divalent metal ions Co²⁺, Fe²⁺, Mn²⁺, Ni²⁺, and Zn²⁺ precipitate as their sulfides, and the trivalent metal ions Al³⁺ and Cr³⁺ precipitate as their hydroxides: Al(OH)₃ and Cr(OH)₃. If the mixture contains Fe³⁺, sulfide reduces the cation to Fe²⁺, which precipitates as FeS.

Group 4: Insoluble Carbonates or Phosphates

The next metal ions to be removed from solution are those that form insoluble carbonates and phosphates. When Na₂CO₃ is added to the basic solution that remains after the precipitated metal ions are removed, insoluble carbonates precipitate and are collected. Alternatively, adding (NH₄)₂HPO₄ causes the same metal ions to precipitate as insoluble phosphates.

Group 5: Alkali Metals

At this point, we have removed all the metal ions that form water-insoluble chlorides, sulfides, carbonates, or phosphates. The only common ions that might remain are any alkali metals (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) and ammonium (NH₄⁺). We now take a second sample from the original solution and add a small amount of NaOH to neutralize the ammonium ion and produce NH₃. (We cannot use the same sample we used for the first four groups because we added ammonium to that sample in earlier steps.) Any ammonia produced can be detected by either its odor or a litmus paper test. A flame test on another original sample is used to detect sodium, which produces a characteristic bright yellow color. As discussed in Chapter 6 "The Structure of Atoms", the other alkali metal ions also give characteristic colors in flame tests, which allows them to be identified if only one is present.

Metal ions that precipitate together are separated by various additional techniques, such as forming complex ions, changing the pH of the solution, or increasing the temperature to redissolve some of the solids. For example, the precipitated metal chlorides of group 1 cations, containing Ag⁺, Pb²⁺, and Hg₂²⁺, are all quite insoluble in water. Because PbCl₂ is much more soluble in hot water than are the other two chloride salts, however, adding water to the precipitate and heating the resulting slurry will dissolve any PbCl₂ present. Isolating the solution and adding a small amount of Na₂CrO₄ solution to it will produce a bright yellow precipitate of PbCrO₄ if Pb²⁺ was in the original sample (Figure 17.11 "The Separation of Metal Ions from Group 1 Using Qualitative Analysis").

As another example, treating the precipitates from group 1 cations with aqueous ammonia will dissolve any AgCl because Ag⁺ forms a stable complex with ammonia: [Ag(NH₃)₂]⁺. In addition, Hg₂Cl₂ disproportionates in ammonia (2Hg₂²⁺ → Hg + Hg²⁺) to form a black solid that is a mixture of finely divided metallic mercury and an insoluble mercury(II) compound, which is separated from solution:

Figure 17.11 The Separation of Metal Ions from Group 1 Using Qualitative Analysis

In (a), the cations of group 1 precipitate when HCl(aq) is added to a solution containing a mixture of cations. (b) When a small amount of Na₂CrO₄ solution is added to a sample containing Pb²⁺ ions in water, a bright yellow precipitate of PbCrO₄ forms. (c) Adding aqueous ammonia to a second portion of the solid sample produces a black solid that is a mixture of finely divided metallic mercury, an insoluble mercury(II) compound [Hg(NH₂)Cl], and a stable [Ag(NH₃)₂]⁺(aq) complex. (d) The presence of Ag⁺ is detected by decanting the solution from the precipitated mercury and mercury complex and adding hydrochloric acid to the decanted solution, which causes AgCl to precipitate.

Any silver ion in the solution is then detected by adding HCl, which reverses the reaction and gives a precipitate of white AgCl that slowly darkens when exposed to light:

Similar but slightly more complex reactions are also used to separate and identify the individual components of the other groups.