16.14 : Qualitative Analysis

When an unknown solution contains a mixture of several different metal ions, the cations can be identified by a systematic series of selective precipitations, called qualitative analysis.

At each step of the analysis, a different precipitating reagent is added. These reagents selectively precipitate some cations as insoluble salts, which can be removed, while the others continue to remain in solution.

In aqueous solutions, there are 22 commonly occurring cations that can be divided into five groups based on the solubility products of their insoluble salts.

Group 1 cations are metal ions that form insoluble chlorides.

Most chloride salts are soluble in water. So, when the aqueous solution of metal ions is treated with 6 M hydrochloric acid, a precipitate would indicate a group 1 metal ion, such as silver, lead, or mercury.

If no precipitate forms, it indicates there are no group 1 cations present in the solution. This mixture is then centrifuged or filtered to separate the solid precipitate and the aqueous supernatant.

Next, hydrogen sulfide gas is bubbled through the acidic supernatant. The reaction between metal ions and hydrogen sulfide produces metal sulfide and protons.

Due to the addition of hydrochloric acid in the previous step, the high concentration of protons shifts the equilibrium towards the reactants.

Thus, under acidic conditions, only group 2 metal ions that form highly insoluble sulfide salts precipitate, while other metal sulfides that are slightly more soluble remain in solution.

Next, group 3 cations, which consist of base-insoluble sulfides and hydroxides, are precipitated.

Sodium hydroxide is added to the supernatant from the previous step to establish basic conditions. This addition depletes protons from the metal sulfide precipitation reaction, and the equilibrium moves towards the products.

As a result, many metal sulfides that were soluble in acidic conditions now become insoluble and form precipitates.

Additionally, metal ions that form insoluble hydroxides, such as iron, aluminum, and chromium, precipitate out of the solution.

When this mixture is separated, only the alkali and alkaline earth metal ions remain in solution.

The alkaline earth metals, which constitute group 4 cations, form insoluble phosphates.

The addition of diammonium hydrogen phosphate to the basic supernatant causes magnesium, calcium, barium, and strontium ions to precipitate.

The liquid decanted from this step contains the group 5 cations. These cations do not form insoluble salts and need to be identified individually.

If adding sodium hydroxide to the solution in the previous steps released a gas with the characteristic smell of ammonia, then ammonium ions were present in the mixture.

Sodium and potassium ions can be identified with a flame test. Sodium ions produce a bright yellow flame while a violet flame indicates potassium ions.