12.5: Qualitative Analysis Using Selective Precipitation — Read

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Learning Objectives

By the end of this section, you will have background information that will help you to be able to:

Predict how stresses in concentration, temperature, and pressure affect the solubility of a sparingly soluble salt.

The composition of relatively complex mixtures of metal ions can be determined using qualitative analysis. This is a procedure that allows for discovering the identity of metal ions present in the mixture (rather than quantitative information about their amounts).

The procedure used to separate and identify more than 20 common metal cations from a single solution consists of selectively precipitating only a few kinds of metal ions at a time under given sets of conditions. Consecutive precipitation steps become progressively less selective until almost all of the metal ions are precipitated, as illustrated in Figure 17.5.1.

Figure 17.5.1 Steps in a Typical Qualitative Analysis Scheme for a Solution That Contains Several Metal Ions

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 2, the other alkali metal ions also give characteristic colors in flame tests, which allows them to be identified if only one is present.

Summary

In qualitative analysis, the identity, not the amount, of metal ions present in a mixture is determined. The technique consists of selectively precipitating only a few kinds of metal ions at a time under given sets of conditions. Consecutive precipitation steps become progressively less selective until almost all the metal ions are precipitated. Other additional steps are needed to separate metal ions that precipitate together.

Key Takeaway

Several common metal cations can be identified in a solution using selective precipitation.

Conceptual Problem

Given a solution that contains a mixture of NaCl, CuCl₂, and ZnCl₂, propose a method for separating the metal ions.

Contributors

Anonymous

Modified by Joshua B. Halpern

This page titled 12.5: Qualitative Analysis Using Selective Precipitation was last modified on Thu, 13 Aug 2026 01:22:55 GMT and is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Staff.

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