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Qualitative Analysis

Introduction to Qualitative Analysis - Identifying Anions and Cations


Qualitative analysis is used to separate and detect cations and anions in a sample substance. In an educational setting, it is generally true that the concentrations of the ions to be identified are all approximately 0.01 M in an aqueous solution. The 'semimicro' level of qualitative analysis employs methods used to detect 1-2 mg of an ion in 5 mL of solution.

First, ions are removed in groups from the initial aqueous solution. After each group has been separated, then testing is conducted for the individual ions in each group. Here is a common grouping of cations:

Group I: Ag+, Hg22+, Pb2+
Precipitated in 1 M HCl

Group II: Bi3+, Cd2+, Cu2+, Hg2+, (Pb2+), Sb3+ and Sb5+, Sn2+ and Sn4+
Precipitated in 0.1 M H2S solution at pH 0.5

Group III: Al3+, (Cd2+), Co2+, Cr3+, Fe2+ and Fe3+, Mn2+, Ni2+, Zn2+
Precipitated in 0.1 M H2S solution at pH 9

Group IV: Ba2+, Ca2+, K+, Mg2+, Na+, NH4+
Ba2+, Ca2+, and Mg2+ are precipitated in 0.2 M (NH4)2CO3 solution at pH 10; the other ions are soluble

Many reagents are used in qualitative analysis, but only a few are involved in nearly every group procedure. The four most commonly used reagents are 6M HCl, 6M HNO3, 6M NaOH, 6M NH3. Understanding the uses of the reagents is helpful when planning an analysis.

Next: Complex Ions and Precipitation Reactions

Common Qualitative Analysis Reagents

Reagent Effects
6M HCl Increases [H+]
Increases [Cl-]
Decreases [OH-]
Dissolves insoluble carbonates, chromates, hydroxides, some sulfates
Destroys hydroxo and NH3 complexes
Precipitates insoluble chlorides
6M HNO3 Increases [H+]
Decreases [OH-]
Dissolves insoluble carbonates, chromates, and hydroxides
Dissolves insoluble sulfides by oxidizing sulfide ion
Destroys hydroxo and ammonia complexes
Good oxidizing agent when hot
6 M NaOH Increases [OH-]
Decreases [H+]
Forms hydroxo complexes
Precipitates insoluble hydroxides
6M NH3 Increases [NH3]
Increases [OH-]
Decreases [H+]
Precipitates insoluble hydroxides
Forms NH3 complexes
Forms a basic buffer with NH4+

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