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.