Introduction

Qualitative analysis of metal cations is a fundamental skill in analytical chemistry. This article explores three main techniques: precipitation tests, flame tests, and complexation reactions, focusing on eight important cations: Ba²⁺, Ca²⁺, Mg²⁺, Pb²⁺, Ag⁺, Cu²⁺, Fe²⁺, and Fe³⁺.

Precipitation Tests: Principles and Applications

Precipitation tests rely on the formation of insoluble compounds when specific anions interact with metal cations. The effectiveness of these tests depends on:

For example, when sulfate ions $SO_4^{2-}$ are added to solutions containing Ba²⁺ or Cu²⁺:

$\text{Ba}^{2+}_{(aq)} + \text{SO}4^{2-}{(aq)} \rightarrow \text{BaSO}4{(s)}$ (white precipitate)

Flame Tests: Theory and Practice

Flame tests utilize the principle of atomic emission spectroscopy. When metal ions are heated in a flame:

The energy difference (ΔE) between levels determines the wavelength (λ) of emitted light:

$\Delta E = \frac{hc}{\lambda}$

where h is Planck's constant and c is the speed of light.

Complexation Reactions

Complexation involves the formation of coordination compounds between metal ions and ligands. These reactions are particularly useful for transition metals due to their:

For example, the formation of the copper-ammonia complex:

$\text{Cu}^{2+}_{(aq)} + 4\text{NH}_{3(aq)} \rightarrow [\text{Cu}(\text{NH}_3)_4]^{2+}_{(aq)}$

Systematic Analysis of Common Cations

Group 1: Silver and Lead

Group 2: Alkaline Earth Metals

Group 3: Transition Metals

Summary Table of Key Tests

Cation	Flame Color	Precipitation with OH⁻	Complex Formation
Ba²⁺	Pale green	White	None
Ca²⁺	Brick red	White	None
Mg²⁺	None	White	None
Cu²⁺	Blue-green	Blue	Deep blue (NH₃)
Fe²⁺	None	Green	Pale green
Fe³⁺	None	Brown	Blood red (SCN⁻)

Safety Considerations