Qualitative Analysis of Anions: A Systematic Approach
Expert reviewed • 23 November 2024 • 5 minute read
VIDEO
Introduction
Testing for anions is a fundamental skill in analytical chemistry. This guide covers systematic methods to identify common anions through precipitation reactions and pH indicators. Understanding these tests is crucial for qualitative analysis in both laboratory and industrial settings.
Common Anion Tests
Halide Tests (Cl⁻, Br⁻, I⁻)
Silver nitrate (AgNO₃) is the primary reagent for identifying halide ions. Each halide produces a characteristic precipitate:
Chloride (Cl⁻): White precipitate (AgCl)
Ag + + Cl − → AgCl ( s ) \text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl}_{(s)} Ag + + Cl − → AgCl ( s )
Bromide (Br⁻): Cream precipitate (AgBr)
Ag + + Br − → AgBr ( s ) \text{Ag}^+ + \text{Br}^- \rightarrow \text{AgBr}_{(s)} Ag + + Br − → AgBr ( s )
Iodide (I⁻): Yellow precipitate (AgI)
Ag + + I − → AgI ( s ) \text{Ag}^+ + \text{I}^- \rightarrow \text{AgI}_{(s)} Ag + + I − → AgI ( s )
Hydroxide (OH⁻) Tests
Precipitation Tests
With copper(II): Blue precipitate
With iron(II): Green precipitate
With iron(III): Brown precipitate
pH Indicators
Universal indicator turns blue
Red litmus paper turns blue
pH > 7
Acetate (CH₃COO⁻) Tests
Acid Test
When acidified with nitric acid:
CH 3 COO ( a q ) − + H ( a q ) + → CH 3 COOH ( a q ) \text{CH}_3\text{COO}^-_{(aq)} + \text{H}^+_{(aq)} \rightarrow \text{CH}_3\text{COOH}_{(aq)} CH 3 COO ( a q ) − + H ( a q ) + → CH 3 COOH ( a q )
pH Testing
In water:
CH 3 COO ( a q ) − + H 2 O ( l ) ⇌ CH 3 COOH ( a q ) + OH ( a q ) − \text{CH}_3\text{COO}^-_{(aq)} + \text{H}_2\text{O}_{(l)} \rightleftharpoons \text{CH}_3\text{COOH}_{(aq)} + \text{OH}^-_{(aq)} CH 3 COO ( a q ) − + H 2 O ( l ) ⇌ CH 3 COOH ( a q ) + OH ( a q ) − Carbonate (CO₃²⁻) Tests
Acid Test
CO 3 2 − (aq) + 2 H + (aq) → H 2 O(l) + CO 2 (g) \text{CO}_3^{2-}\text{(aq)} + 2\text{H}^+\text{(aq)} \to \text{H}_2\text{O}\text{(l)} + \text{CO}_2\text{(g)} CO 3 2 − (aq) + 2 H + (aq) → H 2 O (l) + CO 2 (g)
Limewater Test
Ca(OH) 2 (aq) + CO 2 (g) → CaCO 3 (s) + H 2 O(l) \text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O}\text{(l)} Ca(OH) 2 (aq) + CO 2 (g) → CaCO 3 (s) + H 2 O (l)
Solution turns milky white
Sulfate (SO₄²⁻) Tests
Barium Test
Add acidified barium nitrate solution
Ba 2 + (aq) + SO 4 2 − (aq) → BaSO 4 (s) \text{Ba}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} Ba 2 + (aq) + SO 4 2 − (aq) → BaSO 4 (s)
White precipitate forms
Phosphate (PO₄³⁻) Tests
Barium Test in Basic Conditions
White precipitate in alkaline conditions
Dissolves in acid due to:
PO 4 3 − (aq) + H + (aq) ⇌ HPO 4 2 − (aq) \text{PO}_4^{3-}\text{(aq)} + \text{H}^+\text{(aq)} \rightleftharpoons \text{HPO}_4^{2-}\text{(aq)} PO 4 3 − (aq) + H + (aq) ⇌ HPO 4 2 − (aq)
Yellow precipitate forms
Used for environmental monitoring