All Anions

Qualitative Inorganic Analysis

Overview of Qualitative Analysis

  • Qualitative analysis focuses on detection and identification of different substances alone or in mixtures.

  • Specifically examines anions, defined as negatively charged fragments of salt or compounds (acid radicals).

    • Example: NaCl dissociates into Na⁺ (cation) and Cl⁻ (anion).

  • Analytical chemistry is divided into three main parts: qualitative, quantitative, and applied analysis.

Classification of Anions

  • Anion Groups: Anions are classified into six groups:

    1. Carbonates and Bicarbonates

    2. Sulphur-containing anions

    3. Halides

    4. Cyanogen anions

    5. Arsenic and phosphorus containing anions

    6. Nitrogen-containing anions

Carbonates and Bicarbonates

  • Key Characteristics:

    • Parent Acid: Carbonic acid (H₂CO₃) is a weak, volatile acid stronger than HCN and boric acid.

    • Heating carbonic acid leads to the evolution of CO₂:

      • H₂CO₃ ⇌ CO₂ + H₂O

    • Bicarbonates (HCO₃⁻) are products of the first ionization of H₂CO₃, while carbonates (CO₃²⁻) are from the second.

    • Reactions and solubility:

      • Carbonate salts (excluding alkali metal carbonates) are generally insoluble in water.

      • All bicarbonates are soluble in water.

  • General Reactions:

    1. Dry Reactions:

    • Action of dilute HCl causes effervescence due to CO₂ evolution:

      • CO₃²⁻ + 2H⁺ → CO₂ ↑ + H₂O

      • This is a displacement reaction.

    1. Wet Reactions:

    • A sample with AgNO₃ creates a white precipitate of silver carbonate, soluble in acids and ammonia.

    • Special tests include reactions with BaCl₂, CaCl₂, and MgSO₄ producing precipitates of BaCO₃, CaCO₃, and MgCO₃.

Sulphur-containing Anions

  • Groups of sulphur-containing anions include:

    1. Sulphide (S²⁻)

    2. Sulphite (SO₃²⁻)

    3. Thiosulphate (S₂O₃²⁻)

    4. Sulphate (SO₄²⁻)

    5. Per-sulphate (S₂O₈²⁻)

  • General Properties:

    • Parent acids include:

      • Hydrogen sulfide (H₂S) is a gas with a rotten egg odor, ionizes steps:

      • H₂S → H⁺ + HS⁻ → H⁺ + S²⁻

  • Sulphuric acid (H₂SO₄) is a strong acid, ionizing to:

    • General Reactions:

      • Dilute HCl reaction with sulphides will produce H₂S gas; recognized by a yellow precipitate with cadmium acetate.

Halides

  • Comprising of the following ions:

    1. Fluoride (F⁻)

    2. Chloride (Cl⁻)

    3. Bromide (Br⁻)

    4. Iodide (I⁻)

  • General Properties:

    • Higher electronegativity and decreasing ionic size promotes ease of oxidation: I⁻ > Br⁻ > Cl⁻ > F⁻

  • Parent acids include:

    • Hydrochloric acid (HCl), colorless and highly soluble, forms an acidic solution.

    • Hydrobromic acid (HBr) and Hydroiodic acid (HI) exhibit similar properties and tendencies.

  • General Reactions:

    • Dry reactions with concentrated H₂SO₄ lead to the identification of HX gases, differentiating halides from carbonate and sulphur groups.

Nitrogen-containing Anions

  • Comprising:

    1. Nitrate (NO₃⁻)

    2. Nitrite (NO₂⁻)

  • General Properties:

    • Nitrate ions are strong oxidizing agents, while nitrites can have reducing or oxidizing effects based on their form and reactions.

Analytical Procedures

  • General Reactions:

    • Identifying anions requires knowledge of their characteristics, typical reactions, and the parent acids associated.

    • Solubility assessments and various precipitation tests supported by wet and dry reaction techniques.

  • Special Tests:

    • Include reactions with metallic salts, concentration tests using reagents like AgNO₃, BaCl₂, and specific agent applications to differentiate between anions.