Qualitative Analysis

Analysis of cations (positively charged ions): Cations can be identified through various chemical reactions, such as precipitation, complexation, or color change reactions.

• Common cations and their identification methods:
  1. Ammonium (NH₄⁺): Reaction with sodium hydroxide (NaOH) produces ammonia gas (NH₃) with a pungent odor.
  2. Calcium (Ca²⁺): White precipitate forms when sodium oxalate (Na₂C₂O₄) is added, indicating the presence of calcium ions.
  3. Copper(II) (Cu²⁺): Forms a blue precipitate with sodium hydroxide (NaOH) or a deep blue solution with ammonia (NH₃).
  4. Iron(III) (Fe³⁺): A reddish-brown precipitate forms when sodium hydroxide (NaOH) is added, indicating the presence of iron(III).
  5. Iron(II) (Fe²⁺): Forms a greenish precipitate with sodium hydroxide (NaOH), and can be oxidized to Fe³⁺.

Analysis of anions (negatively charged ions): Anions are identified by testing for specific reactions that produce characteristic precipitates or gases.

• Common anions and their identification methods:
  1. Chloride (Cl⁻): Forms a white precipitate of silver chloride (AgCl) when silver nitrate (AgNO₃) is added.
  2. Sulfate (SO₄²⁻): Forms a white precipitate of barium sulfate (BaSO₄) when barium chloride (BaCl₂) is added.
  3. Nitrate (NO₃⁻): When treated with sodium nitrite (NaNO₂) and acid, it forms a brown ring at the interface, known as the brown ring test.
  4. Carbonate (CO₃²⁻): Reacts with acids to release carbon dioxide gas (CO₂), which can be tested with lime water (Ca(OH)₂).
  5. Phosphate (PO₄³⁻): Forms a yellow precipitate of ammonium phosphate when reacted with ammonium molybdate ((NH₄)₂MoO₄) in an acidic medium.

Test for the presence of cations:

1. Ammonium ion (NH₄⁺):
   1. Add sodium hydroxide (NaOH) solution. Heat gently.
   2. If ammonia gas (NH₃) is evolved, the presence of ammonium ions is confirmed.
2. Calcium ion (Ca²⁺):
   1. Add sodium oxalate (Na₂C₂O₄) solution to the sample.
   2. A white precipitate of calcium oxalate (CaC₂O₄) confirms the presence of calcium ions.
3. Copper(II) ion (Cu²⁺):
   1. Add sodium hydroxide (NaOH) solution to the sample.
   2. A light blue precipitate of copper(II) hydroxide (Cu(OH)₂) indicates copper ions.
   3. Alternatively, ammonia (NH₃) solution will give a deep blue solution of copper(II) ammonia complex if Cu²⁺ is present.
4. Iron(III) ion (Fe³⁺):
   1. Add sodium hydroxide (NaOH) to the sample.
   2. A reddish-brown precipitate of iron(III) hydroxide (Fe(OH)₃) indicates the presence of iron(III).

Test for the presence of anions:

1. Chloride ion (Cl⁻):
   1. Add silver nitrate (AgNO₃) solution to the sample.
   2. A white precipitate of silver chloride (AgCl) indicates the presence of chloride ions.
2. Sulfate ion (SO₄²⁻):
   1. Add barium chloride (BaCl₂) solution to the sample.
   2. A white precipitate of barium sulfate (BaSO₄) confirms the presence of sulfate ions.
3. Nitrate ion (NO₃⁻):
   1. Add sodium nitrite (NaNO₂) solution and dilute sulfuric acid (H₂SO₄) to the sample.
   2. A brown ring forms at the interface between the two layers, indicating the presence of nitrate ions.
4. Carbonate ion (CO₃²⁻):
   1. Add dilute hydrochloric acid (HCl) to the sample.
   2. The formation of carbon dioxide gas (CO₂) confirms the presence of carbonate ions. The gas can be bubbled through lime water to form a milky precipitate of calcium carbonate (CaCO₃), further confirming the presence of carbonate ions.
5. Phosphate ion (PO₄³⁻):
   1. Add ammonium molybdate ((NH₄)₂MoO₄) solution and nitric acid (HNO₃) to the sample.
   2. A yellow precipitate of ammonium phosphomolybdate (NH₄)₃PO₄·12MoO₃ indicates the presence of phosphate ions.

A typical qualitative analysis involves performing systematic tests to identify both cations and anions in the sample. The flowchart below provides a simplified outline of the general steps involved:

1. Preliminary tests:
   1. Physical observations (color, odor, solubility)
   2. Test for the presence of water (if needed)
2. Test for cations:
   1. Perform tests for common cations (e.g., NH₄⁺, Ca²⁺, Cu²⁺, Fe³⁺, Fe²⁺).
   2. Confirm positive results with appropriate reactions.
3. Test for anions:
   1. Perform tests for common anions (e.g., Cl⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, PO₄³⁻).
   2. Confirm positive results with appropriate reactions.
4. Further confirmation (if required):
   1. Additional qualitative tests based on the unknown ions (e.g., flame tests for alkali and alkaline earth metals, solubility tests for salts).

Experiment 1: Identification of cations in a sample

Objective: To identify the cations present in an unknown solution.

1. Prepare the unknown solution.
2. Add sodium hydroxide (NaOH) drop by drop to the sample.
3. Observe the reactions and record the colors and precipitates formed.
4. Test for the presence of specific cations like Ca²⁺, Fe²⁺, Fe³⁺, Cu²⁺, and NH₄⁺ based on known reactions.

Expected results:

1. Ca²⁺: White precipitate of calcium oxalate (CaC₂O₄).
2. Fe²⁺: Green precipitate of iron(II) hydroxide.
3. Fe³⁺: Reddish-brown precipitate of iron(III) hydroxide.
4. Cu²⁺: Blue precipitate of copper(II) hydroxide.
5. NH₄⁺: Ammonia gas evolution upon heating with NaOH.

Experiment 2: Identification of anions in a sample

Objective: To identify the anions present in an unknown solution.

1. Prepare the unknown solution.
2. Perform tests for common anions:
   • For chloride (Cl⁻), add silver nitrate (AgNO₃).
   • For sulfate (SO₄²⁻), add barium chloride (BaCl₂).
   • For carbonate (CO₃²⁻), add hydrochloric acid (HCl) and observe the evolution of CO₂ gas.

Expected results:

1. Cl⁻: White precipitate of silver chloride (AgCl).
2. SO₄²⁻: White precipitate of barium sulfate (BaSO₄).
3. CO₃²⁻: CO₂ gas evolves with HCl and turns lime water milky.