Titration of Sodium Carbonate (Na₂CO₃) with Hydrochloric Acid (HCl) – Class 11 Chemistry

Introduction

This is a typical acid-base titration where sodium carbonate (a basic salt) is titrated against a strong acid, hydrochloric acid (HCl), to determine the concentration of the carbonate.

Chemicals Required

• Sodium Carbonate (Na₂CO₃) – analyte
• Hydrochloric Acid (HCl) solution – titrant
• Methyl orange or phenolphthalein indicator
• Distilled water

Chemical Reactions

Sodium carbonate is a diprotic base (contains two replaceable CO₃²⁻ ions), so it reacts with acid in two steps:

Na₂CO₃ + HCl → NaHCO₃ + NaCl

NaHCO₃ + HCl → NaCl + CO₂↑ + H₂O
    

Overall reaction:

Na₂CO₃ + 2HCl → 2NaCl + CO₂↑ + H₂O
    

Apparatus Required

• Burette (filled with HCl solution)
• Pipette (for measuring sodium carbonate solution)
• Conical flask
• Beaker
• White tile (to observe color change)

Preparation of Solutions

• HCl Solution: Approximately 0.1 M standard solution prepared and standardized.
• Sodium Carbonate Solution: Exactly weighed sodium carbonate dissolved in distilled water.

Procedure

1. Rinse and fill the burette with HCl solution.
2. Using a pipette, transfer a known volume (e.g., 25 mL) of sodium carbonate solution into a conical flask.
3. Add 2-3 drops of methyl orange indicator to the flask (changes color in acidic medium).
4. Place the flask on a white tile for clear observation.
5. Slowly add HCl from the burette to the sodium carbonate solution while swirling.
6. Initially, solution turns from yellow (alkaline) to red as acid neutralizes carbonate to bicarbonate (first endpoint with phenolphthalein may be observed). Continue the addition until the color changes permanently to red (second endpoint).
7. Note the volume of acid used for complete neutralization.
8. Repeat titrations for consistent results.

Observation Table

Titration No.	Volume of Na₂CO₃ (mL)	Volume of HCl Added (mL)	Indicator Used	Color Change at Endpoint	Remarks
1	25.0	24.8	Methyl orange	Yellow to pale pink/red	Accurate endpoint
2	25.0	25.0	Methyl orange	Yellow to pale pink/red	Consistent trial
3	25.0	24.9	Methyl orange	Yellow to pale pink/red	Consistent trial

Calculations

Sodium carbonate requires two moles of HCl per mole for complete neutralization.

Formula:

M₁V₁ × n₁ = M₂V₂ × n₂

Where:

M₁ = Molarity of sodium carbonate,

V₁ = Volume of sodium carbonate solution,

n₁ = Number of replaceable ions in sodium carbonate (2),

M₂ = Molarity of HCl,

V₂ = Volume of HCl used,

n₂ = Number of replaceable H⁺ ions in HCl (1)
    

Result

The concentration of sodium carbonate is calculated from the volume of HCl consumed for neutralization.

Precautions

• Standardize HCl solution if possible before titration.
• Add acid slowly near the endpoint to avoid overshooting.
• Swirl the flask continuously for proper mixing.
• Use an appropriate indicator such as methyl orange.
• Perform multiple titrations for consistent results.