Preparation of a standard solution of sodium carbonate

Introduction

The preparation of a standard solution of sodium carbonate is a fundamental laboratory experiment that forms the backbone of analytical chemistry. This experiment teaches students the essential techniques of volumetric analysis and solution preparation, which are crucial for accurate chemical measurements and titrations.

Aim

To prepare a standard solution of sodium carbonate (Na₂CO₃) of normality 0.1 N with precision and to verify its concentration through titration against a standard acid solution.

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Apparatus Required

Glassware and Instruments:

• 250 mL volumetric flask
• Analytical balance (accurate to 0.0001 g)
• Glass rod for stirring
• Funnel (small)
• Beaker (100 mL)
• Measuring cylinder (50 mL)
• Dropper
• Wash bottle
• Burette (50 mL)
• Pipette (25 mL)
• Conical flask (250 mL)

Chemicals:

• Anhydrous sodium carbonate (Na₂CO₃) - pure and dried
• Standard hydrochloric acid (0.1 N)
• Methyl orange indicator

Theory

A standard solution is a solution of known concentration that is prepared using a primary standard substance. Sodium carbonate serves as an excellent primary standard due to its:

• High purity and stability
• Non-hygroscopic nature
• Definite composition
• Easy availability in pure form

Chemical Principles:

The equivalent weight of Na₂CO₃ = Molecular weight / 2 = 106/2 = 53 g/equivalent

For 0.1 N solution: Amount needed = Normality × Equivalent weight × Volume (in L) ÷ 1000 = 0.1 × 53 × 250 ÷ 1000 = 1.325 g

Titration Reaction:

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Indicator used: Methyl orange (changes from yellow to pink)

Procedure

Part A: Preparation of Standard Solution

1. Weighing the Solute:

   • Clean and dry a small beaker
   • Weigh the empty beaker and record its mass
   • Add approximately 1.325 g of anhydrous Na₂CO₃
   • Record the final mass to calculate exact amount used

2. Dissolution:

   • Transfer the weighed Na₂CO₃ to a 100 mL beaker
   • Add 50 mL distilled water and stir with glass rod until completely dissolved
   • Wash the glass rod and beaker with distilled water

3. Transfer to Volumetric Flask:

   • Pour the solution into a 250 mL volumetric flask through funnel
   • Wash the beaker 2-3 times with distilled water and add washings to flask
   • Add distilled water up to about 2 cm below the calibration mark
   • Allow to cool to room temperature

4. Making up to Volume:

   • Add distilled water carefully drop by drop until the meniscus touches the calibration mark
   • Stopper the flask and shake thoroughly for uniform concentration

Part B: Verification by Titration

1. Rinse and fill burette with 0.1 N HCl solution
2. Pipette out 25 mL of prepared Na₂CO₃ solution into conical flask
3. Add 2-3 drops of methyl orange indicator
4. Titrate against HCl solution until color changes from yellow to permanent pink
5. Record the burette readings
6. Repeat titration for concordant values

Observation Table

Mean titre value = 24.77 mL

Calculation

Using the formula: N₁V₁ = N₂V₂

Where:

• N₁ = Normality of Na₂CO₃ solution (unknown)
• V₁ = Volume of Na₂CO₃ solution = 25 mL
• N₂ = Normality of HCl = 0.1 N
• V₂ = Volume of HCl used = 24.77 mL

N₁ × 25 = 0.1 × 24.77 N₁ = (0.1 × 24.77) ÷ 25 = 0.0991 N

Result

The prepared standard solution of sodium carbonate has a normality of 0.0991 N, which is very close to the target 0.1 N, confirming successful preparation of the standard solution.

Precautions

1. Accurate Weighing:

   • Use analytical balance for precise measurements
   • Close balance doors while weighing
   • Handle weighing bottle with care

2. Proper Transfer:

   • Transfer all Na₂CO₃ completely to volumetric flask
   • Use washings to ensure quantitative transfer

3. Volume Measurement:

   • Read meniscus at eye level
   • Ensure complete mixing by inverting flask multiple times
   • Cool solution to room temperature before making up volume

4. Titration Precautions:

   • Rinse burette with titrant before filling
   • Remove air bubbles from burette
   • Add indicator only after pipetting the solution
   • Titrate slowly near the endpoint

5. General:

   • Use distilled water throughout the experiment
   • Clean all apparatus properly before use
   • Record observations immediately

Viva Questions and Answers

Q1: What is a standard solution?

A: A standard solution is a solution whose concentration is accurately known and can be used for quantitative analysis.

Q2: Why is anhydrous sodium carbonate preferred over hydrated form?

A: Anhydrous Na₂CO₃ has a definite composition and doesn't contain water of crystallization, making its molecular weight constant.

Q3: Why should the solution be cooled before making up to volume?

A: Volume is temperature-dependent. Heating causes expansion, leading to incorrect concentration if volume is marked at elevated temperature.

Q4: What is the equivalent weight of Na₂CO₃?

A: Equivalent weight = Molecular weight / n-factor = 106/2 = 53 g/equivalent (n-factor is 2 as two H⁺ ions are required)

Q5: Why is methyl orange used as indicator?

A: Methyl orange is suitable for strong acid vs weak base titration. It gives sharp color change from yellow to pink at pH 3.1-4.4.

Q6: How can you test the strength of prepared solution?

A: By titrating against a standard acid solution of known concentration.

Q7: What are primary standard substances?

A: Substances that are pure, stable, non-hygroscopic, and have high molecular weight, making them suitable for direct preparation of standard solutions.

Q8: Why is the volumetric flask stoppered and shaken?

A: To ensure uniform concentration throughout the solution and homogeneous mixing.

Conclusion

This experiment successfully demonstrates the preparation and standardization of sodium carbonate solution. The obtained normality of 0.0991 N is very close to the theoretical value of 0.1 N, confirming the accuracy of both preparation and standardization techniques. Mastery of this experiment provides essential skills for volumetric analysis and forms the foundation for more complex analytical procedures in chemistry laboratories.