Titration to determine hydrochloric acid strength using sodium carbonate standard

Aim

To determine the strength of given hydrochloric acid solution by titrating it against a standard solution of sodium carbonate (Na₂CO₃) using methyl orange as an indicator.

Apparatus Required

• Burette (25 mL capacity)
• Pipette (20 mL capacity)
• Conical flask (250 mL)
• Measuring cylinder (100 mL)
• Burette stand with clamp
• White tile (for better color observation)
• Funnel (small)
• Wash bottle with distilled water
• Burette reading card
• Glass rod for stirring

Chemicals Required

• Hydrochloric acid solution (given/unknown strength)
• Standard sodium carbonate (Na₂CO₃) solution (approximately 0.1 N)
• Methyl orange indicator

Theory

Chemical Reaction

The titration is based on the following acid-base neutralization reaction:

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Indicator Selection

Methyl orange is used as an indicator because:

• It changes color in the acidic pH range (3.1-4.4)
• Color change: Yellow (basic) → Reddish orange (acidic)
• The equivalence point falls within this pH range for this titration

Normality Relationship

At the equivalence point: N₁V₁ (Na₂CO₃) = N₂V₂ (HCl)

Therefore: N₂ (HCl) = (N₁ × V₁)/V₂

Strength of HCl = Normality × Equivalent weight of HCl Strength of HCl = N₂ × 36.5 g/L

Procedure

Preparation:

1. Rinse burette with given HCl solution and fill it
2. Rinse pipette with standard Na₂CO₃ solution
3. Fill conical flask with 20 mL standard Na₂CO₃ solution using pipette
4. Add 2-3 drops of methyl orange indicator to the conical flask

Titration Process:

1. Place the conical flask on white tile under burette
2. Initial reading of burette should be noted
3. Add HCl solution slowly while swirling the flask continuously
4. First addition: Add HCl in 1 mL portions with continuous stirring
5. Approaching endpoint: Add HCl drop by drop
6. Endpoint: Color changes from yellow to permanent reddish orange
7. Note the final burette reading
8. Repeat the titration 4-5 times for concordant values

Observation Table

Concordant readings: 15.6 mL (average)

Calculations

Given:

• Volume of Na₂CO₃ solution (V₁) = 20 mL
• Normality of Na₂CO₃ solution (N₁) = 0.1 N
• Average volume of HCl used (V₂) = 15.6 mL

Using formula: N₁V₁ = N₂V₂

N₂ = (N₁ × V₁)/V₂ N₂ = (0.1 × 20)/15.6 = 0.128 N

Strength of HCl = Normality × Equivalent weight Strength = 0.128 × 36.5 = 4.672 g/L

Result

1. Normality of given HCl solution = 0.128 N
2. Strength of given HCl solution = 4.672 g/L (4.672 g per liter)

Precautions

1. Proper rinsing: Always rinse burette and pipette with respective solutions
2. Avoid air bubbles in burette
3. Read meniscus at eye level for accurate readings
4. Add indicator sparingly (2-3 drops only)
5. Continuous swirling during titration for uniform mixing
6. Detect endpoint carefully - look for permanent color change
7. Use white tile for better color observation
8. Do not touch the burette from sides to avoid heating by hands
9. Ensure nozzle of burette is filled with solution
10. Repeat titration to get concordant values

Viva Questions and Answers

Q1: Why is methyl orange used as an indicator in this titration?

A: Methyl orange is used because it shows a sharp color change in the acidic pH range (3.1-4.4), which corresponds to the equivalence point of this acid-base titration.

Q2: What is the equivalent weight of Na₂CO₃?

A: The equivalent weight of Na₂CO₃ = Molecular weight/2 = 106/2 = 53 g/equivalent, because it can accept 2H⁺ ions.

Q3: Why should the burette reading be taken at eye level?

A: Taking readings at eye level prevents parallax error and ensures accurate measurements.

Q4: What is the color change at the endpoint?

A: The color changes from yellow (basic) to permanent reddish orange (acidic).

Q5: Why is the Na₂CO₃ solution considered a primary standard?

A: Sodium carbonate is a primary standard because it is stable, has high purity, easily available, and its equivalent weight can be accurately determined.

Q6: What does concordant value mean?

A: Concordant values are those readings that agree within ±0.02 mL of each other, indicating the reliability of the titration.

Q7: Why should the burette tap be operated with the left hand?

A: It allows better control over the flow rate and enables right-handed persons to swirl the flask with their dominant hand.

Q8: What is the basicity of HCl?

A: The basicity of HCl is 1, as it can donate one proton (H⁺).

Q9: How would you detect the endpoint in this titration?

A: The endpoint is detected by the permanent color change from yellow to reddish orange that does not revert back on swirling.

Q10: Why is it necessary to repeat the titration multiple times?

A: Repetition ensures accuracy and reliability of results by identifying and eliminating any anomalous readings.

Additional Notes

This titration method is widely used in analytical chemistry for acid-base determinations. The key to successful results lies in careful endpoint detection and proper experimental technique. The sodium carbonate method is preferred for HCl standardization due to its stability and precise equivalent weight determination.