Acid–Base Titration: Estimation of Unknown NaOH By Aniket Petrotech

 

CHEMISTRY LAB GUIDE

Acid–Base Titration: Estimation of Unknown NaOH

Using Standard Oxalic Acid and Phenolphthalein Indicator

Complete Practical Notes | Principle | Procedure | Calculations | Viva

Aniket Petrotech

1. Aim of the Experiment

To determine the molarity and strength of an unknown sodium hydroxide (NaOH) solution by titrating it against a standard oxalic acid solution using phenolphthalein as an indicator.

2. Principle

Acid–base titration is a volumetric analytical technique used to determine the concentration of an unknown acid or base by reacting it with a solution of known concentration. In this experiment, oxalic acid reacts with sodium hydroxide in a neutralisation reaction. Phenolphthalein indicates the endpoint by changing from colourless to a permanent pale pink colour when NaOH is added to the acidic solution.

H₂C₂O₄ + 2NaOH → Na₂C₂O₄ + 2H₂O

Reaction ratio: 1 mole of oxalic acid reacts with 2 moles of sodium hydroxide.

3. Chemicals Required

• Standard oxalic acid solution
• Unknown concentration of sodium hydroxide (NaOH) solution
• Phenolphthalein indicator
• Distilled water

4. Apparatus Required

• Burette with stand and clamp
• Pipette with pipette filler
• Conical flask
• Volumetric flask, if solution preparation is required
• Beaker and funnel
• Wash bottle containing distilled water
• White tile for observing the endpoint

5. Step-by-Step Procedure

1. Rinse the burette with the NaOH solution and the pipette with the standard oxalic acid solution.
2. Fill the burette with NaOH solution. Remove any air bubbles from the jet and record the initial burette reading.
3. Using a pipette, transfer a known volume of standard oxalic acid into a clean conical flask.
4. Add 2–3 drops of phenolphthalein indicator to the acid solution.
5. Add NaOH from the burette slowly while continuously swirling the conical flask.
6. Near the endpoint, add NaOH dropwise until a faint pink colour persists for approximately 30 seconds.
7. Record the final burette reading and calculate the volume of NaOH used.
8. Repeat the titration until concordant readings are obtained.

Endpoint: Colourless to permanent pale pink.
Important: The endpoint should be a faint pink, not a deep pink.

6. Observation Table

Record your actual experimental readings in the table below.

Observation	Trial 1	Trial 2	Trial 3
Initial burette reading (mL)	—	—	—
Final burette reading (mL)	—	—	—
Volume of NaOH used (mL)	—	—	—

Concordant readings: Select closely agreeing titre values according to your laboratory's accepted tolerance and calculate their mean.

7. Calculations

Let:

• M₁ = Molarity of oxalic acid
• V₁ = Volume of oxalic acid taken (mL)
• M₂ = Molarity of NaOH
• V₂ = Mean volume of NaOH used (mL)

From the balanced chemical equation:

M₁V₁ × 2 = M₂V₂

Therefore:

M₂ = (2 × M₁ × V₁) / V₂

Strength of NaOH

Strength (g/L) = Molarity × 40

Here, 40 g/mol is the molar mass of NaOH.

Illustrative example only: If oxalic acid is 0.05 M, its volume is 10.0 mL, and the mean NaOH titre is 9.5 mL:

M₂ = (2 × 0.05 × 10.0) / 9.5

M₂ ≈ 0.1053 M

Strength = 0.1053 × 40 ≈ 4.21 g/L

Use these results only if your actual experimental values match these assumptions.

8. Result

The molarity of the given NaOH solution is calculated to be _____ mol/L, and its strength is _____ g/L, based on the experimental titre values.

9. Precautions

• Rinse the burette and pipette with their respective solutions before use.
• Ensure that no air bubbles remain in the burette jet.
• Remove the funnel from the burette before taking readings.
• Read the burette at eye level to avoid parallax error.
• Add NaOH dropwise near the endpoint while swirling the flask.
• Use only 2–3 drops of phenolphthalein indicator.
• Repeat the titration to obtain concordant readings.
• Wear safety goggles and gloves while handling NaOH, as it is corrosive.

10. Important Viva Questions

Q1. What is titration?
Titration is a quantitative analytical method used to determine the concentration of a solution using a solution of known concentration.

Q2. Why is phenolphthalein used?
It provides a clear colour change near the endpoint for this strong-base and weak-acid titration system under the stated experimental conditions.

Q3. What is the endpoint?
The point at which the indicator shows a persistent colour change signalling completion of the titration.

Q4. Why is NaOH solution considered unstable on storage?
NaOH absorbs moisture and carbon dioxide from the air, which can change its effective concentration.

Q5. What is the difference between endpoint and equivalence point?
The equivalence point is the stoichiometric point of reaction, while the endpoint is the observed indicator colour change.

Q6. Why should the conical flask be swirled?
Swirling ensures proper mixing and helps prevent local excess of titrant.

Q7. What is the molar mass of NaOH?
40 g/mol.

11. Learning Outcomes

After completing this experiment, students should be able to:

• Perform an acid–base titration systematically.
• Identify the indicator endpoint correctly.
• Calculate the molarity and strength of an unknown solution.
• Understand stoichiometric relationships in volumetric analysis.
• Apply essential laboratory safety and measurement practices.

Aniket Petrotech

Chemistry Practical • Laboratory Learning • Scientific Concepts

Learn systematically. Experiment accurately. Understand deeply.