Titration to determine sodium hydroxide strength using oxalic acid standard

Aim

To determine the strength of given sodium hydroxide (NaOH) solution using standard oxalic acid (C₂H₂O₄·2H₂O) solution through acid-base titration method.

Keywords: acid base titration, NaOH strength determination, oxalic acid standard, titration experiment, chemistry lab procedure

Apparatus Required

Glassware and Instruments:

• Burette (50 mL)
• Pipette (25 mL)
• Conical flask (250 mL) - 2 nos
• Measuring cylinder (100 mL)
• Wash bottle
• Funnel
• White tile or white glazed tile

Chemicals:

• Standard oxalic acid solution (approximately 0.1 M)
• Given sodium hydroxide solution
• Phenolphthalein indicator

Other Requirements:

• Distilled water
• Burette stand
• Pipette filler

Theory

Chemical Background

This experiment is based on acid-base titration, a quantitative analytical technique used to determine the concentration of an unknown solution using a standard solution of known concentration.

Chemical Reaction

Balanced Chemical Equation:

H₂C₂O₄ + 2NaOH → Na₂C₂O₄ + 2H₂O

This represents a neutralization reaction where one mole of oxalic acid reacts with two moles of sodium hydroxide.

Indicator Selection

Phenolphthalein is used as an indicator because:

• It shows color change in basic pH range (8.2-10.0)
• Colorless in acidic solution, pink in basic solution
• Sharp endpoint detection possible

Key Concepts:

• Standard Solution: Oxalic acid solution of known concentration
• Titrate: Sodium hydroxide solution whose strength is to be determined
• Endpoint: Point at which complete neutralization occurs
• Equivalent Weight: Mass that combines with or displaces one part by mass of hydrogen

Equivalents Concept:

• Oxalic acid (dibasic acid): Equivalent weight = Molecular weight/2
• NaOH (monobasic base): Equivalent weight = Molecular weight/1

Procedure

Preparation:

1. Rinse burette with NaOH solution twice and fill it with the given NaOH solution
2. Remove air bubbles and adjust the meniscus to zero mark
3. Rinse pipette with standard oxalic acid solution
4. Pipette out 25.0 mL of oxalic acid solution into a conical flask

Titration Steps:

1. Add 2-3 drops of phenolphthalein indicator to the conical flask
2. Place white tile below the burette for better color observation
3. Slowly add NaOH solution from burette while swirling the flask continuously
4. When pink color persists for 30 seconds, endpoint is reached
5. Note the final burette reading
6. Repeat the titration 4-5 times for accuracy

Endpoint Identification:

• Before endpoint: Colorless solution
• At endpoint: Light pink color that persists for 30 seconds
• Vigorous swirling required near endpoint to ensure complete mixing

Observation Table

Observations:

• Standard oxalic acid solution concentration: 0.1 M
• Volume of oxalic acid taken: 25.0 mL
• Average burette reading (concordant): 21.8 mL

Calculations and Result

Solution Strength Determination:

Using Normality Equation: N₁V₁ = N₂V₂

N₁ (Oxalic acid) = 0.1 N
V₁ (Oxalic acid) = 25.0 mL
V₂ (NaOH) = 21.8 mL
N₂ (NaOH) = ?

N₂ = (N₁ × V₁) / V₂ = (0.1 × 25.0) / 21.8 = 0.115 N

Since NaOH is monobasic: Molarity = Normality = 0.115 M

Strength of NaOH = Molarity × Molar mass
= 0.115 × 40 g/mol = 4.6 g/L

Result: The strength of the given sodium hydroxide solution is 4.6 g/L or 0.115 M

Precautions

General Precautions:

1. Clean all glassware thoroughly before use
2. Remove air bubbles from burette before starting
3. Use same balance for weighing all chemicals
4. Read meniscus at eye level to avoid parallax error

Specific Precautions:

1. Pipette Handling: Never pipette by mouth; use pipette filler
2. Indicator Use: Add only 2-3 drops of indicator to avoid excess
3. Temperature: Perform titration at room temperature for accuracy
4. Reading Accuracy: Take burette readings up to single decimal place
5. Swirling Technique: Swirl conical flask continuously but gently
6. Endpoint Detection: Approach endpoint slowly for sharp color change

Chemical Precautions:

1. Handle NaOH carefully - avoid skin contact
2. Wash hands immediately if chemicals spill
3. Store chemicals properly after use

Viva Voce Questions and Answers

Q1: What is the principle of acid-base titration?

A: Acid-base titration is based on the neutralization reaction between an acid and a base, where the equivalent amounts of both react completely to form salt and water.

Q2: Why is phenolphthalein used as an indicator?

A: Phenolphthalein changes color in the pH range of 8.2-10.0, making it suitable for strong acid-strong base titrations where the endpoint pH is around 8.7.

Q3: Why is oxalic acid used as a primary standard?

A: Oxalic acid is stable, easily purified, has high molecular weight, and reacts stoichiometrically, making it ideal as a primary standard.

Q4: What is the equivalent weight of oxalic acid?

A: Equivalent weight of oxalic acid = Molecular weight/2 = 126/2 = 63 g/equivalent (since it's dibasic)

Q5: What precautions should be taken during titration?

A:

• Remove air bubbles from burette
• Read meniscus at eye level
• Add indicator in minimal quantity
• Approach endpoint slowly
• Swirl solution continuously

Q6: How do you know that the endpoint has reached?

A: The endpoint is reached when a light pink color appears that persists for at least 30 seconds after adding the last drop of NaOH.

Q7: Why should the funnel be removed from burette before taking final reading?

A: The funnel may contain solution that can drain into the burette, affecting the accuracy of readings.

Q8: What is the significance of concordant readings?

A: Concordant readings (difference ≤ 0.2 mL) ensure reliability and reproducibility of experimental results.

Conclusion

This titration experiment successfully demonstrates the quantitative analysis technique for determining the strength of sodium hydroxide solution. The methodology provides accurate results when performed with proper precautions and careful observations. Understanding this fundamental analytical chemistry technique is crucial for various applications in chemical analysis, quality control, and research laboratories.

The experiment reinforces key concepts of stoichiometry, neutralization reactions, and quantitative analysis, making it an essential component of chemistry curriculum for students at various educational levels.