Studying pH change during titration of strong acid/base using indicator

Aim

To study the pH change during titration of a strong acid (HCl) with a strong base (NaOH) using a suitable indicator and verify the equivalence point through pH measurements.

Apparatus Required

• Burette (50 mL)
• Pipette (25 mL)
• Conical flask (250 mL)
• pH meter or universal indicator paper
• Magnetic stirrer with stir bar (optional)
• Measuring cylinder
• White tile or white background sheet
• Dropping pipette
• Standard hydrochloric acid solution (0.1 M)
• Standard sodium hydroxide solution (0.1 M)
• Phenolphthalein indicator
• Distilled water
• Beakers (100 mL)

Theory

Titration Fundamentals

Titration is a quantitative analytical technique used to determine the concentration of an unknown solution by reacting it with a solution of known concentration. In acid-base titrations, the reaction between H⁺ ions from the acid and OH⁻ ions from the base produces water and salt.

Strong Acid-Strong Base Titration

When a strong acid like HCl is titrated against a strong base like NaOH, the following neutralization reaction occurs:

HCl + NaOH → NaCl + H₂O

pH Changes During Titration

The pH changes during titration follow a characteristic pattern:

1. Initial Stage: Low pH due to excess H⁺ ions from HCl
2. Before Equivalence Point: Gradual pH increase as OH⁻ ions neutralize H⁺ ions
3. Equivalence Point: Sharp pH change (7.0 for strong acid-strong base)
4. After Equivalence Point: High pH due to excess OH⁻ ions from NaOH

Indicators

Phenolphthalein is commonly used for strong acid-strong base titrations. It remains colorless in acidic solutions (pH 10).

Procedure

Preparation

1. Clean all glassware with distilled water.
2. Rinse burette with small amount of NaOH solution.
3. Fill burette with standardized NaOH solution (0.1 M).
4. Record initial burette reading.
5. Pipette 25.0 mL of HCl solution (0.1 M) into a conical flask.
6. Add 2-3 drops of phenolphthalein indicator.

Titration Process

1. Place conical flask on white tile for better color observation.
2. Record initial pH of HCl solution without adding NaOH.
3. Run NaOH solution slowly from burette while continuously swirling the flask.
4. After each 1-2 mL addition, measure and record pH.
5. Note the color change when it appears.
6. Continue additions until pH reaches approximately 12.
7. Record final burette reading.

pH Measurement Method

Using pH Meter: Calibrate pH meter before use. Dip electrode into solution after each addition and note reading. Using Indicator Paper: Dip strip into solution and compare color with standard chart.

Observation Table

Continue recording values beyond equivalence point

Result

Key Findings

1. Equivalence Point: Occurred at approximately 24.5 mL of NaOH addition with pH = 10.2
2. Color Change: Solution turned from colorless to permanent light pink
3. pH Jump: Sharp pH increase observed between 24.0 mL and 24.5 mL
4. Calculated Concentration: If theoretical values match experimental data within acceptable error range

Verification

• The sharp pH change confirms the completion of neutralization reaction
• Color change corresponds to calculated equivalence volume
• pH at equivalence point aligns with theoretical expectations for strong acid-strong base titration

Precautions

Safety Measures

1. Wear safety goggles and gloves throughout the experiment
2. Handle concentrated acids and bases with extreme care
3. Ensure proper ventilation in laboratory area

Experimental Accuracy

1. Use clean, dry glassware to prevent contamination
2. Read burette readings at eye level to avoid parallax error
3. Add NaOH slowly near the equivalence point to avoid overshooting
4. Stir solution continuously but gently to ensure uniform mixing
5. Calibrate pH meter properly before measurements
6. Use fresh indicator for accurate color detection
7. Record observations immediately to avoid memory errors

Equipment Handling

1. Check burette for air bubbles before starting
2. Rinse pipette with sample solution before actual transfer
3. Ensure magnetic stirrer operates smoothly without splashing

Viva Voce Questions and Answers

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

A: Acid-base titration is based on neutralization reaction where hydrogen ions (H⁺) from acid react with hydroxide ions (OH⁻) from base to form water molecules and salt.

Q2: Why is phenolphthalein used as indicator in this titration?

A: Phenolphthalein is ideal because its color change range (8.2-10.0) encompasses the steep portion of pH curve near equivalence point for strong acid-strong base titration.

Q3: What happens to pH at equivalence point?

A: At equivalence point, exactly equal moles of acid and base have reacted. For strong acid-strong base, pH = 7, but due to indicator characteristics, color change occurs slightly above 7.

Q4: Why does pH change rapidly near equivalence point?

A: Near equivalence point, small addition of titrant causes significant change in [H⁺] or [OH⁻] concentration since buffering capacity is minimal, resulting in sharp pH transition.

Q5: How would you detect the endpoint without an indicator?

A: By continuous pH monitoring using pH meter or by observing sudden temperature change (exothermic neutralization) or conductivity change.

Q6: What factors affect the accuracy of titration results?

A: Factors include precision in measuring volumes, speed of titrant addition near endpoint, indicator choice appropriateness, equipment calibration quality, and environmental conditions like temperature.

Q7: Explain the significance of sharp pH change at equivalence point?

A: Sharp pH change indicates completion of stoichiometric reaction, providing precise endpoint detection crucial for accurate quantitative analysis and concentration determination.

Q8: Why should the burette be rinsed with NaOH before filling?

A: Rinsing removes any residual water or impurities that could dilute or contaminate the NaOH solution, ensuring accurate concentration and reliable titration results.

Conclusion

This experiment successfully demonstrates the characteristic pH variation during strong acid-strong base titration. The sharp pH transition at equivalence point validates neutralization theory and provides practical understanding of acid-base reactions, making it fundamental for analytical chemistry applications.