pH Metric Titration Experiment with Viva Questions and Answers | Aniket NCERT

🧪 Determination of pH by Acid-Base Titration

Complete Chemistry Practical | Theory, Procedure, Observations & Viva

🎯 Aim

To determine the pH of a solution during acid-base titration and to plot the pH titration curve for the titration of HCl against NaOH.

🔬 Apparatus Required

• Burette (50 mL)
• Pipette (20 mL)
• Conical Flask (250 mL)
• Beakers
• pH Meter
• Glass Electrode
• Magnetic Stirrer
• Wash Bottle
• Stand & Clamp

⚗️ Chemicals Required

• 0.1 M Hydrochloric Acid (HCl)
• 0.1 M Sodium Hydroxide (NaOH)
• Distilled Water

📖 Theory

pH is the negative logarithm of hydrogen ion concentration present in a solution.

pH = –log [H⁺]

A pH metric titration is a method in which the pH of a solution is measured continuously while a titrant is added. The pH values are recorded and plotted against the volume of titrant added to obtain a titration curve.

In this experiment, hydrochloric acid (HCl) is titrated against sodium hydroxide (NaOH). The hydrogen ions from HCl react with hydroxide ions from NaOH to produce water and sodium chloride.

HCl + NaOH → NaCl + H₂O

Initially the solution is strongly acidic. As NaOH is added, the pH gradually increases. Near the equivalence point a very small addition of NaOH causes a sudden rise in pH. This point indicates complete neutralization of the acid.

📝 Procedure

1. Wash all glassware thoroughly.
2. Fill the burette with 0.1 M NaOH solution.
3. Take 20 mL HCl in a conical flask.
4. Immerse the pH electrode into the solution.
5. Record the initial pH.
6. Add NaOH gradually and note pH after each addition.
7. Add NaOH dropwise near the endpoint.
8. Plot pH versus volume of NaOH added.

✅ Result

The equivalence point was obtained at approximately 20 mL of NaOH and the corresponding pH was approximately 7.0.

⚠️ Precautions

• Calibrate the pH meter before use.
• Rinse the electrode with distilled water.
• Add titrant slowly near the endpoint.
• Stir the solution properly.
• Record only stable pH values.

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🎓 Viva Voce Questions & Answers

Q1. What is pH?

pH is the negative logarithm of the hydrogen ion concentration present in a solution. Mathematically, pH = –log [H⁺]. The pH scale ranges from 0 to 14. A solution with pH less than 7 is acidic, pH equal to 7 is neutral, and pH greater than 7 is basic or alkaline.

Q2. Why is pH important in chemistry?

pH helps determine the acidic or basic nature of a solution. It is important in chemical reactions, biological systems, environmental studies, water quality testing, agriculture, and industrial processes.

Q3. What is a pH metric titration?

A pH metric titration is a method in which the pH of a solution is measured continuously using a pH meter while a titrant is added. The endpoint is determined from the sudden change in pH instead of using an indicator.

Q4. What is the equivalence point?

The equivalence point is the stage in a titration where the amount of acid is chemically equal to the amount of base. At this point, complete neutralization occurs. In a strong acid–strong base titration, the pH at the equivalence point is approximately 7.

Q5. Why is a pH meter used instead of an indicator?

A pH meter provides more accurate and precise results than indicators. It can detect small changes in pH and is useful for colored or turbid solutions where indicators may not work properly.

Q6. What is the reaction between HCl and NaOH?

Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water.

HCl + NaOH → NaCl + H₂O

This reaction is known as a neutralization reaction.

Q7. Why does the pH increase during the titration?

Initially, the solution contains excess hydrogen ions from HCl. As NaOH is added, hydroxide ions neutralize the hydrogen ions. Therefore, the concentration of hydrogen ions decreases and the pH increases gradually.

Q8. Why does pH change sharply near the endpoint?

Near the equivalence point, most of the acid has already been neutralized. Therefore, even a small addition of NaOH causes a large decrease in hydrogen ion concentration, resulting in a sudden rise in pH.

Q9. What is the role of the glass electrode in a pH meter?

The glass electrode detects the hydrogen ion concentration of the solution and converts it into an electrical signal. The pH meter then displays the corresponding pH value.

Q10. What are the advantages of pH metric titration?

The advantages of pH metric titration are:

• High accuracy and precision.
• No indicator is required.
• Suitable for colored solutions.
• Easy determination of equivalence point.
• Useful for weak acid–weak base titrations.