Lecture Notes on Acid-Base Titration and Indicators

Overview

• Discussed using acid-base indicators to find the equivalence point in titrations.
• Indicators change color over specific pH ranges to help detect the equivalence point.

Common Acid-Base Indicators

• Methyl Red: Changes from red to yellow between pH 4.4 to 6.2 (approximately 4 to 6).
• Bromthymol Blue: Changes from yellow to blue between pH 6 to 7.6 (approximately 6 to 8).
• Phenolphthalein: Changes from colorless to pink/magenta between pH 8.2 to 10 (approximately 8 to 10).

Titration Curves and Indicators

Strong Acid with Strong Base

• Equivalence Point: pH = 7
• Example: 20 mL of strong base needed to reach equivalence.
• Suitable Indicator: Bromthymol Blue (color change from yellow to blue around pH 6 to 8).
• The curve is steep from pH 4 to 10, allowing for use of any of the three indicators to approximate the equivalence point.

Weak Acid with Strong Base

• Example: Acetic acid with sodium hydroxide.
• Equivalence Point: pH > 7 (for this example, close to 9).
• Reason: The formation of a conjugate base increases hydroxide ion concentration.
• Suitable Indicator: Phenolphthalein (color change from colorless to magenta around pH 8 to 10).
• Unsuitable Indicator: Methyl Red (changes at pH 4 to 6).

Weak Base with Strong Acid

• Example: Ammonia (NH3) with a strong acid.
• Equivalence Point: pH < 7 (for this example, a little over 5).
• Reason: Formation of NH4+ increases hydronium ion concentration.
• Suitable Indicator: Methyl Red (changes from yellow to red at pH 4 to 6).
• Unsuitable Indicator: Phenolphthalein (changes at pH 8 to 10).

Conclusion

• Selection of an appropriate acid-base indicator is crucial for accurately determining the equivalence point in a titration.
• Understanding the pH ranges where different indicators change color helps in selecting the right one for various types of titration.