Answer
Determining the Best Indicator for the Second Equivalence Point in Alanine Titration
Alanine is an amino acid with two key functional groups:
- A carboxylic acid group (–COOH), which loses a proton first (low pKa ≈ 2.3).
- An amino group (–NH3+), which loses a proton second (higher pKa ≈ 9.7).
During titration with sodium hydroxide (NaOH), alanine undergoes a two-step deprotonation:
- First Equivalence Point: Loss of proton from –COOH (pKa ≈ 2.3).
- Second Equivalence Point: Loss of proton from –NH3+ (pKa ≈ 9.7).
🔍 Choosing an Appropriate Indicator
To accurately detect the second equivalence point, the indicator’s transition (color change) should occur around the pH of that point — near pH 9.7–10.
🧪 Analysis of Options
| Indicator | pKb | Corresponding pH Range | Suitability |
|---|---|---|---|
| Methyl violet | 13.0 | pH ≈ 1–2 (very acidic) | ❌ Too low; unsuitable |
| Methyl yellow | 10.5 | pH ≈ 3–4 | ❌ Far from pH 9.7; unsuitable |
| Thymol blue | 12.0 | Second transition: pH 8.0–9.6 | ✔ Acceptable, close to 9.7 |
| Phenolphthalein | 4.9 | pH 8.3–10.0 | ✅ Best match; clear transition near 9.7 |
✅ Final Conclusion
Phenolphthalein is the most suitable indicator to determine the second equivalence point of alanine during titration with sodium hydroxide. Its pH transition range of 8.3 to 10.0 closely matches the second pKa of alanine (~9.7), ensuring a clear and visible color change at the correct stage of the titration.