Theory of Indicators
Ostwald's Theory & Quinonoid Theory | Ionic Equilibrium
1. What is an Acid-Base Indicator?
An indicator is a substance that indicates the end point of a titration by changing its color. Chemically, indicators are typically weak organic acids or weak organic bases that have different colors in their unionized and ionized forms.
2. Ostwald's Theory (Ionization Theory)
According to this theory, the color change is due to the ionization of the indicator. The unionized molecule has one color, while the ion has a different color.
Mechanism of Action (e.g., Phenolphthalein):
Phenolphthalein ($HPh$) is a weak acid.
$$ HPh (\text{Colorless}) \rightleftharpoons H^+ + Ph^- (\text{Pink}) $$
- In Acidic Medium: Excess $H^+$ ions suppress the ionization of $HPh$ due to the Common Ion Effect. The equilibrium shifts to the left. The solution remains Colorless.
- In Basic Medium: $OH^-$ ions from the base react with $H^+$ to form water. This decreases concentration of $H^+$, shifting equilibrium to the right to produce more $Ph^-$ ions. The solution turns Pink.
3. Quinonoid Theory (Modern Theory)
According to this theory, the acid-base indicators exist in two tautomeric forms: Benzenoid and Quinonoid. The color change is due to the interconversion of one structural form into another.
- Benzenoid Form: Generally lighter in color.
- Quinonoid Form: Generally deeper in color (contains a chromophore).
Change in pH causes the conversion from one form to the other, resulting in a color change.
4. pH Range of Indicators
The color change does not happen at a single pH point but over a specific range. Using the Henderson-Hasselbalch equation for an indicator $HIn$:
The visible color change generally occurs when the ratio $[\text{In}^-]/[\text{HIn}]$ changes from $0.1$ to $10$. This gives the working pH range:
$$ pH = pK_{In} \pm 1 $$Common Indicators Table
| Indicator | Nature | pH Range | Acidic Color | Basic Color |
|---|---|---|---|---|
| Methyl Orange | Weak Base | 3.1 - 4.4 | Red | Yellow |
| Methyl Red | Weak Base | 4.2 - 6.3 | Red | Yellow |
| Phenolphthalein | Weak Acid | 8.3 - 10.0 | Colorless | Pink |
5. Selection of Indicator
An indicator is suitable if its pH range lies on the steep part of the titration curve (vertical section) where pH changes rapidly.
- Strong Acid vs Strong Base: Any indicator (Methyl Orange or Phenolphthalein).
- Weak Acid vs Strong Base: Phenolphthalein (Range 8-10).
- Strong Acid vs Weak Base: Methyl Orange (Range 3-5).
Practice Quiz
Test your knowledge on Indicator Theory.
No comments:
Post a Comment