Acid-base indicators are substances that change color depending on the pH of the solution they are in.
They are typically weak organic acids or bases that exhibit different colors in their protonated and deprotonated forms.
These indicators are crucial in titrations as they help signal the endpoint of the reaction by undergoing a visible color change.
There are two main theories that explain the behavior of acid-base indicators:
1. Ostwald Theory (Ionization Theory)
The Ostwald theory explains the behavior of acid-base indicators based on the ionization equilibrium of weak acids or bases.
According to this theory:
Ionization Equilibrium: In the presence of a base, a weak acidic indicator (HIn) can lose a proton (H⁺) to form its conjugate base (In⁻).
HIn ↔ H+ + In-
Color Change: Both the protonated form (HIn) and the deprotonated form (In⁻) have different colors due to differences in their electronic structures.
pH Dependence: The equilibrium shifts based on the pH of the solution, causing a color change as the concentration of H⁺ ions changes.
Example:
Phenolphthalein:
Ionization Equilibrium:
HIn (colorless) ↔ H+ + In- (pink)
Behavior:
In acidic solutions, the equilibrium favors the colorless protonated form (HIn).
In basic solutions, the equilibrium shifts towards the pink deprotonated form (In⁻).
Methyl Orange:
Ionization Equilibrium:
HIn (red) ↔ H+ + In- (yellow)
Behavior:
In acidic solutions, the equilibrium favors the red protonated form (HIn).
In basic solutions, the equilibrium shifts towards the yellow deprotonated form (In⁻).
2. Quinonoid Theory (Resonance or Quasi-Valence Theory)
The quinonoid theory, also known as the resonance (quasi-valence) theory, explains the color change of indicators based on the resonance structures and electron distribution in the molecules.
According to this theory:
Resonance Structures: Indicators can exist in two tautomeric forms—benzenoid and quinonoid structures—which have different colors.
Electron Distribution: Gaining or losing a proton changes the electron distribution, altering the absorption of light in the visible spectrum and thus the color.
Color Dependence: The color of the indicator is determined by the predominant resonance structure at a given pH.
Example:
Phenolphthalein:
Structural Change:
In acidic solutions, phenolphthalein exists in its colorless lactone (benzenoid) form.
In basic solutions, it loses a proton to form a pink-colored quinonoid structure.
Color Change Mechanism: The shift from the lactone form to the quinonoid form changes the electron delocalization, resulting in a color change.
Methyl Orange:
Structural Change:
In acidic conditions, methyl orange exists in a red-colored benzenoid form.
In alkaline conditions, it shifts to a yellow-colored quinonoid form.
Color Change Mechanism: The change in resonance structures alters the wavelengths of light absorbed, leading to a visible color change.