Introduction
An indicator is a substance that exhibits a distinct color change at a particular stage of a chemical reaction, usually during a titration. Indicators are most commonly used in acid-base titrations to signal the end point of the reaction.
Indicators are typically weak acids or weak bases that exist in two forms — an undissociated form and its conjugate ion — each having a different color. The color of the solution thus depends on the relative concentrations of these two forms, which vary with pH.
For accurate titrations, the indicator chosen must have its color change (transition interval) occur near the equivalence point pH of the titration.
Theories of Indicators
Two major theories explain how indicators work:
- Ostwald Theory
- Resonance Theory
1. Ostwald Theory of Indicators
The Ostwald Theory was the first theory to explain the behavior of indicators. It states that:
“The undissociated indicator acid (HIn) or base (InOH) has a color different from its ionized form.”
Explanation:
Let the indicator be a weak acid represented as:
HIn ⇌ H⁺ + In⁻
Here:
- HIn = Undissociated acid form (one color)
- In⁻ = Ionized base form (another color)
In acidic solution, due to the common ion effect, ionization of the indicator is suppressed and the solution shows the color of the undissociated form (HIn). In alkaline solution, ionization increases, and the solution shows the color of the ionized form (In⁻).
By applying the Law of Mass Action:
The dissociation constant of the indicator (KIn) is given by:
KIn = [H⁺][In⁻] / [HIn]
Rearranging gives:
pH = pKIn + log([In⁻]/[HIn])
Thus, the color of the indicator depends on the ratio of its ionized and unionized forms. The transition interval (the pH range over which the color change occurs) generally lies between pKIn ± 1.
Condition for Ideal Indicator:
The chosen indicator should have its transition range near the equivalence point of the titration to ensure a sharp and distinct color change.
2. Resonance Theory of Indicators
The Resonance Theory applies to the structural behavior of organic indicators. According to this theory, the difference in color of an indicator in acidic and basic media is due to structural resonance changes within the molecule.
Most acid-base indicators are organic compounds that can exist in two resonance-stabilized structures — one predominant in acidic conditions and another in basic conditions. The structural rearrangement leads to changes in electron distribution, which affects light absorption and results in a color change.
Explanation:
- In acidic solution, the indicator exists mainly in its unionized form with one color.
- In basic solution, ionization and resonance within the structure increase, leading to another color.
For example, phenolphthalein is colorless in acidic medium but turns pink in alkaline medium. This is due to increased resonance between the ionic forms in alkaline conditions, allowing the compound to absorb visible light and appear colored.
Detailed Notes:
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