An indicator is a chemical substance that changes color depending on the pH of the solution in which it is placed.
Indicators help to determine whether a solution is acidic, basic, or neutral.
Most indicators are weak organic acids or bases that exist in two forms — one color in acidic medium and another in basic medium.
Common Acid-Base Indicators
Indicator
Color in Acidic Medium
Color in Neutral Medium
Color in Basic Medium
pH Range
Methyl Orange
Red
Orange
Yellow
3.1 – 4.4
Phenolphthalein
Colorless
Colorless
Pink
8.2 – 10.0
Litmus
Red
Purple
Blue
5.0 – 8.0
Bromothymol Blue
Yellow
Green
Blue
6.0 – 7.6
Methyl Red
Red
Orange
Yellow
4.2 – 6.2
Thymolphthalein
Colorless
Colorless
Blue
9.3 – 10.5
Indicator Suitability
Type of Titration
Indicator Used
Reason
Strong Acid vs Strong Base
Phenolphthalein or Methyl Orange
The pH change at the equivalence point is rapid (pH ≈ 7).
Strong Acid vs Weak Base
Methyl Orange
Equivalence point lies in the acidic range (pH ≈ 4 – 6).
Weak Acid vs Strong Base
Phenolphthalein
Equivalence point lies in the basic range (pH ≈ 8 – 10).
Weak Acid vs Weak Base
No suitable indicator
pH change at equivalence is too small to observe color change.
Buffer Solutions
A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added.
It usually contains a weak acid and its conjugate base or a weak base and its conjugate acid.
Examples:
- Acidic buffer: Mixture of ethanoic acid (CH₃COOH) and sodium ethanoate (CH₃COONa).
- Basic buffer: Mixture of ammonia (NH₃) and ammonium chloride (NH₄Cl).
Applications of Buffer Solutions:
Maintain constant pH in blood (≈7.4).
Used in fermentation and biochemical reactions.
In calibration of pH meters.
Maintain pH in photographic and dyeing industries.
Conductance of Solution
Conductance refers to the ability of a solution to conduct electric current due to the movement of ions.
Acids: Conduct electricity in aqueous solution because they produce H⁺ ions.
Example: $$ \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- $$
Bases: Conduct electricity because they release OH⁻ ions in water.
Example: $$ \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- $$
Salts: Conduct electricity when dissolved or molten due to free movement of both cations and anions.
Example: $$ \text{NaCl} \rightarrow \text{Na}^+ + \text{Cl}^- $$
Summary:
- Conductivity increases with ion concentration.
- Strong electrolytes (like HCl, NaOH, NaCl) conduct better than weak ones (like CH₃COOH, NH₄OH).
Note: Pure water does not conduct electricity — only when ions are present can conduction occur.
Interactive Indicator
This simulator lets you explore how indicators behave at different pH values and view titration curves for various acid–base combinations.
Adjust the pH slider to see color changes or choose a titration type to view its curve and suitable indicators.