Indicators

Core Concept

Definition: Acid-base indicators are substances that change color depending on the pH of the solution.

Key Property: They exist in equilibrium between two forms:

Acidic form (HIn): Dominates in acidic solutions.
Basic form (In⁻): Dominates in basic solutions.

Key Tips

Indicators are weak acids or bases that change color at specific pH ranges.
The choice of an indicator depends on the reaction's equivalence point.
Natural and synthetic indicators can be used for titrations and pH testing.

Test Yourself

Assorted Multiple Choice

Methyl red is an acid-base indicator that changes color from red to yellow across a pH transition range of 4.4 to 6.2. For which of the following titrations would methyl red be the most appropriate indicator to signal the equivalence point?

Podcast Episode

Episode

Indicators

Companion Guides

Coming soon

Student Guide Teacher Guide

Practice Problems & Worked Out Examples 🔒

Browse by category

01

Indicator Selection and Endpoint Theory

→ 02

The Chemical Nature of Indicators (Weak Acid Equilibrium)

→ 03

The pH Transition Range and pKIn

→ 04

Endpoint vs. Equivalence Point and Titration Error

→ 05

Other / Uncategorized

→ 06

Assorted Multiple Choice

→

How Do Indicators Work?

Indicators are weak acids or bases that dissociate in solution according to the following equilibrium:

$$\text{HIn} \rightleftharpoons \text{H}^+ + \text{In}^-$$

Color Change

In acidic solutions ($\text{pH} < \text{p}K_a$): The equilibrium shifts left (Le Châtelier's principle). The acidic form ($\text{HIn}$) dominates, giving one color.
In basic solutions ($\text{pH} > \text{p}K_a$): The equilibrium shifts right. The basic form ($\text{In}^-$) dominates, giving a different color.

Key Terms

pH Range: The range of pH values over which the indicator changes color.
Transition Point: The pH at which the indicator is halfway between its acidic and basic forms (pH=pKa\text{pH} = \text{p}K_apH=pKa).

  
    Acid-Base Indicators
    
                Indicator
                Color in Acid
                Color in Base
                Transition Range (pH)
            
                Litmus
                Red
                Blue
                4.5–8.3
            
                Methyl orange
                Red
                Yellow
                3.1–4.4
            
                Bromothymol blue
                Yellow
                Blue
                6.0–7.6
            
                Phenolphthalein
                Colorless
                Pink
                8.2–10.0
            
                Universal indicator
                Red
                Violet
                4.0–10.0 (varies)

Choosing the Right Indicator

Equivalence Point: The indicator must change color close to the pH of the reaction's equivalence point.
- For strong acid-strong base titrations: Use indicators like phenolphthalein or bromothymol blue (pH ≈ 7).
- For weak acid-strong base titrations: Use phenolphthalein (pH > 7 at equivalence).
- For strong acid-weak base titrations: Use methyl orange (pH < 7 at equivalence).

Common Acid-Base Indicators

Topic Related Resources

Labs

Demos

Activities

Simulations

LAB — 01

Lab

Coming Soon...