Acid-Base Indicators

Interactive visualization of acid-base indicators - Explore pH color changes, titration with multiple indicators, pH-to-color mapping, and indicator transition ranges

Titration Scene

Current pH: 7.00
Base Added: 0.00 mL

pH Meter

7.00
0 7 14

Current Indicator Color

Indicator: Phenolphthalein
Hex: #FFFFFF

Multiple Indicators Comparison

pH Color Spectrum

Acidic (pH < 7)
Neutral (pH = 7)
Basic (pH > 7)

Indicator & Titration Controls

pH Control

Titration Control

Select Indicator

Color Transition Range: 8.2 - 10.0
pKa: 9.7
Acid Color:
Base Color:

Display Options

Acid-Base Indicator Equations

Indicator Equilibrium: HIn ⇌ H⁺ + In⁻
Acid Dissociation Constant: Ka = [H⁺][In⁻]/[HIn]
Henderson-Hasselbalch: pH = pKa + log([In⁻]/[HIn])
Color Transition Range: pH ≈ pKa ± 1
Color Ratio (midpoint): [In⁻] = [HIn] at pH = pKa

What are Acid-Base Indicators?

Acid-base indicators are organic compounds that exhibit different colors in acidic and basic solutions. They are weak acids or bases that exist in equilibrium between two forms with distinct colors: the protonated form (HIn) and the deprotonated form (In⁻). The color change occurs over a specific pH range, typically centered around the indicator's pKa value.

How Indicators Work

Equilibrium Mechanism: Indicators establish an equilibrium between their acidic form (HIn) and basic form (In⁻).
Color Transition: At low pH, the acidic form HIn predominates. As pH increases, more HIn loses protons to form In⁻.
Transition Range: Most indicators change color over approximately 2 pH units centered at their pKa.
Human Eye Perception: We typically perceive a complete color change when one form is about 10 times more concentrated than the other.

Common Indicators and Their Ranges

Phenolphthalein (pKa 9.7, range 8.2-10.0): Colorless in acid, pink in base.
Methyl Orange (pKa 3.7, range 3.1-4.4): Red in acid, yellow in base.
Litmus (pKa 6.5, range 5.0-8.0): Red in acid, blue in base.
Bromothymol Blue (pKa 7.1, range 6.0-7.6): Yellow in acid, blue in base.

Choosing the Right Indicator

Strong Acid + Strong Base: Equivalence pH = 7. Use bromothymol blue or litmus.
Weak Acid + Strong Base: Equivalence pH > 7. Use phenolphthalein.
Strong Acid + Weak Base: Equivalence pH < 7. Use methyl orange or methyl red.

pH Calculation During Titration

Before Equivalence: pH = pKa + log([A⁻]/[HA]).
At Equivalence: pH depends on base hydrolysis.
After Equivalence: pH = 14 + log[OH⁻].

Applications of Acid-Base Indicators

Volumetric analysis, pH paper, biological applications, food industry, environmental testing, educational demonstrations.

Universal Indicator

Universal indicator is a mixture of several indicators that exhibits a continuous range of colors across the entire pH scale (0-14): red, orange, yellow, green, blue, indigo, violet.