Acid-Base Indicators Chart
Simulate color transitions with our Virtual pH Lab. Essential for accurate titration endpoint detection.
Virtual pH Lab
Drag the slider to test indicator colors at different pH levels.
Methyl Violet
Acid IndicatorUsed for precise titration of weak bases.
Malachite Green
Acid IndicatorDye used as a biological stain and pH indicator.
Thymol Blue (Acid)
Acid IndicatorFirst transition range of Thymol Blue. It has two distinct color changes.
Methyl Orange
Acid IndicatorCommon indicator for strong acid-weak base titrations.
Bromocresol Green
Acid IndicatorUsed in DNA agarose gel electrophoresis.
Methyl Red
Acid IndicatorUsed in microbiology to identify bacteria producing stable acids.
Bromothymol Blue
Neutral IndicatorMeasuring pH of respiration (CO2) and photosynthesis.
Phenol Red
Base IndicatorStandard indicator for cell culture media.
Thymol Blue (Base)
Base IndicatorSecond transition range of Thymol Blue.
Phenolphthalein
Base IndicatorMost common indicator for strong acid-strong base titrations.
Thymolphthalein
Base IndicatorUsed in disappearing ink.
Alizarin Yellow R
Base IndicatorUsed for high pH titrations.
Indigo Carmine
Base IndicatorUsed as a redox indicator and food dye.
The Chemistry of Color Change
Indicators are not magic; they are weak acids (represented as HIn). They exist in a dynamic equilibrium between their associated (acidic) form and dissociated (basic) form. Each form has a distinctly different color.
The Equilibrium Equation
- Adding Acid (H⁺): Shifts equilibrium to the left. Dominant species: HIn (Color A).
- Adding Base (OH⁻): Removes H⁺, shifting equilibrium to the right. Dominant species: In⁻ (Color B).
Choosing the Right Indicator
The "perfect" indicator changes color exactly at the equivalence point of your titration.
- Strong Acid + Weak BasepH < 7 (Methyl Orange)
- Weak Acid + Strong BasepH > 7 (Phenolphthalein)
Universal Indicator
Unlike simple indicators which toggle between two colors, a Universal Indicator is a complex cocktail of Thymol Blue, Methyl Red, Bromothymol Blue, and Phenolphthalein. This blend creates a smooth rainbow spectrum across the entire pH 4-10 range, making it ideal for checking the approximate pH of an unknown solution.
Frequently Asked Questions
What is the difference between End Point and Equivalence Point?
The Equivalence Point is the theoretical moment when moles of acid equal moles of base. The End Point is the physical moment when the indicator actually changes color. Ideally, these two points should coincide.
How do acid-base indicators work?
Indicators are weak acids (HIn) that exist in equilibrium. The acid form (HIn) has one color, and its conjugate base (In-) has another. Changing pH shifts this equilibrium (Le Chatelier's Principle), changing the dominant color.
Which indicator is used for Strong Acid vs. Strong Base titration?
For Strong Acid-Strong Base titrations (pH ≈ 7 at equivalence), almost any indicator works (Phenolphthalein, Methyl Red, Bromothymol Blue) because the pH change is very steep.
Why use Phenolphthalein for Weak Acid vs. Strong Base?
The equivalence point for a weak acid-strong base titration is basic (pH > 7). Phenolphthalein is ideal because it changes color in the pH 8.2-10.0 range, capturing this basic endpoint.
Why use Methyl Orange for Strong Acid vs. Weak Base?
The equivalence point for a strong acid-weak base titration is acidic (pH < 7). Methyl Orange is suitable as it changes color in the acidic pH 3.1-4.4 range.
What is a Universal Indicator?
A Universal Indicator is a mixture of several different indicators (like thymol blue, methyl red, bromothymol blue, phenolphthalein) that exhibits a smooth spectrum of color changes across the entire pH 1-14 scale.
Why is the pH range usually 2 units?
Human eyes need the ratio of one colored form to the other to be about 1:10 to 10:1 to perceive a full color change. This logarithmic shift corresponds to a pH change of approximately 2 units (pKIn ± 1).
Can titration be done without an indicator?
Yes, using a pH meter or conductivity meter (potentiometric titration) allows you to plot the pH curve precisely without relying on visual color changes.
What color is Phenolphthalein in acid?
Phenolphthalein is colorless in acidic and neutral solutions. It turns pink only in basic solutions (pH > 8.2).
Why are indicators not used in excess?
Indicators themselves are weak acids or bases. If you add too much, they can actually participate in the reaction and slightly alter the titration volume, introducing error.