Acid-Base Indicators
Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases. They change color depending on the pH of the solution they are in, making them useful for determining whether a solution is acidic or basic.
How Acid-Base Indicators Work
Acid-base indicators are weak organic acids or bases that exhibit different colors in their protonated and deprotonated forms. The color change occurs over a specific pH range, known as the transition interval.
Common Acid-Base Indicators
| Indicator | Color in Acid | Color in Base | pH Range |
|---|---|---|---|
| Phenolphthalein | Colorless | Pink | 8.3-10.0 |
| Litmus | Red | Blue | 5.0-8.0 |
| Methyl orange | Red | Yellow | 3.1-4.4 |
| Bromothymol blue | Yellow | Blue | 6.0-7.6 |
Example: Phenolphthalein
Phenolphthalein is one of the most commonly used acid-base indicators. In acidic solutions (pH < 8.3), it remains colorless. As the solution becomes basic (pH > 8.3), it turns pink. This makes it particularly useful for strong acid-strong base titrations.
Natural Indicators
Many natural substances can also serve as acid-base indicators. For example:
- Red cabbage juice: Changes from red (acid) to purple (neutral) to green-yellow (base)
- Turmeric: Yellow in acid, red in base
- Beetroot juice: Red in acid, yellow in base
Acid-base indicators are essential tools in chemistry laboratories, particularly in titration experiments where they help determine the equivalence point of acid-base reactions.
Acid-Base Indicators
Understanding pH detection through color changes
What Are Acid-Base Indicators?
Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases. These substances change color depending on the pH of the solution they’re in, allowing chemists to visually determine whether a solution is acidic, neutral, or basic.
Indicators are typically weak acids or bases themselves, where the acidic and basic forms have different colors. The color change occurs over a specific pH range known as the indicator’s transition interval.
Synthetic Indicators
These are chemically manufactured indicators commonly used in laboratories:
Colorless in acid (pH < 8.3)
Pink in base (pH > 8.3)
pH range: 8.3-10.0Commonly used in strong acid-strong base titrations.
Red in acid (pH < 3.1)
Yellow in base (pH > 4.4)
pH range: 3.1-4.4Used in titrations of strong acids with weak bases.
Yellow in acid (pH < 6.0)
Blue in base (pH > 7.6)
pH range: 6.0-7.6Often used to measure neutral pH ranges.
Red in acid (pH < 5.0)
Blue in base (pH > 8.0)
pH range: 5.0-8.0One of the oldest known pH indicators.
Natural Indicators
Many plants contain natural pigments that change color with pH, making them excellent eco-friendly indicators:
Red/Pink in acid
Green/Yellow in base
pH range: 2-12Contains anthocyanins that change color across the entire pH spectrum.
Yellow in acid/neutral
Red/Brown in base
pH range: 7.4-9.2Curcumin changes color in basic solutions.
Red in acid
Yellow in base
pH range: 10-12Betanin pigment changes at very high pH.
Red in acid
Blue in base
pH range: 3-7Contains anthocyanins similar to red cabbage.
Universal Indicator
A universal indicator is a mixture of several indicators that gives a different color for each pH value over a wide range. It provides a full spectrum of colors corresponding to different pH levels:
| pH Range | Color |
|---|---|
| 0-3 | Red |
| 3-6 | Orange/Yellow |
| 7 | Green |
| 8-11 | Blue |
| 11-14 | Purple |
How to Use Indicators
Indicators can be used in several ways:
- Solution form: A few drops are added to the test solution
- Paper strips: Indicator-impregnated papers (like litmus paper) are dipped into solutions
- Indicator mixtures: Universal indicator solutions or papers show approximate pH
- Natural extracts: Crushed plant material or juices can be used directly
When choosing an indicator for a titration, select one whose color change range includes the expected pH at the equivalence point.
