Understanding Acid-Base Indicators
Acid-base indicators are essential tools in chemistry for detecting the pH of solutions. These substances change color depending on the pH of the solution they are added to. Let’s explore how they work and how to use them effectively.
What Are Acid-Base Indicators?
Definition: Indicators are substances that change color according to the pH of the solution to which they are added.
Acid-base indicators are typically weak acids or weak bases whose acid form (HIn) and conjugate base form (In⁻) have different colors.
How Indicators Work
An indicator (HIn) undergoes partial dissociation in solution:
HIn(aq) ⇌ H⁺(aq) + In⁻(aq) Colour A Colour B
In Acidic Medium (High [H⁺]):
- The equilibrium shifts to the LEFT
- More HIn molecules are present
- The acid color (Colour A) is observed
In Alkaline Medium (Low [H⁺]):
- The equilibrium shifts to the RIGHT
- More In⁻ ions are present
- The base color (Colour B) is observed
Mathematical Expression for Indicators
The equilibrium constant for an indicator dissociation is:
Kin = [H⁺][In⁻] / [HIn]
Rearranging:
[H⁺] = Kin([HIn] / [In⁻]) Taking logarithms: log[H⁺] = log Kin + log([HIn] / [In⁻]) Multiplying by -1: -log[H⁺] = -log Kin - log([HIn] / [In⁻]) pH = pKin + log([In⁻] / [HIn])
Color Change Range of Indicators
For a complete color change from acid color to base color:
[In⁻] / [HIn] = 10
When [In⁻] = 10[HIn], the color change from color A to color B is complete.
Important Note: The color change is most distinct when the pH range is such that there is a significant difference in the ratio of [In⁻] to [HIn].
Common Acid-Base Indicators
| Indicator | Acid Color | Base Color | pH Range | Best Used For |
|---|---|---|---|---|
| Methyl Orange | Pink/Red | Orange/Yellow | 3.0 – 4.6 | Strong acid-weak base titrations |
| Methyl Red | Red | Yellow | 4.4 – 6.2 | Weak acid-strong base titrations |
| Phenolphthalein | Colorless | Red/Pink | 8.2 – 10.0 | Weak acid-strong base titrations |
| Litmus | Red | Blue | 6.0 – 7.6 | General pH testing |
Selecting the Right Indicator for a Titration
The choice of indicator depends on the pH at the equivalence point of the titration:
1. Strong Acid – Strong Base Titration (e.g., HCl + NaOH)
- Equivalence point pH ≈ 7
- pH range at equivalence point: 4-10
- Suitable indicators: Any indicator (methyl orange, methyl red, phenolphthalein)
- Most common: Methyl orange or phenolphthalein
2. Strong Acid – Weak Base Titration (e.g., HCl + NH₃)
- Equivalence point pH < 7 (acidic, around pH 5-6)
- Suitable indicators: Methyl orange or methyl red
- NOT suitable: Phenolphthalein (color range too high)
3. Weak Acid – Strong Base Titration (e.g., CH₃COOH + NaOH)
- Equivalence point pH > 7 (alkaline, around pH 8-10)
- Suitable indicators: Phenolphthalein
- NOT suitable: Methyl orange (color range too low)
4. Weak Acid – Weak Base Titration
- Equivalence point pH varies depending on Ka and Kb values
- No suitable indicator exists with a sharp color change
- These titrations are difficult and not recommended
Why Indicators Show pH-Dependent Color Changes
The indicator molecule has conjugate pairs with different colors. The ratio of these forms changes with pH:
At low pH: Mostly HIn (acid color)
At high pH: Mostly In⁻ (base color)
At intermediate pH: Mixture of HIn and In⁻ (intermediate color)
Important Considerations
- Sensitivity: Most indicators are sensitive to pH changes of only about 2 units
- Volume: Only a few drops of indicator should be used to avoid affecting the pH
- End Point: The color change marks the end point, not necessarily the equivalence point
- Accuracy: For accurate titrations, choose an indicator whose range includes the expected equivalence point pH
Summary
| Titration Type | Equivalence Point pH | Marked pH Range | Indicator |
|---|---|---|---|
| Strong acid-strong base | 7.0 | 4-10 | Any indicator |
| Strong acid-weak base | 5-6 | 3.5-6.5 | Methyl red/Methyl orange |
| Weak acid-strong base | 8-10 | 7.5-10.5 | Phenolphthalein |
| Weak acid-weak base | Variable | Variable | None suitable |
