Introduction

pH indicators are essential tools in chemistry that allow us to determine the acidity or basicity of solutions through distinct color changes. These specialized chemicals provide a quick and visual method for measuring pH levels, making them invaluable in laboratory work and chemical analysis.

What Are pH Indicators?

pH indicators are weak acids that change color at specific pH values, providing a visual indication of a solution's acidity or alkalinity. These color changes occur at distinct pH ranges, making them particularly useful in analytical chemistry and especially in acid-base titrations.

Common pH Indicators and Their Ranges

Here are some frequently used pH indicators in chemistry laboratories:

Indicator	Acidic Color	Basic Color	pH Range
Methyl Orange	Red	Yellow	3.1 - 4.4
Bromothymol Blue	Yellow	Blue	6.0 - 7.6
Phenol Red	Yellow	Red	6.8 - 8.0
Phenolphthalein	Colorless	Pink	8.3 - 10.0

The Science Behind pH Indicators

pH indicators function through a chemical equilibrium between their protonated (HIn) and deprotonated (In⁻) forms:

$$\text{HIn}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightleftharpoons \text{In}^-_{(aq)} + \text{H}_3\text{O}^+_{(aq)}$$

The ratio between these forms determines the observed color:

Using Multiple Indicators

For accurate pH determination, using multiple indicators is recommended. For example:

Consider a solution tested with multiple indicators showing:

By combining these observations, we can narrow the solution's pH to between 6.8 and 7.6.

Understanding Indicator Strength

The strength of an indicator is characterized by its pKa value - the pH at which the indicator is 50% ionized. At this point:

Examples: