Difference Between Acid Base Indicator and Universal Indicator

Main Difference – Acid Base Indicator vs Universal Indicator

A chemical indicator is a substance that can show a change of a measurable parameter in response to a change in the solution. The change should be observable such as a color change, temperature change, precipitate formation, etc. The substances that cause a color change in response to the pH of the medium are called acid-base indicators because these substances are used to identify the termination of an acid-base reaction. Indicators show different colors at different pH values. The term universal indicator is used to name a chemical compound that can act as an indicator for a wide range of pH values. The main difference between acid base indicator and universal indicator is that acid base indicators show color changes at a certain pH range whereas universal indicators show color changes at a wide variety of pH values ranging from 0 to 14.

Key Areas Covered

1. What is Acid Base Indicator
   – Definition, Theory, Examples
2. What is Universal Indicator
   – Definition, Common Formulation, Different Forms
3. What are the Similarities Between Acid Base Indicator and Universal Indicator
   – Outline of Common Features
4. What is the Difference Between Acid Base Indicator and Universal Indicator
   – Comparison of Key Differences

Key Terms: Acid Base Indicators, Chemical Indicator, Color, Endpoint, pH indicators, Titration, Universal Indicator

What is an Acid Base Indicator

Acid base indicators are chemical substances that can give a color change in a reaction medium as a response to a change in pH. These indicators are used to identify the termination of an acid-base reaction. They are also called pH indicators. They are usually either weak acids or weak bases.

Acid base indicators show these color changes when they get dissociated. The following equation shows the dissociation of a weak acid that can be used as an acid base indicator.

HIn_(aq)   +   H₂O_(l)      ↔       In^–_(aq)    +    H₃O⁺_(aq)

HIn acid has a different color from its conjugated base: In^–. Therefore, this dissociation shows a color change. The dissociation reaction of a weak acid is partial. Hence, there is an equilibrium between the acid and its conjugated base. If the pH is high, then the reaction moves to the left, forming more acid molecules. Then the color of the medium would be the acidic color of the indicator. If the pH of the medium is low, the equilibrium moves to the right forming more protons. Then the color of the medium is the basic color of the indicator. But the color change is given in a certain pH range.

Figure 1: Color Change of Methyl Red; Acidic, Neutral and Basic Colors from Left to Right

Acid base indicators are used to determine the endpoint of a titration. The endpoint is approximately similar to the point where the reaction is terminated. Following table shows some examples of acid base indicators.

What is a Universal Indicator

Universal indicator is a type of pH indicator that gives its color changes for a wide variety of pH values ranging from 0 to 14. Therefore, it can be used to determine the acidity or the alkalinity of a solution. There are many universal indicators available, but the most common universal indicator is a mixture of following pH indicators.

• Thymol blue
• Methyl red
• Bromothymol blue
• Phenolphthalein

Figure 2: Different Colors in Different Ph Values When the Universal Indicator is Added.

Universal indicators come in two types: paper form and solution form. The paper form of the universal indicator can give a color change when it is immersed in a particular solution. Or else few drops of the solution can be put on to a strip of paper. In the solution form, few drops of the solution can be added to the reaction mixture.

Similarities Between Acid Base Indicator and Universal Indicator

• Both acid base indicator and universal indicator are pH indicators.
• Both indicators give color changes in response to a change in pH.

Difference Between Acid Base Indicator and Universal Indicator

Definition

Acid Base Indicator: Acid base indicators are chemical substances that can give a color change in a reaction medium as a response to a change in pH.

Universal Indicator: Universal indicator is a type of pH indicator that gives its color changes for a wide variety of pH values ranging from 0 to 14.

pH Range

Acid Base Indicator: Acid base indicators show color changes at a certain pH range.

Universal Indicator: Universal indicators show color changes over a wide range of pH from 0 to 14.

Color

Acid Base Indicator: Acid base indicators have an acidic color and a basic color at acidic and basic pH values respectively.

Universal Indicator: Universal indicators show different colors at different pH values.

Formulation

Acid Base Indicator: Acid base indicators are individual chemical compounds.

Universal Indicator: Universal indicators are prepared by mixing few chemical compounds together.

Conclusion

Acid base indicators and universal indicators are pH indicators that can show a color change as a response to a change of pH. The main difference between acid base indicator and universal indicator is that acid base indicators show color changes at a certain pH range whereas universal indicators show color changes at a wide variety of pH values ranging from 0 to 14.

Reference:

1. Helmenstine, Anne Marie. “What Is a Chemical Indicator?” ThoughtCo, Aug. 6, 2017, Available here.
2. “Indicators.” Chemistry LibreTexts, Libretexts, 9 Feb. 2017, Available here.
3. Helmenstine, Anne Marie. “Universal Indicator Definition.” ThoughtCo, Dec. 9, 2017, Available here.

Image Courtesy:

1. “Color transition of Methyl red solution under different acid-base conditions” By LHcheM – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Skala boja 2” By Dejan Jovic DJ – Own work (CC BY-SA 4.0) via Commons Wikimedia