How Does an Acid Base Indicator Work?

Chemical Identities of Indicators

Acid-base indicators are chemicals that change color when they react with acids and bases. They change color depending on the pH (level of acidity) of a solution.

Most indicators are in fact weak acids themselves, so they are able to react with either acids or bases. Each reaction produces a product of a different color. That is why indicators are able to change the color of a solution.

Reactions of Indicators with Acids and Bases

Indicators are present in solutions in two forms, noted as HIn and In-. Both of these have a distinct color. There are two possible reactions, depending on whether acid or base is introduced into the solution containing the indicator.

If acid is added to the solution, more HIn is made. This changes the color of the solution to match that of HIn.

If base is added instead, more In- is made, changing the color of the solution to that of In-.

When reacted, the indicator simply changes to its other form, changing the color as well. To learn more about why this occurs, see the link in Resources.

Visibility of Indicator Reactions

In order to be a suitable indicator for acid-base reactions, HIn and In- must have different colors. This means that most dyes will not work as an indicator does. Indicators change color gradually across a pH range, and it is common for one of the indicator's forms to be colorless.

Since indicators are weak acids, care should be taken to avoid adding too much indicator (more than 3 to 4 drops per 100-mL) to a solution of acid or base. Otherwise, the indicator may interfere with the chemical reaction.

References

• Facts About Chemical Indicators

Resources

• Le Chatelier's Principle