Unit D

Module 6 ~ Lesson 1

Using Indicators

In Chemistry and Biology, the use of chemical indicators in experiments is common. A chemical indicator is a compound that gives a visible (or olfactory) sign indicating a particular type of chemical species is present. You probably remember using acid-base indicators in earlier science courses: red litmus paper, blue litmus paper or pH paper.

In Biology 20, you will still be required to use acid-base indicators. However, you are also introduced to a number of new indicators that indicate the presence of a specific macromolecule. Acid-base indicators change colour within a specified pH range, so you may need to use more than one indicator to determine the specific pH. For example, if thymol blue turns yellow in a solution, it only tells us that the solution has a pH greater than 2.8. It could still be either an acid or a base.

The chart below shows some common indicators used to test for macromolecules.

The positive result on the left, negative result on right.

By Flo Sorg [Public domain], from Wikimedia Commons

Biuret Reagent 
Positive result.

By Ozone aurora/Philip Evans [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons

Benedict's Solution

By Thebiologyprimer [CC0], from Wikimedia Commons