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1.3 Testing for Macromolecules
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.
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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.

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

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

By Flo Sorg [Public domain], from Wikimedia Commons
Biuret Reagent
Positive result.

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