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November 25 - December 2

Question of the Week

Name: Manisha
Status: student
Grade: 9-12
Country: India
Date: Fall 2014


Question

I am curious to know that how universal indicator shows different colours on different acids and bases?

Answers from Our Expert Staff

Manisha,

Universal indicators are made by combining several different compounds that produce color changes at various pH ranges. For example, phenolphthalein - which you are familiar with as an indicator that changes from colorless to pink at around pH=8.2, could be combined with bromthymol blue - another indicator which is yellow below pH=6, and blue above pH=7.6. So we would have a new indicator solution that will give color changes at least three different pH's. The trick then is to choose indicators carefully so that they produce distinct color changes and that the colors of the various indicators do not interfere with the viewing of specific color changes at particular pH's.

Greg (Roberto Gregorius)
Canisius College



Hi Manisha,

Thanks for the question. There is no single indicator that works for all pH values. So, a universal indicator is made up of a few indicators. Each indicator changes color at a specific pH value. So, if you want to tell the pH by using the color, you will need several indicators.

I hope this helps.
Thanks
Jeff Grell


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