How Indicators Work

The inspiration of this post is http://vivekspace.wordpress.com/2006/09/28/iits-in-news/

When I was in school, our Chemistry teacher told us a story about the phrase “caught red-handed”. He said that the police actually puts a chemical (some sort of mild acid in powdered form) on the currency notes which are handed over in an anti-corruption operation. Clearly, the term “sting operation” hadn’t caught on in those days. Later, when they wash the hands of the culprit with a chemical they turn red.

So what is the powder that is applied in the notes and is the other chemical

If the initial powder is an acid, the indicator would be
Thymol Blue (which is actually red in Acid and Yellow in Base)
Methyl Orange
Methyl Red

If it (the powder on the note) is a base, the indicator would have been phenophthalein

Or the powder can be an indicator and the second chemical acid / base.

Table of Indicators