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Question #154 posted on 08/14/2003 midnight
Q:

Dear 100 Hour Board,
Why are so many liquid laundry soaps blue? It seems to be almost universal.
- Color-safe

A: Dear Color-safe,
Thanks to the University of Chicago and the US DOE (Department of Energy) your question has an answer! http://www.newton.dep.anl.gov/askasci/gen99/gen99787.htm

Question - Why is most liquid laundry detergent the color blue? Does it have anything to do with active enzymes or substrates?

Nothing so functional. Most all detergents contain "brightening agents". Compounds that absorb U.V. light just outside the visible range and fluoresces it back in a broad visible band edged toward the blue part of the visible. Organic stilbene compounds are a typical class of such compounds. The effect is to give a "whitening effect". This is also common in paper. If your school has a "black light" U.V. lamp you can see the effect very dramatically, even with the regular lights on.
- Vince Calder