# Why do bleached whites turn purple in sunlight? **URL:** \ **Category:** Factual Questions\ **Created:** [May 9, 2002, 7:06am UTC](https://boards.straightdope.com/t/why-do-bleached-whites-turn-purple-in-sunlight/108040 "2002-05-09T07:06:34Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![kambuckta](https://avatars.discourse-cdn.com/v4/letter/k/53a042/32.png) [@kambuckta](https://boards.straightdope.com/u/kambuckta)\ **Post date:** [May 9, 2002, 7:06am UTC](https://boards.straightdope.com/t/why-do-bleached-whites-turn-purple-in-sunlight/108040/1 "2002-05-09T07:06:34Z")
How come, when I’ve soaked white knickers etc in a bleach (sodium hypochlorite) solution, they turn a vivid shade of purple when I hang them out in the sunlight to dry. It’s only a temporary phenomenon (they rapidly return to white) but I was wondering what the deal is. Any clues? ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [May 9, 2002, 11:09am UTC](https://boards.straightdope.com/t/why-do-bleached-whites-turn-purple-in-sunlight/108040/2 "2002-05-09T11:09:00Z")
Maybe some additive in the detergent you use becomes photosensitive when oxidized by bleach 😕. That sort of reaction is rare, but it can happen. Here in the northern hemisphere, I’ve hung bleached whites out in the sun and they don’t turn purple, so it’s not just something that bleach and cloth do when you hang them outside. There must be another factor, such as your laundry detergent, involved in the purpling. You might try contacting the company that makes your soap and asking them if they know what’s going on. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [May 10, 2002, 12:49am UTC](https://boards.straightdope.com/t/why-do-bleached-whites-turn-purple-in-sunlight/108040/3 "2002-05-10T00:49:59Z")
This may be a different phenomenon, but detergents typically have anti-yellowing ingredients that fluoresce blue in UV. The idea is that ambient light will cause just enough blueish glow to cancel out the natural yellowing of the fabric, so it looks white. Bleach, though? I dunno about that, unless it’s just removing some of the yellow, making the blue more obvious… ---
**Author:** ![femtosecond](https://avatars.discourse-cdn.com/v4/letter/f/f08c70/32.png) [@femtosecond](https://boards.straightdope.com/u/femtosecond)\ **Post date:** [May 10, 2002, 12:26pm UTC](https://boards.straightdope.com/t/why-do-bleached-whites-turn-purple-in-sunlight/108040/4 "2002-05-10T12:26:00Z")
[http://naio.kcc.hawaii.edu/chemistry/everyday\_bleach.html](http://naio.kcc.hawaii.edu/chemistry/everyday_bleach.html) > [@](#): > > Bleaches are often combined with “optical brighteners”. These compounds are quite different from bleaches. They are capable of absorbing wavelengths of ultraviolet light invisible to the human eye, and converting these wavelengths to blue or blue-green light. The blue or blue-green light is then reflected by the substance making the fabric appear much “whiter and brighter” as more visible light is seen by the eye. The brighteners are to conceal the last bit of yellowing the bleach wasn’t able to remove. They just overdo their adding of blue in direct sunlight. But I can’t say why the color disappears. Perhaps some of the fresh brightener gets oxidized, or maybe you look at your knickers in another light, so to speak, when you take them off the line. Here’s a recent thread about bluing: [Bluing used in laundry. Why? How did it work?](http://boards.straightdope.com/sdmb/showthread.php?threadid=113007) **Squink** , I wish that _my_ 1000th post will contain “4-Methoxy-N-methyl-1,8-naphthalimide or 7-(2H-Napthol[1,2-d]triazol-2-yl)-3-phenylcoumarin”. 🙂