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We have some relatively new white towels that were getting dirty, so we decided to clean them with bleach (standard Clorox). When we sprayed the diluted bleach on the towels they immediately turned bright purplish pink, but the color faded back to white within seconds. We initially thought it was somehow transferring color from the pink nozzle on the spray bottle, but we noticed the same effect when the diluted bleach was poured directly onto the towels. No harm appears to be done, but what would cause the towel to turn pink, especially since it fades almost immediately?

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    $\begingroup$ Although Dr. Laundry says it happens to the incoming brighteners of a detergent and not to those fixed to the fabric, I think that your new towels should have enough brighteners - deposited into and/ or onto them - to react as well. Additional note : brighteners absorb light in the near UV and emit in the blu tail of vis, so making the fabric "extrawhite". clorox.com/dr-laundry/detergentbleach-reaction $\endgroup$ – Alchimista Jul 26 '17 at 22:20
  • $\begingroup$ @Alchimista That could quite possibly be it, and would be worth posting as an answer. $\endgroup$ – David K Jul 27 '17 at 11:58
  • $\begingroup$ I do it, just it is not evident to me which oxidation product (dye) is formed. $\endgroup$ – Alchimista Jul 27 '17 at 13:04
  • $\begingroup$ @Alchimista If it's helpful, this is the detergent we use. $\endgroup$ – David K Jul 27 '17 at 13:07
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Although Dr. Laundry says it only happens to the incoming brighteners within a detergent and not to those fixed to the fabric, I think that your new towels should have enough brighteners - deposited into and/or onto them - to react as well.

Optical brightners and bleach reaction: https://www.clorox.com/dr-laundry/detergentbleach-reaction/

Additional note : brighteners absorb light in the near UV and emit in the blu tail of the Vis spectrum, so making the fabric "extrawhite". Search for Optical brighteners in Google or Wikipedia.

Perahps someone here could know or propose the ongoing reaction and is transient behaviour. As the OP said in a comment, the towels were washed with a detergent containing a stilbenedisulfonic triazine derivative(s).

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