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Liquid soap that sits in metal pump handles turns blue after a week or so, as in this photo comparing the soap in its initial clear state and then after pumping from the dispenser.

Clear soap turned blue

The pumps are presumably brass with either nickel or chrome electroplating on the exterior. Can't say for sure whether or to what degree the interior where the soap makes contact is plated. They have been in use for years.

The ingredients of this particular soap are:

  • water
  • sodium laureth sulfate
  • cocamidopropyl betaine
  • decyl glucoside
  • sodium chloride
  • "fragrance"
  • DMDM hydantoin
  • polyquaternium-7
  • tetrasodium EDTA
  • citric acid
  • sodium sulfate
  • PEG-120 methyl glucose dioleate
  • PEG-7 glyceryl cocoate
  • poloxamer 124
  • D&C red #33
  • FD&C blue #1
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The color of the precipitate is strongly reminiscent of copper(II) hydroxide. I hypothesize:

  1. Chloride corrosion of copper from exposed brass (ref 1 || ref 2)

    a. Per ref 2 above, sulfate may also participate

  2. Alkaline precipitation of $\ce{Cu(OH)2}$ in the neutrally-buffered soap

  3. Insufficient $\ce{Na_4EDTA}$ to complex the amount of $\ce{Cu}$ corroded from the brass

It's also possible the blue material is $\ce{Ni(II)}$ complexed in such a way that its color is changed from the typical green-blue of $\ce{Ni(OH)2}$.

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  • $\begingroup$ Is it clear where the free chlorine would come from? The system is not exposed to any electrical charge and is held at a stable room temperature. (Don't assume I know anything special about the chemistry of solutions like this. E.g., if sodium chloride has some steady-state dissociation rate, or something in there is or could be ionic, I don't know about it.) $\endgroup$ – feetwet Jun 2 '15 at 17:48
  • $\begingroup$ You don't need free chlorine for brass to corrode. Chloride ion is sufficient. See the new "ref 2", added above in-edit. Quote: "It was reported previously that ions as thiozionates, bromides, and iodides did not participate in dezincification process, while chlorides and sulphates enhanced dezincification process." Etc. $\endgroup$ – hBy2Py Jun 2 '15 at 18:03
  • $\begingroup$ Good finds on those references! $\endgroup$ – feetwet Jun 2 '15 at 19:31
  • $\begingroup$ I guess, to cover my bases, it's possible there are chloride-resistant varieties of brass I don't know about. But, there are definitely chloride-susceptible alloys. $\endgroup$ – hBy2Py Jun 3 '15 at 10:44

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