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A client of mine has installed an ion exchange water purifier to de-harden her tap water. After that, however, she noticed that it has negatively affected her watercolor painting. From what she describes, it seems that the pigments have flocculated. I have a theory of what may have caused it, but I wish to run it by the community here/inquire about any alternative theories.

The ion exchange filter captures Ca2+ and Mg2+ and exchanges them for Na+, while carbonate and sulfate ions slip through. The original hard water had in other words the presence of partially dissociated CaCO3/CaSO4/MgCO3/MgSO4 species. The ion exchange of Ca2+ and Mg2+ for Na+, however, leaves a different ionic profile consisting of Na+, carbonate, and sulfate ions. This resembles the ions present when N2SO4/N2CO3 dissociate completely, forming a strong electrolyte solution.

If that is correct and we assume that the new water behaves like a strong electrolyte solution compared to regular hard water, then the electrical double layer surrounding the pigment particles ought to be affected --> electrostatic repulsion decreases --> pigments flocculate. However, the DVLO theory states that it is the valency of the electrolytes present and their concentrations that affect particle dispersion, not whether the solution is considered a strong or weak electrolyte. Therefore I am not entirely sure whether I am on the right track.

Am I on the right track thinking that it is the change in electrostatic repulsion that causes my client's watercolor problems or is there something else I am missing? If it is the electrostatic repulsion that is affected, then how exactly?

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What you state seems reasonable: salting-out can be used to precipitate some substances. If it were tempera paint, you could point out that proteins in egg albumin might precipitate, but I don't know about water color pigments.

It would be easy to test, though. Mix up batches of paint with a few choices of solvent, perhaps:

  • Distilled water (not just deionized water)
  • The client's treated tap water
  • The client's untreated tap water, if readily available, perhaps from a garden hose that bypasses the ion-exchange filter
  • Distilled water with specific amount(s) of the salt used in the purifier added.

She should be party to these tests, and would not only learn what solvents are acceptable for her use, but could help others if that were reported here as an answer!

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