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I'm building a dehumidifier based on $\ce{CaCl2}$-solution as descant, which is to be regenerated by heating and evaporation.

I thought it would be a good idea to add some citric acid to combat chalk-buildup due to rather hard tap water.

To test for possible reactions of $\ce{CaCl2}$ and citric acid, I mixed a 30% $\ce{CaCl2}$ solution by weight, added different amounts of acid (10%, "rather more", "very much") and boiled them. (My main fear being that some Ca might react with Citric acid to form a precipitate, which did NOT happen.) While heating, the solutions with more citric acid turned dark turquoise, but remained clear. Using vinegar did not cause a color-change. Heating $\ce{CaCl2}$ or citric acid alone does not cause color change. No other reactants (I know of) were present. It seems that cooking for longer turns the solution darker, but I may be mistaken on that.

The reactions I could think of should all be colorless/white, not blue/green/turquoise: Ca-citrate, decomosed citric acid, I didn't find sources for citric $\ce{Ca}$ complexes. Maybe a $\ce{Cl}$-complex is formed, but $\ce{Ca}$ is the only available metal, and again I couldn't find resources for chlorine $\ce{Ca}$ complexes. Impurities are probably not at fault, since the color is very strong/dark.

What I'm asking is specifically what reactions might occur between $\ce{CaCl2}$ and citric acid. I know of enough ways to get around my hard-water problem.

Edit:
The pot I used for heating was at fault. Heating in an enamel pot showed no color change. Thanks @poutnik and @m-farooq

I would still be very curious as to what reaction might cause a green/blue color change. From what I can tell, the metal pot is made of stainless steel.

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  • $\begingroup$ Can you add a picture? These blue/green salts indicate transition metals. What was the container made of in which you were heating? $\endgroup$ – M. Farooq Jan 6 at 8:10
  • $\begingroup$ Calcium forms a citrate complex, but the real issue may be thermal stability of it. There can occur a bunch of complicated organic reactions. As the acid alone does not get such colour, transition metal interference is excluded, unless present in CaCl2. It may be the complex thermal decomposition along an organic chemistry path. $\endgroup$ – Poutnik Jan 6 at 9:35
  • $\begingroup$ Thank you for your input. My kettle indeed had a metal bottom which I thought to be stainless steel. Heating in an email-pot had no color-changing effect. $\endgroup$ – GammaSQ Jan 6 at 9:46
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    $\begingroup$ Is email pot enamel? $\endgroup$ – James Gaidis Jan 6 at 13:54

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