# Fluoride precipitation

#### dearknarl

Gday,

I only have basic chemistry knowledge (highschool and a bit of exposure to first year undergrad material).
I need some help figuring out what theory and techniques to study so I can work out the following problem:

If I mix sodium fluoride, calcium carbonate powder, and water, under what conditions (if any) would I get a stable concentration of dissociated fluoride ions?
Ie. Assume we start out with fully dissociated Na + F (aq), because it's fairly soluble. But when I add the CaCO3, it's only slightly soluble, so assume an excess of solid CaCO3 and a small amount dissolved.
Will that be stable with NaF concentration much greater than the saturated concentrations of CaCO3, or will CaF start (and continue) to precipitate, which is also poorly soluble, and I'll be left with much less dissociated fluoride?

I'm wondering if it's the relative solubilities of CaCO3 and CaF that will drive the equilibrium (if that's what it is) one way or the other. I can't work out how to apply the theory of an equilibrium constant because of the excess solid reactant and corresponding possible precipitate because it messes up how concentrations balance out in a normal equilibrium.... And I'm wondering if the absolute concentration of NaF effects things as well...

Any pointers appeciated. Thanks.

Neil.

PS. if you haven't guessed, I'm just starting to formulate a toothpaste (tablet) recipe. And I chose calcium carbonate as a medium abrassiveness cleanser, but I'm worried it will affect the eventual available fluoride ions, either when I went process the mixture, on the shelf, or in the mouth.

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#### dearknarl

Sorry, realised I said CaF when I meant $$\displaystyle CaF_{2}$$, and I can't figure out how to edit my post so I can fix that and add some latex formatting :-(

#### Woody

The amounts of fluoride in toothpaste are tiny.
any more than just the smallest trace can lead to (rather horrible) toxic effects.
People who live in areas with a naturally high level of fluoride (in the water supply) have to avoid toothpastes with added fluoride.

Fluoride toothpastes provide most benefit only in places where the natural fluoride levels are particularly low.

topsquark