[APD] Re:Triple PO4 and Apatite solubility

["Jim Seidman" <js5 at seidman_net>]

Neil Frank asks:
"Jim, would any of the phosphate rock dissolve at all if there was a
moderate amount of Ca already in solution. We are talking about an aquarium
not distilled water?"

Neil, that's an excellent point. Looking at Chicago's water supply (the
Jardine plant comprehensive report from 8/03), Calcium was present at
around 33 mg/L, which is around 8.3E-4 Molar. Fluoride is present at around
1 mg/L, or about 5.2E-5 M. (As it turns out, this amount of fluoride has
almost no effect on the results.)

Assuming you started with no phosphate in the water, this calcium would
only allow you to dissolve about 4E-12 M of fluorapatite, which would give
you about 1.2E-11 M of phosphate, equal to about 0.000001 ppm.

Needless to say, 0.000001 ppm phosphate is less than most people recommend.
:-) And again, that's only a starting level. As the phosphate gets
consumed, the now higher calcium and fluoride levels will make the
phosphate level keep going south, not that you could detect this on a
hobbyist test kit. You might not even be able to see the fluorapatite
dissolving, since so little would be affected.

So, now that you mention it, fluorapatite is even less useful than I
thought in typical aquarium conditions.

- Jim


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