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re: apatite solubility
> Under favorable conditions, hydroxylapatite is essentially insoluble. I
> turned around a calculation (perhaps a little too quickly?) and found that
> at pH 8 with 4 degrees of general hardness (all as calcium)
> hydroxylapatite solubility would allow something like 0.6 part per
> trillion (not a typo) of HPO4 in solution. With 8 degrees of general
> hardess this drops to about 0.2 parts per trillion.
As you might guess, once I got home and had more time to look at these
calculations I did find that I went through it too fast. I think the
real number is 27 parts per billion at 4 degrees hardness and 8 parts per
billion at 8 degrees hardness.
At any rate, the concentration is 2 orders of magnitude (or so) lower
than the concentrations people often report in their aquariums.