I agree with waterbear. Let me do some calculations, mostly for the case where the water isn't already saturated with calcium carbonate.
Calcium phosphate, Ca
3(PO
4)
2, has a solubility product of 1x10
-26. At a typical CH of 300 ppm (about 3x10
-3 moles/liter), this implies a phosphate saturation level of 6x10
-10. At a pH of 7.5, this implies a hydrogen phosphate, HPO
42-, of 9x10
-5 and a dihydrogen phosphate, H
2PO
4-, of 4.6x10
-5 and a phosphoric acid, H
3PO
4, of 2x10
-10. So the total phosphate level is around 1.4x10
-4 or about 13 ppm or 13,000 ppb. This is far higher than would normally be found in pools. [EDIT] The solubility product of calcium phosphate is all over the map depending on source where a search I made found
1x10-26,
2.07x10-33,
2.83x10-30,
1x10-33,
2.0x10-29. At 300 ppm CH with a pH of 7.5 this results in a total phosphate level at saturation of 8170 ppb, 3.7 ppb, 137 ppb, 2.6 ppb, 265 ppb, respectively. The 8170 ppb vs. 13,000 ppb difference is probably due to somewhat different dissociation constants for phosphoric acid when I originally made the calculation until now. [END-EDIT]
At higher pH, the same phosphate ion level (so same saturation level) implies a lower total phosphate level, so solubility decreases at higher pH. At a pH of 8.0, hydrogen phosphate is 2.8x10
-5, dihydrogen phosphate is 4.5x10
-6, phosphoric acid, 6x10
-12. So the total phosphate level is around 3.3x10
-5 or about 3 ppm or 3000 ppb. This is a level that could be found in pools and just means that high pH could precipitate calcium phosphate. Of course, it's also likely to precipitate calcium carbonate if the water is saturated as with plaster pools.
In an SWG cell at the cathode where hydrogen gas is generated, the pH gets high which is why scaling usually occurs at that plate. So it's possible to scale with calcium phosphate if the water were not saturated with calcium carbonate, though I would ask, "so what?". Many SWG cells change polarity every so many hours to remove scale that may have built up and calcium phosphate scale should dissolve in the acidic conditions of the (now) anode just as calcium carbonate would dissolve.
Richard