Calcium phosphate effects on overall water profile

[cheier]

I've just finished brewing a pale ale where I had done some water modifications. One of the main mods that I did was to soften the water through use of phosphoric acid. The result of this addition is what I'm more curious about.

From what I've found, the phosphoric acid reacts with the CaCO3 to produce tricalcium phosphate (Ca3(PO4)2), H2O and CO2. Now, does this binding stay as is in the overall grand sceme of things, or does the calcium in this molecule contribute to the overall calcium content in the wort?

Right now, my municipal water profile has 170 ppm CaCO3 and 45ppm of Calcium. I believe I added enough calcium to the water to ring it up to about 60 ppm, but I'm not sure if the resulting calcium from the phosphoric acid reaction also makes its contribution.

 

[ajdelange]

I've just finished brewing a pale ale where I had done some water modifications. One of the main mods that I did was to soften the water through use of phosphoric acid. The result of this addition is what I'm more curious about.

Adding trisodium phosphate to water softens it through the formation of hydroxyl apatite

10Ca++ + 18Na+ + 6PO4--- + 2 H2O --> Ca10(PO4)6(OH2) + 2 H+ + 18Na+

and it was commonly used for that purpose until eutrophication became a concern. Adding phosphoric acid to water does not soften it except under unusual condition but it does reduce alkalinity as does any acid.

From what I've found, the phosphoric acid reacts with the CaCO3 to produce tricalcium phosphate (Ca3(PO4)2), H2O and CO2. Now, does this binding stay as is in the overall grand sceme of things, or does the calcium in this molecule contribute to the overall calcium content in the wort?

Where did you find this? When phosphoric acid reacts with bicarbonate ion, the source of most of the alkalinity in most brewing water, the reaction is the same as with any other acid

3H+ + PO4--- + HCO3- --> CO2 + H2PO4- + H2O

It is only if the alkalinity is great enough that a lot of phosphoric acid must be used to neutralize it and when the water is very hard (lots of calcium) that the reaction between the dibasic phosphate and calcium results in the precipitation of apatite:

6H2PO4- + 10Ca++ + H2O --> Ca10(PO4)6(OH)2 + 14H+

There are some curves in the back of Palmer's new Water book that show when this occurs.

Right now, my municipal water profile has 170 ppm CaCO3 and 45ppm of Calcium. I believe I added enough calcium to the water to ring it up to about 60 ppm, but I'm not sure if the resulting calcium from the phosphoric acid reaction also makes its contribution.

You mean that your water has alkalinity of 170 ppm as calcium carbonate. This is not at all the same thing as 170 ppm calcium arbonate.

Once apatite precipitates its calcium is unavailable. Apatite is an extremely insoluble compound (your teeth, for example, are made of it)