I understand that malt phosphates number in the 1 percent range in the typical wort. I haven't really studied malt phosphates in detail, but I know that AJ has. Those phosphates are the basis for the Ca and Mg reactions and subsequent proton production and pH drop.
The number I always see is that phosphate, as the pentoxide, comprises 2% of the weight of malt. As phosphate that would be about 1.4% as phosphate. So at 1 quart (2 lbs) of water per pound of grain and assuming that all the phosphate got released into solution you would indeed have approximately 1% w/w.
AJ, are you saying the phosphatic ions from the dissolution of phosphoric acid are similar to those of the malt phosphates?
AFAIK yes. They are held in the grain as a salt of inositol hexaphosphoric acid which, subject to the action of phytase releases inorganic phosphate ions and myoinositol. Furthermore, apparantly some of the phosphate is able to chelate calcium and magnesium even when still attached to inositol hexaphosphoric acid.
If they are, then would the addition of minor amounts of phosphoric acid actually promote the additional Ca compound precipitation that Colin is mentioning?
It would in Colins case because he is adding phosphate to water which contains no other phosphate i.e. he's doing it before adding any malt. Looking at the numbers I ginned up last night, however, it doesn't appear that much calcium is being stripped out by the phosphate and that's assuming a pretty high pH (8.3) to start. Given that his pH were say 7.7 to start it would take less H+ to get to 7.2 and require less precipitation of calcium with phosphate. If one holds off on using phosphoric acid until the mash, for example to move a mash pH of 6 or 6.2 to 5.4 or so then I would think even less would precipitate. In any event as the example calculation seems to indicate the amount of phosphate added to move pH is small compared to the amount of phosphate in the malt.
I'm really trying to get a better grasp on the reason that adding phosphoric acid would be the genesis of additional Ca precipitation.
Me too. It's pretty complicated to figure out what's really going on as you have to simultaneosly satisfy Henry, Henderson Hasselbalch for 5 pK's (2 for carbonate and 3 for phosphate), 2 solubility criteria and neutrality in the simplest model which is that carbonic and phosphoric are the only acids present. But the general principal is that appatite is extremely insoluble to the extent that where phosphate is present at the gram/L level even minute amounts of calcium saturate the solution at pH's above 5.4 or so and precipitation occurs. Phosphate softens water - even at relatively low pH.
The other thing I'm trying to grasp is if precipitating some extra Ca from solution by phosphoric acid addition is really that detrimental to the ferment performance?
That will depend on how much alkalinity you need to combat and how much calcium is available. If you have water with alkalinity 300 ppm as CaCO3 it's going to take about 6 meq/L (120 mg/L) calcium to drop it to pH 5.2 with phosphoric (acid added plus acid from malt). OTOH if your alkalinity is 50 it's only going to take out 1 meq/L (20 mg/L). If you do what Colin is doing and my hypothesized characteristics for his water apply then phosphoric acid doesn't remove that much calcium because not that many protons (0.247 meq/L) are needed to drop the pH of 100 ppm alkalinity water from 8.3 to 7.2. But then the alkalinity isn't reduced much either - 12 ppm.
Eventually most of the rest of the alkalinity (94% of it) has to come out to get to mash pH 5.2 so assuming he starts with 100 ppm (2 meq) he'll need 94/50 = 1.84 mEq protons and 1.84 meq calcium (all /L) precipitated to get the water
to mash pH by the phosphate mechanism. That's 37 mg/L which isn't so much. But then there's the acid needed to get the mash
from distilled water pH (say 5.75) to desired pH (say 5.4). That's a shift of 0.35 pH which divided by Kolbach's 0.00186 pH/ppm_as_CaCO3 gives another 208 ppm calcium hardness which needs to be sacrificed. This is 83 mg/L which is quite a bit and which, with many waters, would have to be added.