In that second scenario, I have neutralized all the alkalinity that was due to the bicarbonate content of the strike water, haven't I eliminated all of the bicarbonate?Right. This is what I originally asked about. I am not sure if I neutralize the bicarb with acid and get to a mash pH of 5.4 if the bicarb is GONE or if it's just at a lower ppm.
I feel like this question is still outstanding, and also there has been some divergence in the discussion between (i) acidifying strike water and (ii) residual alkalinity/mash pH.
If you reduce the mash pH to at or below the pH that would have resulted if you had used 100% distilled water with no bicarbonate as your strike water, what is the mechanism by which bicarbonate can still be present? Put another way, if you have reduced residual alkalinity to zero (or below), have you converted all the bicarbonate to water and CO2?
I get that it doesn't really matter for all practical brewing purposes, but I want to understand it just for the sake of understanding it.
The pH scale for the carbonate/bicarbonate/carbonic acid equilibrium are:
4.3 - below this pH, everything is in the form of Carbonic acid/dissolved CO2. There are no bicarbonate or carbonate ions in solution
4.3-8.3 - everything exists as a mixture of carbonic acid and bicarbonate, with the proportion of bicarbonate increasing as pH increases
6.3 - this is a buffer with equal concentrations of carbonic acid and bicarbonate
8.3 - at this value everything exists as bicarbonate
8.3-12.3 - everything exists as a mixture of bicarbonate and carbonate, with the proportion of carbonate increasing as pH increases
10.3 - this is a buffer with equal concentrations of bicarbonate and carbonate
12.3 - above this pH, everything is in the form of carbonate, no bicarbonate or carbonic acid is present
Note that these values are directly related to the pKa values for H2CO3 <> H+ + HCO3-; and HCO3- <> H+ + CO3(2-). This isn't related to the pH of distilled water. Also, the presence of ions like calcium and mag make this a bit less straightforward, as they can bind to carbonate and bicarbonate ions, sequestering them from this equilibrium.