Thanks Silver. I’ve got same water as Kinggolf so that is a big help.
For the sake of people who don't have 115 ppm alkalinity water, but want to calculate this on their own, I offer this "generally close enough" (I.E, somewhat corner cutting) method.
I will stick with 115 ppm alkalinity (as CaCO3) and 5 gallons of such water for these examples:
Molecular weight of CaCO3 = 100.0869 g/mol = 100.0869 mg/mmol
Charge (or valence) of the Ca++ ion = +2
Equivalent Weight of CaCO3 = 100.0869 / +2 = 50.04345 g/Eq = 50.04345 mg/mEq
115 ppm alkalinity (as CaCO3) = 115 mg/L
115mg/L / 50.04345 mg/mEq = 2.298 mEq/L of alkalinity as CaCO3
2.298 mEq/L x 5 Gal. water x 3.7854 L/Gal. = 43.4942 mEq's of alkalinity as CaCO3
10% phosphoric acid = ~1.0746 mEq/mL at pH 5.4
88% lactic acid = ~11.45 mEq/mL at pH 5.4
43.4942 mEq alkalinity / 1.0746 mEq/mL = 40.475 mL of 10% phosphoric acid to neutralize 5 Gal. of this water to ~pH 4.3
40.475 mL x 0.90 = ~36.4 mL of 10% phosphoric acid to neutralize this water to ~pH 5.4
43.4942 mEq alkalinity / 11.45 mEq/mL = 3.7986 mL of 88% lactic acid to neutralize 5 Gal. of this water to ~pH 4.3
3.7986 mL x 0.90 = ~3.4 mL of 88% lactic acid to neutralize this water to ~pH 5.4
NOTE: A.J. deLange has detailed the precise method for computing this on this forum, and it is well more complex and complicated and confusing than the above, but for most waters the end results are generally close between the two methods, such that this method can be considered good enough for most practical purposes (strictly IMHO). The shortcuts mainly involve ignoring the waters initial pH and only targeting ~5.4 pH. If your source water has a mainstream normal pH, and targeting pH 5.4 to rid yourself of pesky alkalinity is OK with you, then (in my opinion, always subject to change or correction) this method should work fine.
If you want to use other acids, you will need to discover their mEq/mL acid strength and substitute it into this method.
These values should be close:
80% Lactic Acid, mEq/mL ~= 10.4
75% Phosphoric Acid, mEq/mL ~= 12.08
85% Phosphoric Acid, mEq/mL ~= 14.65
AMS (CRS), effective mEq/mL ~= 3.66
The below is a bit more speculative (I.E., less confidence on my part, albeit that it seems intuitively to be correct):
If you want to target a specific remaining alkalinity in your water make the two following revisions:
1) Subtract the specific alkalinity you want to have remaining in your water post acid adjustment from your starting alkalinity.
2) Target pH 4.3 and then do not back-adjust to pH 5.4.
