OK this is a great question; and with most great questions the answer is... yes complex.
This is parts per thousand, not trillion. factor of 10^6 better but not really waht we need to know.
pH... well it does relate to pH in a round about way... Oh, why did I ever do this...
TL; DR; -> it is about titration g/liter to neutralise a solution, well to trip a marker of known pH(often to about pH 8.2).
But, you want to know about pH like a brewer who does not do titration as a random passtime. In a former life I did do titrations, but never as a passtime.
Therefore we need some maths [this was more annoying than I expected it to be, but it came out witha answer that seems reasonable.]
When titrating a solution for acidity, we need to find hydrogen (H+) ions in the solution, we choose an acid as a reference in order to express the acidity as a concentration [ppt].
Most winemaking texts (some differ) use tartaric acid as the reference, with units of either percent or grams/liter (parts per thousand, or ppt).
We will be doing all this in moles, grams and litres
- USGal -> 3.785 litres
- Imperial Gal -> 4.454 litres
- Ounce -> 28.35g
pH in Mol H+/litre
- pH 1 -> 0.1 Mol/l H+
- pH 2 -> 0.01 Mol/l H+
- pH 3 -> 0.001 Mol/l H+
- pH 4 -> 0.0001 Mol/l H+
- pH 5 -> 0.00001 Mol/l H+
I know you want to go from 2.5 to 4 but we are going to keep this simple.
To go from 3 to 4 we need to reduce the concentration of H+ ions from 0.01 to 0.001 mol/l.
Now we can find out that Tartaric Acid 150.09g/mol and yields 2 H+ ions per unit.
If acidity is in grams per litre of TA then if 1 gram of TA yields (1/150)*2 mol of H+, which is 0.013mol of H+, then one ppt 0.013mol/litre.
Increasing pH3 to 4 requires a reduction of 0.009 mol per litre H+ so I guess this would need 0.009/0.013 ppt, which is 0.69 ppt.
Therefore you would want 0.69/1.5 -> g/gal to increase pH from 3 to 4
Which is roughly 0.5 g/gallon.
I am not entirely certain that is right.
I know ideal range from a google search - "Preferred titratable acidity levels are 7 - 9 g/L for white wines, and 6 - 8 g/L for red wines"
I would suggest adding 0.2g/gal, wait 24H and test again, then 0.2g and checking over a few days how the pH changes.
If anyone has some good references for this rather than me going of 20 year old chemistry and some googled bits and pieces please add on, also if anyone notices any glaring holes in my working please drop a comment below.