Well, the endpoint pH depends on the concentration of the bicarbonate . . . Without going into too much of the chemistry, carbonic acid is a weak acid, so the pH of a solution of carbonic acid (which is what you have at the equivalence point) depends on both the Ka (equilibrium constant) and the concentration of the acid. Here is a basic equation (assuming C is greater than about 0.0001M):
[H+] = [-Ka + SQRT((Ka^2) + 4(Ka)(C))]/2
pH = -log[H+]
Ka = equilibrium constant. For H2CO3, Ka=4.46*10^-7
C = concentration of H2CO3 (for this special case, this is CO2 + H2CO3).
If you put this into Excel and plug in values of C between 0.1M and 0.0001M you will find pH ranges from 3.7 to 5.2. Since aquarium water usually has a relatively low kH (<0.008M), the equivalence point should be between 4.2 and 5. Since the titration curve is steep at that point, there won't be much error in using an indicator that changes color at 5 or 4.5.
72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.