Carbonic Acid Titration with Sodium Hydroxide
The two pKa values for carbonic acid are given as 6.37 and 10.25, so you may want to pick the midpoint between these (pH 8.31) as your equivalence point. At that pH, a tiny amount of base will change the pH a lot, so the amount of base needed should not depend strongly on the exact value you select for the equivalence point. But the pH range is rather narrow, so I would recommend that you use a pH meter instead of a visual indicator for the titration.