For my school lab, I'm determining the solubility of carbon dioxide at different temperatures. I will determine the amount of dissolved carbon dioxide by titrating carbonated water with sodium hydroxide. I'm confused as to how I can determine the pH at equivalence point (for indicator)? I don't know the concentration of the carbonated water so how would I go about determining the appropriate indicator?
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.
The pH would be basic at the equivalence point. This makes the standard phenolphthalein an even more ideal choice. It goes from clear to purple at around a pH of 8.