Determining Mono/Diprotic Acids?

Question

How would one (in theory) be able to verify or test to determine the monoprotic or diprotic nature of an unknown acid without a pH probe for a titration (assuming the acid is not any other polyprotic)?

This is probably one of those "duh" questions, but I can't think of anything.

Answer 1

Using an indicator of small pKin value (eg methyl orange) will give you a certain titre. If using an indicator with a much higher pKin value (eg phenolphthalein) gives you a titre which is double, then it's certain that the acid was at least diprotic. If the titres are the same, it was monoprotic.