How do you calculate buffer pH for polyprotic acids?

All buffer consist of a mixture of an acid with its conjugate base. Calculate using the Henderson-Hasselbalch equation, which requires the Ka of the acid and the relative concentrations of acid and base.

This works for polyprotic acids, too.

Example: For a mixture of sodium carbonate and sodium bicarbonate, the bicarbonate is the acid and the carbonate is the base. Use Ka for bicarbonate and the relative concentrations as usual:

##pH = pKa + log([A-]/[HA])##