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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])##