Which form of the Henderson-Hasselbalch equation should be used for a solution containing H₃A and H₂A-?

• A. pH = pK_a2 + log([HA^2-]/[H₂A^-])
• B. pH = pK_a3 + log([H₃A]/[A^3-])
• C. pH = pK_a1 + log([H₂A^-]/[H₃A])
• D. pH = pK_a3 + log([A^3-]/[H₂A^-])

Answer: (C)