Henderson–Hasselbalch Equation (Weak Base Buffer)
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Run Henderson and Hasselbalch's argument on the base side and everything mirrors: pOH is set by the base's pKb plus the logarithm of the conjugate-acid-to-base ratio. An ammonia buffer (pKb 4.75) holding 0.200 M NH₄⁺ against 0.100 M NH₃ has pOH = 4.75 + log₁₀(2) = 5.05, so at 25 °C its pH is 14 − 5.05 = 8.95 — comfortably alkaline, which is why ammonia/ammonium buffers are the standard choice for EDTA titrations of calcium and magnesium hardness.
The one thing to keep straight is which species goes on top. In the acid form the base is the numerator; here the conjugate acid is, because adding more BH⁺ makes the solution less basic. Get it upside down and your answer is wrong by twice the log term. Many chemists sidestep the whole issue by converting pKb to pKa (pKa = 14 − pKb at 25 °C) and using the acid form throughout — for ammonia that gives pKa(NH₄⁺) = 9.25, and 9.25 + log₁₀(0.100/0.200) = 8.95, the same answer by a different road.
- = pOH of the buffer
- = pKb of the weak base
- = Conjugate acid concentration
- = Weak base concentration
- pOH of the buffer — pH and pOH Relation, Henderson–Hasselbalch Equation (Weak Acid Buffer)
- pKb of the weak base — pKb from Base Dissociation Constant, Henderson–Hasselbalch Equation (Weak Acid Buffer)
- Conjugate acid concentration — Henderson–Hasselbalch Equation (Weak Acid Buffer), pH of a Weak Acid from Ka
- Weak base concentration — Henderson–Hasselbalch Equation (Weak Acid Buffer), Molarity (C = n/V)