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Henderson–Hasselbalch Equation (Weak Base Buffer)

pOH=pKb+log⁡10 ⁣[BH+][B]\mathrm{pOH} = \mathrm{p}K_b + \log_{10}\!\frac{[\mathrm{BH^+}]}{[\mathrm{B}]}
sig figs

Worked example: Ammonia buffer 0.200 M NH4+ / 0.100 M NH3 → pOH 5.0510. Press Try an example to run it live, then adjust anything.

Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value. Change any unit and the answer follows.

Where Henderson–Hasselbalch Equation (Weak Base Buffer) fits

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pH and pOH Relation
In this toolkit

See also

pKb from Base Dissociation Constant: Both use pKb of the weak base.pH and pOH Relation: Both use pOH of the buffer.pOH from Hydroxide Ion Concentration: Both use pOH of the buffer.
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Henderson–Hasselbalch Equation (Weak Base Buffer) explained

[BH+][B]pOHpKb

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.

Henderson–Hasselbalch Equation (Weak Base Buffer)

pOH=pKb+log⁡10 ⁣[BH+][B]\mathrm{pOH} = \mathrm{p}K_b + \log_{10}\!\frac{[\mathrm{BH^+}]}{[\mathrm{B}]}
Where
  • pOH\mathrm{pOH}= pOH of the buffer
  • pKb\mathrm{p}K_b= pKb of the weak base
  • [BH+][\mathrm{BH^+}]= Conjugate acid concentration (M)
  • [B][\mathrm{B}]= Weak base concentration (M)
In this toolkit

The buffer equations

4 formulasHenderson–Hasselbalch in both its acid and base forms, plus the pKa and pKb conversions that feed them — buffer pH from a ratio.

Revised 31 October 2026
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formula.expert. (2026, October 31). Henderson–Hasselbalch Equation (Weak Base Buffer) Calculator. https://formula.expert/solvers/henderson-hasselbalch-base

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