Equation and purpose
pH of buffer from conjugate pair ratio; approximations limited.
\[pH = pK_a + \log\frac{[A^-]}{[HA]}\]
pH of buffer from conjugate pair ratio; approximations limited.
REARRANGEMENTS
$[A-]/[HA]=10^{pH-pKa}$
Dimensional check: log ratio ✓
Variables and units
| Symbol | Quantity | Unit | Notes |
|---|---|---|---|
| pH | pH | — | — |
| pKa | pKa | — | — |
| [A-] | [A-] | — | — |
| [HA] | [HA] | — | — |
Conditions and limitations
Approximate; fails at extreme dilution, very low/high pH, when [HA] or [A-] not >>x, activity corrections ignored.
Worked example
pKa=4.76, [A-]=[HA] → pH=4.76.
Henderson–Hasselbalch
pH of buffer from conjugate pair ratio; approximations limited.
Equation
Inline: $pH = pK_a + \log\frac{[A^-]}{[HA]}$$
Variables
| Symbol | Quantity | Unit | Notes |
|---|---|---|---|
| pH | — | — | |
| pKa | — | — | |
| base concentration | M | — | |
| acid concentration | M | — |
Conditions and limitations
Approximate; fails at extreme dilution, very low/high pH, when [HA] or [A-] not >>x, activity corrections ignored.
Rearrangements
Dimensional check: log ratio ✓
Worked example
pKa=4.76, [A-]=[HA] → pH=4.76.
Steps: write equation → substitute with units → check dimensional consistency → report with correct significant figures.
Common errors
- Using outside buffer region
- Ignoring dilution effects
Related resources
- acids-bases-buffers topic
- Calculators — linked only when a real calculator implements this formula.
- Constants — sourced constants with version when values are required.
Sources
- NIST Physical Constants 2022 — CODATA.
Reviewed 2026-08-21 by Editorial Team; due 2027-08-21.
Common errors
- Using outside buffer region
- Ignoring dilution effects
RELATED FORMULAS
C
Chemistry Fundamentals Editorial TeamAuthor
E
Editorial Review BoardReviewer
Editorial Team — transparent review placeholder
Reviewed August 21, 2026
SOURCES
- [nist-2022]NIST Physical Constants 2022 (2022). NISThttps://physics.nist.gov/constants