Henderson-Hasselbalch Buffer
Conjugate-base / acid ratio and moles for a target pH.
Example
You enter
- pKa 7.4
- Target pH 7.4
- Total buffer concentration (M) 0.1
- Total volume (L) 1
You get
- [A-] / [HA] ratio 1
- Fraction base 0.5
- Moles conjugate base 0.05 mol
- Moles acid 0.05 mol
Checked against published worked examples from: LibreTexts Chemistry (South Puget Sound Community College, Chem 121); King Saud University, Biochemistry Department.
Details, formula, and sources
ScopeBundled pKa for common laboratory buffers.
Formula or tablepH = pKa + log10([A-] / [HA]). ratio = 10^(pH - pKa). fraction_base = ratio / (ratio + 1). Moles each side from total buffer concentration * total volume * fraction.
Source and editionHenderson-Hasselbalch equation. First principles. Common laboratory buffer pKa values bundled in BUFFER_PKA (data/lab/buffer-pka.json) with per-entry citation (Good et al. 1966 names the Good's buffers, tabulated at 20 C; their 25 C values per PanReac AppliChem IP-022EN; CRC Handbook 95th ed. for Tris / phosphate / acetate / bicarbonate).
Free accessGood et al. 1966 historical paper free at the journal archive; CRC Handbook is a commercial reference book; the bundled pKa values are public physical constants cited only by name.
What governsVerify protocol against your lab's SOP before pipetting. A miscalculated dilution can ruin a run or a sample.
API field namespKa, target_pH, total_buffer_concentration, total_volume, ratio_base_acid, fraction_base, moles_base, moles_acid
- Temperature pKa values at 25 CCRC Handbook 95th ed.; PanReac AppliChem IP-022EN for HEPES / MES / MOPS / PIPES (Good et al. tabulate at 20 C)
- Activity coefficients ignored (concentrations approximate activities at low ionic strength)approximation