Henderson-Hasselbalch equation
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
PH = pKa + log([A⁻]/[HA]); buffers function best within pH = pKa ± 1.
Henderson-Hasselbalch equation in plain English
A buffer is a mix that resists changes in acidity, and this equation just predicts how acidic (the pH) that mix will be. In plain terms, pH depends on the ratio of the 'used-up' acid form to the 'reserve' base form: pH = pKa + log([A^-]/[HA]). The neat takeaway is that when those two forms are roughly equal, the buffer is at its strongest and can soak up added acid or base without the pH swinging much — which is why your blood keeps its pH almost perfectly steady. It works well only within about one pH unit above or below the acid's own pKa value.
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