14.7Buffers
Section · module m68808
What this needs
Learn these first — this section will not make sense without them.
pKa = −log Ka is itself a logarithm evaluation, needed before the Henderson-Hasselbalch equation can be used numerically, e.g. pKa = 6.4 for carbonic acid in the blood-buffer example.
The Henderson-Hasselbalch derivation takes −log of both sides of [H3O+] = Ka × [HA]/[A⁻] and splits −log(Ka × [HA]/[A⁻]) into −log Ka − log([HA]/[A⁻]), then flips the ratio to +log([A⁻]/[HA]) — log-of-a-product and log-of-a-reciprocal used back to back, and this is one of the sharpest prerequisite edges in the book because the whole equation collapses if either rule is misapplied.
What this touches
Algebra that turns up here. Not a blocker, but this is where you will see it used.
- Applies4.2Linear Functions
pH = pKa + log([A⁻]/[HA]) is a linear function of the quantity log([A⁻]/[HA]) with slope 1 and intercept pKa — recognizing this is what makes it obvious that equal concentrations (ratio = 1, log = 0) give pH = pKa exactly, the fact used repeatedly at titration half-equivalence points.