pKₐ of Acetic Acid

A pK_a is a pH: it is the pH at which an acid is exactly half dissociated, and Henderson–Hasselbalch says pH = pK_a + log([A⁻]/[HA]). At pH 4.756 an acetate buffer is a 50:50 mixture, which is also where it resists added acid or base most strongly. Move one unit either side and the ratio is 10:1, the buffer capacity has collapsed, and the useful window has closed — hence the rule of thumb to pick a buffer whose pK_a is within one unit of the target pH.

Lawrence Henderson wrote the underlying relation in 1908 while studying how blood holds its pH, and Karl Hasselbalch recast it in logarithmic form in 1917 once Sørensen had introduced the pH notation. Acetic acid's own pK_a of 4.76 is worth memorising as a yardstick: formic acid is stronger at 3.75, carbonic acid weaker at 6.35, and where an unfamiliar acid falls relative to acetate tells you immediately how it will behave.