Acidic Buffer
A buffer made from a weak acid and one of its salts, giving a pH below 7.
Formula / equation
pH = pKa + log([salt]/[acid])
Unit
Dimensionless
In depth
The weak acid supplies protons to neutralise any added base while its conjugate anion mops up any added acid. The buffer works best when the two components are present in roughly equal amounts, at which point the pH equals the acid's pKa.
Examples in the real world
An ethanoic acid and sodium ethanoate mixture buffering at pH 4.76; a citrate buffer in soft drinks.