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The buffer equation

A buffer is a weak acid and its conjugate base in the same solution. The pH lies near the pKa of the acid and changes little when you add a little strong acid or base. The buffer equation says that pH is pKa plus the base-10 logarithm of base divided by acid. If the two are equal, the logarithm is zero and the pH equals pKa. The buffer then resists the most. Acetic acid has pKa 4.76, the value the problems use. The units in the fraction can be moles or moles per litre, as long as they match.

\mathrm{pH} = \mathrm{p}K_a + \lg\frac{[\mathrm{base}]}{[\mathrm{acid}]}

the buffer equation

Symbols

$\mathrm{p}K_a$	the acid constant on log form
$[\mathrm{base}]$	conjugate base	mol/L
$[\mathrm{acid}]$	weak acid	mol/L

Example

An acetic acid buffer with pKa 4.76, base 0.20 mol/L and acid 0.10 mol/L:

$\mathrm{pH} = 4.76 + \lg(0.20/0.10) \approx 5.06$ .

If the base is ten times the acid, pH rises by 1 from pKa. If the acid is ten times the base, pH falls by 1.

Practise buffers and titration for free →

← Ethanol · A buffer when the ratio is ten →

Part of Chemistry 2: Buffers and titration.