Henderson/Hasselbach Derivation">


In a weak acid solution:
K_a = ([H⁺][A¯])/[HA]

Solve for [H⁺]:
[H⁺] = K_a ([HA]/[A¯])

Take the log of both sides:
log ([H⁺]) = log (K_a) + log ([HA]/[A¯])

Multiply by -1:
-log ([H⁺]) = -log (K_a) - log ([HA]/[A¯])

And voila!
pH = pK_a - log ([HA]/[A¯])