pH and pKa

pH and pKa formula

pH
= pKa + log([proton acceptor]/[proton donor])

For acid,

pH = pKa + log([A-]/[HA])

For base,

pH = pKa + log([B]/[HB+])

Derivation of pH and pKa formula

Dissociation of acid:
* HA ----> H+ + A-

Ka = acid dissocation constant
= [H+][A-]/[HA]

Then,

Log Ka = log([H+][A-]/[HA])
= log([A-]/[HA]) + log[H+]

Then,

-log[H+] + log Ka = log([A-]/[HA])

Thus,

pH-pKa = log([A-]/[HA])

NB:

• pH = -log[H+]
• pKa = -logKa