Ionisation

[PHW2:p7]

Acid

(e.g. barbiturate, aspirin)

HA -----> H+ + A-

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

 

Thus,

For acids, (pH - pKa) is proportional to the degree of ionisation

 

Therefore,

High pH --> Weak acid becomes ionised

Low pH --> Weak acid becomes non-ionised

Base

(e.g. local anaesthetics, opioids)

BH+ -----> B + H+

 

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

 

Thus,

For bases, (pH - pKa) is proportional to the degree of non-ionisation

 

Therefore,

High pH --> Weak base becomes non-ionised

Low pH --> Weak base becomes ionised

 

Alternatively

 

[Unprotonated form]/[Protonated form] = 10^(pH - pKa)

 

When pH = pKa
--> Unprotonated form = Protonated form

When pH > pKa
--> Unprotonated form > Protonated form

When pH < pKa
--> Unprotonated form < Protonated form