Solubility and pH

pH formula

pH = -Log [H+] or Log 1/[H+]

Strong acids and bases

Substances that completely dissociate in aqueous solution without forming equilibrium.

Weak acids

Substances that partially dissociate in water due to lower capacity to donate protons.

Weak bases

Substances that partially dissociate in water due to lower capacity to accept protons.

Bronsted-Lowry definition of acids

Acids are proton donors.

Bronsted-Lowry definition of bases

Bases are proton acceptors.

Acid dissociation constant (Ka)

The equilibrium constant of a weak acid in its ionised and unionised forms.

pKa definition

The negative logarithm of the acid dissociation constant (pKa = -log Ka).

Henderson-Hasselbalch equation for acids

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

Acidic drug ionisation and pH

Solubility and ionisation of acidic drugs increase above their pKa.

Basic drug ionisation and pH

Solubility and ionisation of basic drugs increase below their pKa.

Intrinsic solubility (So)

The solubility of an ionisable drug in its unionised form. Solubility of drugs greatly increases in its dissociated charged form.

Relationship between pKa and solubility

Log D vs Log P

Log D accounts for pH-dependent partitioning, whereas Log P does not.

Amphoteric drug

A drug that exhibits both acidic and basic properties.

pKa at 50% ionisation

The pKa is the pH at which a drug is 50% ionised and 50% unionised.