Potash alum is a double salt, its aqueous solution shows the characteristics of :

Potash alum in aqueous solution has an acidic pH as a result of cationic hydrolysis. A potassium and aluminium salt with sulphate is known as potash alum. Al³⁺ ions are responsible for the acidity.

 Al³⁺ ions are solvated by water molecules because of their high charge, which draws electron density toward them and reduces it between O and H in the solvating water molecules. As a result, water is more prone to absorb protons.