Can anybody tell why does alum lower the pH of water?

Question

What are the characteristics of alum that enable it to lower the pH of water? Thanks in advance!!

Answer 1

Al2SO43 is a salt formed by a weak base Al(OH)3 and a strong acid H2SO4 so its normal that it will be acid and not neutral

Answer 2

The chemical formula for alum is KAl(SO4)2.12H2O

KAl(SO4)2 dissociates in water to make K+, Al3+ and SO4^2-.

K+ is the cation of a strong base and it does not hydrolyze.

Al3+ + 4HOH --> [Al(OH4)]- + 4H+

This lowers the pH, by increasing the concentration of H+

SO4^2- + HOH <==> HSO4- + OH-

This raises the pH, by increasing the concentration of OH-

The net effect is that the production of H+ is greater than the production of OH-, resulting in an overall increase in H+ and a lowering of pH.