Purification of water with ferric chloride
If I'd state that the pH drops, would you know why?

If you dissolve $\ce{FeCl3}$ in water, a hexaqua complex is formed. These complexes are known as cation acids:

$$\ce{[Fe(H2O)6]^{3+} + H2O -&gt; [Fe(OH)(H2O)5]^{3+} + H3O+}$$