If you have a divalent metal the reaction is
M + 2HCl ==> H2 + MCl2
As M reacts H^+ is used which decreases the acidity and make the pH increase. One reason the increase may be slow at first could be due to a coating of the metal oxide on the surface of the metal. It proceeds at a faster rate as the acid eats into the metal and exposes a purer surface.
Identification of an unknown powdered metal by reaction with HCl:
Why does the pH of the HCl solution initially increase very slowly when an unknown metal is first added to the acid solution, but gradually increase at a faster rate as the reaction proceeds to completion?
2 answers
pH increases exponentially. Thus, smaller amounts of metal will make greater differences in H3O+ present as metal is added.
5 answers