[ajax0604]

An online resource I've been reading states that the pH falls rapidly as HCl is added initially but then the rate stabilizes due to a buffer solution being set up between NH3 and NH4+. I've attached the pH curve to this post.
I am not 100% sure why the pH drops quickly then the rate slows down and it would be appreciated if you could tell me if I'm on the right track.

When HCl is added at the beginning, there will be a lot more H+ in solution than there was before resulting in a quick drop in pH.

The H+ reacts with NH3 to produce NH4+ which helps remove some of the H+ in solution so the pH doesn't drop as fast as it did before.
I feel as I 'm not getting the full picture. Thank you.

[Borek]

At the very beginning there is almost no NH₄⁺ in the solution. Even small amounts of HCl added change the concentration of NH₄⁺ rapidly (its concentration grows by almost an order of magnitude during addition of first milliliters of the acid). That means pH changes must be relatively fast as well.

[ajax0604]

Right. Can I ask you another question? After awhile, the concentration of NH4+ builds up which reduces the ionization of NH3 as per the equation NH3 + H2O --> NH4+ + OH-. Is this why the pH drop slows down after the initial quick drop?

[Borek]

The faster the change of the ratio of concentrations, the faster the pH change. Initially concentration of NH₄⁺ grows rapidly, so teh pH changes fast. Later, ration changes slowly, so the pH changes slowly. Close to the end point ratio starts to change rapidly again.