Fluctuation in pH reading on pH meter I was conducting a titration of a saturated $\ce{Ca(OH)_2}$ solution against $\ce{HCl}$ using a pH meter (the kind with a glass electrode). At the beginning of the titration, the pH reading would drop a bit every time I added a drop of $\ce{HCl}$. However, near the equivalence point, when I added a drop of $\ce{HCl}$, the pH reading would drop rapidly, but then it would rise again at a steady pace for a few minutes. Why does the pH rise again? My teacher said that the pH meter's membrane is somehow becoming "saturated" with the solution, after being left in the solution for awhile. But what's actually going on in terms of the ions and the membrane?
Your teacher gave you a poor answer.

You are observing three different things. 

(1) The solution isn't instantly homogeneous when HCl is added. It takes a couple of seconds of stirring to mix it thoroughly. 

(2) The calcium hydroxide solution initially has considerable buffer capacity. As the solution gets less basic then a small amount of HCl can change the pH by a relatively large amount. So a drop of HCl in 50 ml of 0.1 molar calcium hydroxide won't change the pH much, but a drop of HCl in a  neutral solution at pH 7 would change the pH by quite a bit. 

(3) The electrode and meter have a response time. So if you could move the pH electrode instantly from an acid solution to a basic solution it would take the electrode and meter themselves a few seconds to equilibrate. (The pH electrode has a hydrated silica layer that forms the boundary on the surface which takes time to equilibrate. The pH meter itself will have some RC time constant so that electrical noise is minimized.)