 I've got another sample of the .1 molar acetic acid, .1 molar sodium acetate buffer. I'm now going to add one molar HCl. I've added one drop and the pH has changed only slightly, whereas with the water, the pH dropped rapidly, two drops. I've added 10 drops and the pH has only dropped slightly. A buffer does not maintain absolutely constant pH. What it does is moderate the change in pH. I've now added 20 drops of one molar HCl and the pH is only down now to 4.63. Eventually, if I add enough strong acid, I will neutralize all the conjugate base and I will destroy the buffer and the pH will drop. After adding 30 drops of one molar HCl, the pH has started to drop rapidly. I have a new solution of the .1 molar acetic acid, .1 molar sodium acetate buffer solution. I'm now going to add sodium hydroxide to it. Notice that after adding about three drops of sodium hydroxide, the pH is essentially unchanged. The pH would have risen very rapidly with pure water. After 10 drops, the pH is slightly higher. Again, the buffer does not maintain absolutely constant pH. It just moderates or keeps the pH change relatively small. Notice that the pH is going up as I add the strong base. After 20 drops, it's up to 5.3. It will maintain relatively constant pH until I add enough sodium hydroxide to use up all the weak acid. After about 45 drops, the pH is starting to rise more rapidly.