Buffers :(

Hi guys I need help understanding buffers made from a weak acid and a salt. In my book it says to make up this buffer you need a large "reservoir" of the weak acid and salt (conjugate base) to stop any substantial change in pH. I really dont understand why you need a large reservoir.

Because a small reservoir would be less effective.

When you add an acid/alkali, you need a large amount of HA or A- to react with the added OH-/H+ so that the [H+] doesn't change by much and the pH change is resisted.

Imagine if you had a load of HA, but only a little bit of A-. If you added some OH-, then more HA could dissociate and replace the H+ lost and the pH wouldn't change by much. BUT, if you added some H+, there is only so much A- that could react (making HA) and the [H+] would increase and pH would drop quickly.

HA <-> H+ + A-

You can consider the OH- reacting with the H+, lowering [H+]. OR you can consider the OH- reacting with the HA, then the Le C.P. says the equilibrium will go to the LHS, lowering [H+]