how do buffers work?? for example i have 2 solutions. solution 1: Na2HPO4 and solution 2: NaH2PO4..?

Question

what will happen to the pH of each solution,assuming that the initial pH( without the HCl or NAOH) is 8,when i add HCl to solution 1 and NaOH to solution 2?

Answer 1

"Buffer solutions are solutions which resist change in hydronium ion and the hydroxide ion concentration (and consequent pH) upon addition of small amounts of acid or base, or upon dilution. Buffer solutions consist of a weak acid and its conjugate base (more common) or a weak base and its conjugate acid (less common). The resistive action is the result of the equilibrium between the weak acid (HA) and its conjugate base (A-):

HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)
Any alkali added to the solution is consumed by hydronium ions. These ions are mostly regenerated as the equilibrium moves to the right and some of the acid dissociates into hydronium ions and the conjugate base. If a strong acid is added, the conjugate base is protonated, and the pH is almost entirely restored. This is an example of Le Chatelier's principle and the common ion effect. This contrasts with solutions of strong acids or strong bases, where any additional strong acid or base can greatly change the pH."

from http://en.wikipedia.org/wiki/Buffer_solution

So in your case

Na2HPO4 + H2O <=> PO4(-3)+ H3O(+) + 2Na(+)
Adding a small amount of HCl will not decrease the pH much because the H+ from HCl will bind to PO4(-3) forming HPO4(-2)

NaH2PO4 + H2O <=> HPO4(-2)+ H3O(+) + Na(+)
Adding a small amount of NaOH will result in H3O+ being neutralized but this will shift the equilibrium to the right, more H2PO4(-) will dissociate and replenish the H3O+ that reacted with the base.

Answer 2

A buffer solution is a mixture of two solutions.i.e; a weak acid and salt of it with a strong base or vice versa.And a weak base and salt of it with a strong acid.A buffer sol. has the ability to maintain its pH when a strong acid or base is added to it. This happens because when a strong base is added to it;acid reacts with it & converts it into a weak base which can't alter its pH to an appreciable extent.Reverse is true when a strong acid is added,provided addition is not in large quantity.