What are the two components of a buffer system? How does a buffer system work to hold pH nearly constant?

Answer 1

Buffer solutions are solutions which resist change in hydronium ion and the hydroxide ion concentration (and consequently 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.

When writing about buffer systems they can be represented as salt of conjugate base/acid, or base/salt of conjugate acid. It should be noted that here buffer solutions are presented in terms of the Brønsted-Lowry notion of acids and bases, as opposed to the Lewis acid-base theory (see acid-base reaction theories). Omitted here are buffer solutions prepared with solvents other than water.

Answer 2

Buffer systems are either a weak acid and a alkali-metal salt of the acid or a weak base an ionic salt of the base. Generally, equal moles of each ingredient are added to the solution. A common example is acetic acid and sodium acetate. The equilibrium is given by:
[H+][OAc-]/[HOAc] = 1.8x10^-5.
If equal moles of acetate and HOAc are added, the [H+] is approximately 1.8x10-5.
When relatively SMALL amounts of a strong acid is added, the large amount of salt reacts with strong acid to convert it to more of the weak acid, and if strong base is added, the large amount of acid reacts with strong base to convert it to more of the salt. The changes in concentration are relatively slight compared to the those prior to the addition, which keep the pH nearly constant.