pH and Buffers

In solutions that are NOT buffered When an acid is added: the pH decreases. When a base is added: the pH increases.

A buffer allows an acid or base to be added to a solution without causing a major change in pH. DOES NOT ALWAYS MAINTAIN IT AT NEUTRAL! Stomach buffers keep the stomach at 2; blood buffers keep blood at 7.35-7.45 Buffers defined

A buffer solution contains a weak acid (HA) and its conjugate base (A-) Both these components remain present in solution because they don’t undergo any reactions that significantly alter their concentration (they rarely react with water & when they react with themselves, the concentrations don’t change) However, they will react with any excess H+ or OH- ions in solution. Structure of a Buffer

Structure of a Buffer

A buffer works by soaking up (reacting with) any excess H+ or OH- depending on what was added. By reacting with the ions, a change in the pH of the system is prevented. Generalized Summary

If a strong acid is added to a buffer, the weak base will react with the H+ from the strong acid to form the weak acid (HA): H+ + A-  HA The H+ gets absorbed by the A- instead of reacting with water to form H3O+ (H+), so the pH changes only slightly. Adding Acid to a Buffer

Adding a base to a buffer When a strong base is added to a buffer, the weak acid (HA) will give up its H+ in order to transform the base (OH-) into water (H2O) and the conjugate base (A-): HA + OH-  A- + H2O Since the added OH- is consumed by the reaction, the pH will change only slightly Adding a base to a buffer

The ratio of [A-]/[HA] influences the pH ; the actual concentrations of A- and HA influence the effectiveness of the buffer. The more A- and HA available, the less of an effect the addition of acid or base will have on the pH. (The buffer will work longer). Buffer Capacity