Buffers

• A buffer is a solution that resists changes in pH when small amounts of acid or base are added, or it’s diluted.

• Acidic buffers have a pH lower than 7 and consist of a mixture of a weak acid and one of its salts.

• Basic buffers have a pH higher than 7 and consist of a mixture of a weak base and one of its salts

Acidic buffers

• Rely on the weak acid equilibrium

• The [HA] is high and it is assumed that almost all HA molecules don’t dissociate

• The [A^-] is high when the salt is added to the solution and it is assumed that almost all A^- ions have come from the added salt (not HA dissociation)

• When a small amount of acid is added, equilibrium shifts to the left and [A^-] is high and reacts with the added H⁺ ions so [H⁺] hardly changes

• When a small amount of base is added, OH^- will react with H⁺ shifting equilibrium to the right. There is a high [HA] and so some can dissociate so [H⁺] hardly changes

Everyday applications of buffers

• Shampoos have a slightly acidic buffer for shiny hair

• Blood needs to be kept at a constant pH just above 7 so enzymes don’t denature

• Buffers are used in food product as pH changes cause by bacteria or fungi can cause food to deteriorate

Buffer calculations

• To calculate the pH of a buffer solution, the K_a expression is used

• For buffers [H⁺] ≠ [A^-]

• [H⁺] = K_a[HA] / [A^-]

Techniques and procedures to measure pH

• A pH probe can be used to find the pH of a solution

• After using the pH probe in one solution, rinse it with distilled water before testing another as some acid/base could remain on the pH probe