L7 - Buffers

objectives

• The composition and action of a buffer solution.

Buffer Definition

'A solution which resists changes in pH when small quantities of acid or alkali are added'

Acidic Buffers

• Have a pH of < 7
• Can be made using a Weak Acid and it's Sodium or Potassium Salt

Alkaline Buffers

• Have a pH of > 7
• Can be made using a Weak Base and it's Chloride

Uses of Buffers

• Standardising pH Meters
• Buffering Biological Systems (e.g. Blood)
• Maintaining the pH of Shampoos

How Buffers Work - Buffer Action

Buffer action is the term given to show how buffers react with strong acids or bases (small amounts only!) to minimise the pH change - this can be best demonstrated with the following buffer:

Ethanoic Acid and Sodium Ethanoate

Here the Ethanoic acid is the weak acid

Sodium Ethanoate is the weak base

If we add small amounts of HCl or NaOH these should be able to react to prevent the pH from changing drastically

Addition of a Strong Acid:

CH₃COO^- + H⁺ → CH₃COOH

Here we can see that the Strong acid has been converted into a weak acid - this means that the pH will go down but only by a small amount.

The weak base has reacted completely with the strong acid.

Addition of a Strong Base:

CH₃COOH + OH^- → CH₃COO^- + H₂O

Here we can see that the Strong base has been converted into a weak base - this means that the pH will go up but only by a small amount.

The weak acid has reacted completely with the strong base.

Buffer Solutions and Concentration

The concentration of a buffer solution is very important as:

• If the concentration is too low there won't be enough of the weak acid or base to react with the string acid or base