Buffer action (3.1.12.6)

What is a buffer solution and what are the different types?

Buffer solution - a solution that resists changes in pH when small amounts of acids or alkalis are added

There are two types of buffer solution:

• Acidic - keep solution below 7 pH

• Basic - keep solution above 7 pH

How can an acidic buffer be made?

Route 1:

An acidic buffer can be made from a mixture of a weak acid and one of its salts (HA and A^-)

Route 2:

An acidic buffer can be made by mixing an excess of a weak acid with a strong alkali, resulting in a mixture of HA and A^-

In acidic buffers the [acid] and [salt] are much higher than [H⁺]

How can a basic buffer be made?

Route 1:

A basic buffer can be made from a mixture of a weak alkali and one of its salts (e.g. ammonia and ammonium chloride)

Route 2:

A basic buffer can be made by mixing an excess of a weak alkali with a strong acid

In basic buffers the [base] and [salt] are much higher than [OH^-]

How do acidic buffer solutions work?

HA ←→ H⁺ + A^-

If a little H⁺ is added:

The added H⁺ is removed by the reaction with A^- to form HA

[H⁺] increases so the equilibrium shifts to the LHS to reduce [H⁺]

[A^-] falls slightly and [HA] rises slightly but since [A^-] and [HA] are much greater than [H⁺] the ratio between them remains roughly constant

If a little OH^- is added:

The OH^- reacts with H⁺,forming water, and some HA breaks down to replace that H⁺

[H⁺] decreases so the equilibrium shifts to the RHS to increase [H⁺]

[A^-] rises slightly and [HA] falls slightly but since [A^-] and [HA] are much greater than [H⁺] the ratio between them remains roughly constant

How do basic buffer solutions work?

Weak base (B) + H₂O←→ OH^- + Salt (B⁺)

If a little H⁺ is added:

The added H⁺ is removed by the reaction with OH^- to form water, so some B reacts to replace the OH^-

[B] falls slightly and [B⁺] rises slightly but since [B] and [B⁺] are much greater than [OH^-] the ratio between them remains roughly constant

If a little OH^- is added:

The OH^- reacts with B⁺, forming B

[B] rises slightly and [B⁺] falls slightly but since [B] and [B⁺] are much greater than [OH^-] the ratio between them remains roughly constant

What are the applications of buffer solutions?

Buffer solutions are used to control the pH of blood:

• In humans HCO₃^- ions act as a buffer to keep the pH of the blood between 7.35 and 7.45

• Body cells produce CO₂ during aerobic respiration

• This CO₂ will combine with water in the blood to form a solution containing H⁺ ions

CO₂ (g) + H₂O (l) ⇌ H⁺ (aq) + HCO₃^- (aq)

• If the concentration of H⁺ isn’t regulated the blood pH would drop and cause acidosis which can lead to coma

• It acts as a basic buffer

How can the pH of a buffer be calculated?

If the buffer is made using HA and A^-:

1) Work out moles of HA/B and A^-/B⁺ added

2) Determine [HA] and [A^-]

3) Rearrange and use K_a = [H⁺][A^-]/[HA] to find H⁺

4) pH = -log[H⁺]

If buffer is made using excess of weak alkali/acid and strong acid/alkali:

1) Work out moles of HA/B and OH^-/H⁺ (Steps will be using HA and OH^-)

2) Use ICE table to work out excess moles of HA and A^- (all OH^- must be used as HA must be in excess)

3) Work out [HA] and [A^-]

4) Rearrange and use K_a = [H⁺][A^-]/[HA] to find H⁺

5) pH = -log[H⁺]

If H⁺ or OH^- is added to the buffer use an ice table

If water is added pH stays the same as [salt]/[acid] is unchanged