Buffer solutions

what is a buffer solution?

a buffer is a solution that is resistant to changes in pH on the addition of small amounts of acid or base

what are buffers made of

• A buffer can consists of weak acid - conjugate base or weak base - conjugate acid

how a buffer works eg aqueous mixture of ethanoic acid and sodium ethanoate

• Ethanoic acid is a weak acid and partially ionises in solution to form a relatively low concentration of ethanoate ions

• Sodium ethanoate is a salt which fully ionises in solution

• There are reserve supplies of the acid (CH₃COOH) and its conjugate base (CH₃COO^-)

  • The buffer solution contains relatively high concentrations of CH₃COOH (due to partial ionisation of ethanoic acid) and CH₃COO^- (due to full ionisation of sodium ethanoate)

• In the buffer solution, the ethanoic acid is in equilibrium with hydrogen and ethanoate ions

buffer action equation

1) The buffer equation: CH3COOH—> ←— CH3COO- + H+

2) Raising the pH (adding OH-)

CH3COOH + OH- —> CH3COO- + H2O

The OH- ions are mopped up by the undissociated acid. The equilibrium of the buffer (Eqn 1) moves to the right. and more CH₃COOH molecules ionise to form more H⁺and CH₃COO^- until equilibrium is re-established.

• As there is a large reserve supply of CH₃COOH the concentration of CH₃COOH in solution doesn’t change much when CH₃COOH dissociates to form more H⁺ ions

  • As there is a large reserve supply of CH₃COO^- the concentration of CH₃COO^- in solution doesn’t change much

  • As a result, the pH remains reasonably constant

3) Lowerign the pH (adding H+)

CH3COO- + H+ —→ CH3COOH

The H+ ions are mopped up by the conjugate base/salts ions

The eqm of the buffer equation moves to the left.

• As there is a large reserve supply of CH₃COO^- the concentration of CH₃COO^- in solution doesn’t change much as it reacts with the added H⁺ ions

• As there is a large reserve supply of CH₃COOH the concentration of CH₃COOH in solution doesn’t change much as CH₃COOH is formed from the reaction of CH₃COO^- with H⁺

• As a result, the pH remains reasonably constant

for a buffer to work effectively, two assumptions are made:

• almost all of the A- ions come from the salt

• Almost all of the weak acid, HA molecules remain undissociated. Remember for buffers (H+) does not equal (A-)

what are biological buffers

the buffer maintains a steady pH in biological systems

the environment require a narrow pH range, so that enzymes work efficiently

why buffer needed in blood

• the pH of blood must be maintained in the range pH 7.35-7.45 using buffer HCO3-

• Body cells produce CO₂ during aerobic respiration

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

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

• this ensures oxygen is delivered efficiently to the tissues

how HCO3- ensures pH of blood remains constant

• If there is an increase in H⁺ ions

• The equilibrium position shifts to the left until equilibrium is restored

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

• This reduces the concentration of H⁺ and keeps the pH of the blood constant

• If there is a decrease in H⁺ ions

  • The equilibrium position shifts to the right until equilibrium is restored

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

• This increases the concentration of H⁺ and keeps the pH of the blood constant

what happens when pH of blood becomes too low < 7.2

• acidosis: lactic acid production/ excess acidic products. Lead to kidney/lung disease

what happens when pH of blood too high > 7.6

• alkalosis. Dehydration/sickness and diarrhoea

• lung disease/ embolisms and meningitis

why greater proportion of conjugate base is needed

• Most substances released into the blood following respiration are acidic, so a greater proportion of the conjugate base is needed

why buffers used in food?

buffers are used to maintain pH. which slows microorganisms multiplying and spoiling food

Bacteria and moulds can produce waste products that are poisonous

buffers: citric acid/ sodium citrate in jam

what is buffer capacity?

the amount of acid or base required to significantly change the pH of the food (or buffer solution)

The more protein there is in the food, the higher the buffer capacity

• This is because the amino acids present have both acidic and basic properties

• This means it takes longer for the pH of the food to change enough for the bacteria to begin to multiply