5.2.7 Buffers

• A buffer solution is a solution which resists changes in pH when small amounts of acids or alkalis are added
  • A buffer solution is used to keep the pH almost constant
  • A buffer can consists of weak acid - conjugate base or weak base - conjugate acid

Ethanoic acid & sodium ethanoate as a buffer

• A common buffer solution is an 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

• When H⁺ ions are added:
  • The equilibrium position shifts to the left as H⁺ ions react with CH₃COO^- ions to form more CH₃COOH until equilibrium is re-established
  • 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

When hydrogen ions are added to the solution the pH of the solution would decrease; However, the ethanoate ions in the buffer solution react with the hydrogen ions to prevent this and keep the pH constant

• When OH^- ions are added:
  • The OH^- reacts with H⁺ to form water

OH^- (aq) + H⁺  (aq) → H₂O (l)

• • The H⁺ concentration decreases
  • The equilibrium position shifts to the right and more CH₃COOH molecules ionise to form more H⁺and CH₃COO^- until equilibrium is re-established

CH₃COOH (aq) → H⁺ (aq) + CH₃COO^- (aq)

• • 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

When hydroxide ions are added to the solution, the hydrogen ions react with them to form water; The decrease in hydrogen ions would mean that the pH would increase however the equilibrium moves to the right to replace the removed hydrogen ions and keep the pH constant

• The pH of a buffer solution can be calculated using:
  • The K_a of the weak acid
  • The equilibrium concentration of the weak acid and its conjugate base (salt)

• To determine the pH, the concentration of hydrogen ions is needed which can be found using the equilibrium expression

• To simplify the calculations, logarithms are used such that the expression becomes:

• Since -log₁₀ [H⁺] = pH, the expression can also be rewritten as:

• This is known as the Hendersen-Hasselbalch equation

Answer

Ethanoic acid is a weak acid that ionises as follows:

CH₃COOH (aq) ⇌ H⁺ (aq) + CH₃COO^- (aq)

Step 1: Write down the equilibrium expression to find K_a

Step 2: Rearrange the equation to find [H⁺]

Step 3: Substitute the values into the expression

   = 1.02 x 10-5 mol dm^-3

Step 4: Calculate the pH

   pH = - log [H+]

   = -log 1.02 x 10-5

   = 4.99

How to make a buffer solution with a required pH

• To make a buffer solution with a pH of less than 7, you need to use a mixture of a weak acid and its conjugate base
• Conversely, you can make a buffer solution with a pH greater than 7 by using a mixture of a weak base and its conjugate acid
• Imagine we want to make a buffer solution with a pH of 5.00 at a temperature of 298K
• This would require a hydrogen ion concentration of

      [H⁺_(aq)] = 1.00 x 10^-5 mol dm^-3

• The hydrogen ion concentration of a buffer solution of a weak acid and its conjugate base is calculated using the formula

• We will use ethanoic acid as our weak acid of choice

      K_a = 1.74 x 10^-5 mol dm^-3

• Substituting our known values into the equation we get

• This gives a value for the ratio of the concentrations of acid and base needed in our buffer solution

• Mixing an equal volume of ethanoic acid with a concentration of 0.575 mol dm^-3 and a sodium ethanoate solution of 1.00 mol dm^-3 would allow us to make this buffer solution
• This would give us a solution with an acid concentration of 0.2875 mol dm^-3 and a salt concentration of 0.500 mol dm^-3

Controlling the pH of blood

• In humans, HCO₃^- ions act as a buffer to keep the blood pH between 7.35 and 7.45
• 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 equilibrium between CO₂ and HCO₃^- is extremely important
• If the concentration of H⁺ ions is not regulated, the blood pH would drop and cause ‘acidosis’
  • Acidosis refers to a condition in which there is too much acid in the body fluids such as blood
  • This could cause body malfunctioning and eventually lead to coma

• If there is an increase in H⁺ ions
• The equilibrium position shifts to the left until equilibrium is restored

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

• 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