Buffer solutions

• Buffer- a solution that minimises changes in pH when small amounts of acid or base are added. They can’t cope with larger amounts of acid/base.

  • They’re made by setting up an equilibrium between a weak acid and its salt or it‘s conjugate base.
• Acidic buffer- a solution of a weak acid and it’s salt.
• Alkaline buffer- a solution of a weak base and it’s salt.
• Making a buffer from a weak acid and the salt of its conjugate base:

  • a small amount of the acid will dissociate, but some will remain undissociated.
  • The salt will fully dissociate to form the conjugate base of the acid in the solution.
  • So the solution will contain a mixture of a weak acid and its conjugate base, and can act as an acidic buffer.

  
• Making a buffer from an excess of weak acid and some strong alkali:

  • If you mix an excess of a weak acid with a small amount of a strong alkali then some of the acid is neutralised to make a salt, but some is left un-neutralised.
  • The reaction mixture would then contain a weak acid and its conjugate base, and so would act as an acidic buffer.

  
• How an acidic buffer works:

  • The buffer solution contains lots of undissociated acid (HA), lots of the acid's conjugate base (A-) and enough H+ ions to make the solution acidic.
  • When you alter the concentration of H+ or OH- ions in the buffer solution the equilibrium position between the conjugate pair moves to counteract the change (Le Chatelier's principle).
  • The conjugate base can react to mop up any excess H+ ions and shift the equilibrium to the left. The conjugate acid can dissociate and release H+ ions if there's too much base around and shift the equilibrium to the right.

  
• How a buffer resists an Acid:

  • If you add a small amount of acid the H+ concentration increases.
  • Most of the extra H+ ions join with A- ions to form HA. This shifts the equilibrium to the left, reducing the H+ concentration to almost its original value. So the pH doesn't change much.
• How a buffer resists a Base:

  • If a small amount of base is added to the buffer solution, the OH- concentration increases. Most of the extra OH- ions react with H+ ions to form water — removing H+ ions from the solution.
  • This causes more of the weak acid, HA, to dissociate to form H+ ions - shifting the equilibrium to the right. There's no problem doing this as there's loads of spare undissociated HA molecules. The H+ concentration increases until it's close to its original value, so the pH doesn't change much.
• The ions involved in buffer action in the blood:

  • Carbonic acid dissociates into H+ ions and HCO3- ions

  H2CO3 (aq) <=> H+ (aq) + HCO3- (aq)
• The carbonic acid-hydrogencarbonate buffer system (1)

  • If the concentration of H+ rises in blood, then HCO3- ions will react with the excess H+ ions, and the equilibrium will shift to the left, reducing the H+ concentration to almost its original value. This stops the pH of blood from dropping.
  • If the concentration of H+ ions falls in blood, then more H2CO3 molecules will dissociate, and the equilibrium will shift to the right, increasing the H+ concentration to almost its original value. This stops the pH of blood from rising.
• The carbonic acid-hydrogencarbonate buffer system (2)

  • The levels of H2CO3 are controlled by respiration.
  • By breathing out CO2, the level of H2CO3 is reduced, as it moves this equilibrium to the right:

  H2CO3 (aq) <=> H2O (l) +CO2 (aq)

  • The levels of HCO3- are controlled by the kidneys, with excess being excreted in the urine.
• To calculate the pH of buffers you need to make the following assumptions:

  • The salt of the conjugate base is fully dissociated, so the equilibrium concentration of A- is the same as the initial concentration of the salt.
  • HA is only slightly dissociated, so its equilibrium concentration is the same as its initial concentration.
• Calculating the pH of a buffer:


  1. Write out the expression for the Ka of the weak acid.
  2. Rearrange the equation to give an expression for [H+]. (Kacid/salt)
  3. Substitute the value for Ka and the equilibrium concentrations of the acid and salt into the equation.
  4. Solve the equation to find a value for [H+].
  5. Substitute your value for [H+] into the pH equation.

  
• Calculating the pH of a buffer when given volumes and concentrations added:

  1. Use moles=concentration x volume
  2. Find the concentrations of the each solution by dividing those moles by the total volume
  3. The find the pH of the buffer like normal
• Finding the equilibrium concentrations when calculating the pH of a buffer:

  1. Write out the equation for the neutralisation reaction:

  acid + base → salt + water.

  1. Calculate the number of moles of acid and base at the start of the reaction using the volumes and concentrations given in the question.
  2. Use the molar ratios in the equation to work out the moles of acid and salt left at the end of the reaction.
  3. Calculate the concentration of the acid and salt in the buffer solution by dividing by the total volume of the solution.
  4. Calculate the pH.

  
• Calculate pH: