Buffer Solutions (A2 Only)
Chemistry: AQA A Level, topic 2.5.7
Buffer Solutions
Buffer solutions oppose small changes in pH.
What is a buffer solution?
- A buffer solution is a solution that opposes pH changes.
- This means that if you add acid, the buffer solution removes it to keep a constant pH.
- In reality, they don't keep a completely constant pH, but they reduce the size of any changes.
How does a buffer solutions work?
- Buffers work through changes in equilibria.
- An acidic buffer will contain a weak acid and a salt of the weak acid.
- E.g. methanoic acid and sodium methanoate.
- The important equilibrium is:
- CHOOH ⇌ CHOO- + H+
- As you have sodium methanoate, you have lots of CHOO-.
- According to le Chatelier’s principle, this will remove lots of H+ from solution to reduce the concentration of CHOO-.
- This means you have a lot of un-ionised methanoic acid
Adding acid to an acid buffer
- If you add acid to this acid buffer solution, then the methanoate ions will react with the hydrogen ions and remove them.
- This will reduce any change in pH.
Adding a base to an acid buffer
- If you add a base to the acid buffer solution, the hydrogen ions will react with it and neutralise it.
- The un-ionised methanoic acid will ionise to produce more H+ ions and this will reduce the pH change.
Basic buffers
- A basic buffer has a pH above 7 and works on a similar principle.
- A good example is ammonium hydroxide.
- The equilibrium for this reaction is:
- NH3 + H2O ⇌ NH4+ + OH-
- Adding base shifts the equilibrium left to remove base.
- Adding acid shifts the equilibrium right to remove the acid.
Calculations of Buffer Solutions
Buffer solutions have a specific pH, which can be calculated.
Assumptions
- All you need to know to calculate the pH of a buffer solution is its Ka.
- You need to make a couple of reasonable assumptions:
- That the salt is fully dissociated.
- That the acid is pretty much un-ionised.
Method
- Write down the equation for Ka:
- Ka =
- Rearrange it for the hydrogen ion concentration:
- [H+] = Ka ×
- We assume the salt is fully dissociated, so [A-] is just the inital salt concentration.
- We assume the [HA] is un-ionised, so that’s just the initial acid concentration.
Test yourself on Buffer Solutions (A2 Only)
What happens when you add an acid to an acid buffer?
What do you need to know to calculate the pH of a buffer solution?