2.5.7

# Buffer Solutions (A2 Only)

Test yourself

## 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 = $\frac{[H^+][A^-]}{[HA]}$
• Rearrange it for the hydrogen ion concentration:
• [H+] = Ka × $\frac{[HA]}{[A^-]}$
• 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.