Buffer solutions
Buffer solutions
- A buffer resists a change in pH when a little acid or alkali is added.
- It is made from a weak acid and its conjugate base.
- It is vital in living things — like your blood.
Practice
A buffer solution is made from:
A weak acid and its conjugate base (e.g. ethanoic acid + sodium ethanoate) make a buffer.
Practice
A buffer keeps the pH almost constant when a small amount of acid or alkali is added.
That is exactly what a buffer does — it resists changes in pH.
How a buffer works
It holds a store of both partners:
- added $\text{H}^+$ is removed by the conjugate base: $\text{CH}_3\text{COO}^- + \text{H}^+ \rightarrow \text{CH}_3\text{COOH}$.
- added $\text{OH}^-$ is removed by the weak acid: $\text{CH}_3\text{COOH} + \text{OH}^- \rightarrow \text{CH}_3\text{COO}^- + \text{H}_2\text{O}$.
So the pH barely changes.
Practice
When a little acid (H⁺) is added to a buffer, it is removed by:
The conjugate base mops up added H⁺; the weak acid mops up added OH⁻, so the pH barely changes.
Finding the pH and uses
- Put the concentrations of the acid and its salt into the $K_a$ expression.
- Buffers matter in life: $\text{HCO}_3^-$ keeps the pH of blood close to 7.4.
Practice
In blood, the pH is kept close to 7.4 by the buffer ion:
The hydrogencarbonate ion buffers blood, keeping its pH around 7.4.
You've got it
Key idea
- a buffer = weak acid + its conjugate base; it resists pH change
- added $\text{H}^+$ is mopped up by A⁻; added $\text{OH}^-$ by HA
- find its pH from the $K_a$ expression with the acid and salt concentrations
- blood is buffered by $\text{HCO}_3^-$ near pH 7.4