what is a buffer
a buffer is a solution that minimises changes in pH when small amounts of acid or base are added
acid buffer
acid buffers has a pH of less than 7
how to make an acidic buffer (1)
1) mix a weak acid with the salt of its conjugate base
- salt fully dissociated into its ions when it dissolves
- weak acid only slightly dissociates
how to make an acidic buffer (2)
2) mix an excess of weak acid with a strong alkali
- all the base reacts with the acid
- weak acid was in excess so there’s still some left in solution once all the base has reacted, this acid slightly dissociates
acid buffer equilibrium
CH3COOH <-> H+ + CH3COO-
- lots of undissociated weak acid (HA)
- lots of the acids conjugate base (A-)
- enough H+ ions to make the solution acidic
what happens if you add a small amount of acid
- H+ concentration increases
- most of extra H+ ions combine with CH3COO- ions to form CH3COOH
- this shifts equilibrium to the left reducing the H+ concentration close to original value
- pH doesn’t change much
what happens if you add a small amount of alkali
- the OH- concentration increases
- most of the extra OH- ions react with H+ ions to form water to remove H+ ions from the solution
- this causes more CH3COOH to dissociate to form H+ ions which shifts equilibrium to the right
- H+ concentration increases until close to original value so pH doesn’t change much
buffer solutions in blood
- blood needs to be kept between 7.35 and 7.45
- pH controlled by using a carbonic acid-hydrogen carbonate buffer system
- levels of H2CO3 are controlled by respiration, CO2 is breathed out and HCO3 is reduced so equilibrium shifts right
- HCO3- levels are controlled by the kidneys with excess being excreted in urine
equilibrium reactions in blood
H2CO3 <-> H+ + HCO3-
H2CO3 <-> H2O + CO2
how to calculate pH of a buffer solution
- write expression for Ka of the weak acid
- rearrange expression and put data in to calculate [H+]
- convert [H+] to pH
pH= -log10[H+]
to calculate pH of a buffer, 2 assumptions
- salt of the conjugate base is fully dissociated, equilibrium concentration of A- is the same as the initial concentration of the salt
- HA is only slightly dissociates so its equilibrium concentration is the same as its initial concentration
