Buffer Solution: A buffer solution is a system that minimises pH changes when small amounts of
acid or base added. Buffer solutions contains 2 things…
Buffer solution contains weak acid, HA, to remove added alkali.
Buffer solutions contain conjugate base, A-
, removes added acid.
When no Buffering: When acid and alkalis added to buffer, two components react and used up,
and solution loses its buffering.
Preparing Weak Acid Buffer Solutions: Two methods for preparing…. Name HA and A-.
Preparation from Weak Acid and its Salt: A buffer solution prepared by mixing a solution of
ethanoic acid CH3COOH with a solution of one of its salts e.g. sodium ethanoate CH3COONa.
– Weak Acid: Ethanoic acid is weak acid component in buffer. In water acid partially
dissociates to ethanoate ion CH3COO- in solution, but very small.
– Conjugate Base: Salts of weak acid is source for conjugate base. Salt completely dissociates
in water- CH3COONa -> CH3COO- + Na+
as it’s an ionic compound.
Preparation by Partial Neutralisation of Weak Acid: A buffer solution can also be prepared by
adding aq alkali (e.g. NaOH or NH3) to excess of weak acid.
– Conjugate Base: The weak acid reacts with alkali in neutralisation reaction to form salt of
weak acid therefore forming conjugate base/ salt ions. NAME everything.
– Weak Acid: Some of the excess weak acid remains unreacted so HA present.
In all Q’s name HA and A-. Also write down equilibrium equation with HA as reactant.
Organic carboxylic acids classed as acids, so the conjugate base is COO-.
Equilibrium System: CH3COOH H+ + CH3COO-.
Conjugate Base Removes Added Acid:
– [H+ (aq)] increases, added acid.
– So H+ reacts with A- to form HA. Name A- and HA.
– Equilibrium shifts to the left towards HA, removing most of the H+.
Weak Acid Removes Added Alkali:
– [OH- (aq)] increases, added alkali.
– The small concentration of H+ reacts with OH- (added alkali) to form H2O.
– HA dissociates, shifting equilibrium position to right to restore H+.
NH3 is an alkali which can remove added acid in a buffer. NH3 accepts a proton OR H+ + NH3 OR
H+ + OH-. So equilibrium position shifts right to restore H+.
Equal Concentrations: Buffer most effective at removing added acid or alkali when equal
concentrations of weak acid and its conjugate base. [HA(aq)] = [A-(aq)]
pH Ranges: Different weak acids result in buffer solutions that operate over different pH ranges.
– When [HA(aq)] = [A-(aq)], pH of buffer solution is same as pKa value of weak acid. Ka = [H+
(aq)]
– Also when [HA(aq)] = [A-(aq)], the operating pH value is +/- 1 of the pKa of that pH.
Can then adjust concentration of weak acid and conjugate base to fine tune the pH of buffer
solution.
Therefore the pH of a buffer solution depends on 1) pKa value of weak acid 2) ratio of
concentrations of HA weak acid and A- conjugate base.
Calculating pH of Buffer Solution:
Remember to convert [H+(aq)] to pH using –log.
Calculating pH of Buffer Solution from Preparation
of Weak Acid and its Salt:
– Weak acid (HCCOH) and salt (HCOO-). Only know
concentration of preparation solutions but need
to concentrations of them in buffer solution.
Calculating pH of Buffer Solution from Preparing by
Partial Neutralisation: If mentions an alkali e.g.
NaOH.
Use what’s given in question to work out
moles, and then use molar ratio to work out A-.
Multiply moles x2 if H2 as two H’s since dibasic.
Work out excess HA, all the HA in buffer
solution. HA added– alkali added (due molar
ratio).
Go from Step 2 in previous example.
Likewise if excess alkali, do big take small to find excess i.e. the pH of solution.
Don’t need to times by 1000 if volume in 1dm3
.
Establish if alkali or salt. If alkali need to find excess, otherwise leave it.
Can draw a table to represent information above.
When given all of above but need to work out HA or A-:
– Substitute what I know into formula and find unknown.
– Be careful if have moles of substance normally where use volume of substance. OR have
moles of substance within buffer where use total volume of both components.