Brown's Chemistry Advanced A Level Notes - Theoretical–Physical
Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 6
6.3 Buffer solutions – definition, formulation and
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Part 6 sub–index
Acid–base indicator theory, pH curves and
Buffers – definition, formulation
and action (this page)
of buffer function
What is a buffer? How do buffers work?
Buffers are defined and their actions explained with appropriate
examples such as ethanoic acid/sodium ethanoate and ammonia/ammonium
Buffer solutions – definition, formulation and action
6.3.1 A buffer is a
solution that minimises pH change on the addition of small amounts
of acid or alkali.
their chemical reactions must obey Le Chatelier's Equilibrium
Concentration Principle, and act in a way to remove H+
and OH– ions. BUT, they cannot theoretically
prevent the pH being lowered/raised by the addition of acid/alkali,
however small the change.
Note that any buffer will eventually be 'used up' if
large quantities of acid or alkali are added to the solution.
buffers and their action.
action example 6.3.2a
A mixture of
a weak acid and the salt of the weak acid with a strong base.
like methanoic, ethanoic, propanoic, citric, benzenedicarboxylic
etc. are frequently used in buffer mixtures i.e. those with the
carboxylic acid functional group –COOH
The salts are
usually those of the strong base–alkalis sodium and potassium
acid CH3COOH and sodium ethanoate CH3COO–Na+
gives buffers in the range pH 3.7–5.6
and CH3COO– constitute a conjugate
most of the weak acid is NOT ionised and the relatively high
concentration of the CH3COO– ion actually
The salt is
fully ionised in solution to give a relatively high concentration of
the ethanoate ions.
How to choose the best
weak acid and its corresponding salt is explained in
action example 6.3.2b
A mixture of
a weak base and the salt of the weak base with a strong acid.
e.g. ammonia NH3
and ammonium chloride NH4+Cl–
and NH3 constitute a conjugate acid–base pair.
In solution most
of the ammonia is NOT ionised (and even suppressed by the ammonium
ions from the salt).
The salt is
fully ionised in solution giving relatively high concentrations of
the ammonium ion.
several solutions of salts, weak acids/bases are prepared and then
mixed in different ratios to provide buffers of a wide pH range.
single salt will do to give a single accurate pH value for calibrating a pH
Case study 6.5.1)
Advanced Equilibrium Chemistry Notes Part 1. Equilibrium,
Le Chatelier's Principle–rules
* Part 2. Kc and Kp equilibrium expressions and
calculations * Part 3.
Equilibria and industrial processes * Part 4
Partition between two
phases, solubility product Ksp, common ion effect,
ion–exchange systems *
Part 5. pH, weak–strong acid–base theory and
calculations * Part 6. Salt hydrolysis,
acid–base titrations–indicators, pH curves and buffers * Part 7.
Redox equilibria, half–cell electrode potentials,
electrolysis and electrochemical series
pressure, boiling point and intermolecular forces watch out for sub-indexes
to multiple sections or pages
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