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Brown's Advanced A Level Chemistry Revision Notes
Theoretical–Physical
Advanced Level
Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 6.3
6.3 Buffer solutions – definition, formulation and
action
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
chloride mixtures.
Chemical Equilibrium Notes Parts 5 & 6 Index
6.3
Buffer solutions – definition, formulation and action
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6.3.1 A buffer is a
solution that minimises pH change on the addition of small amounts
of acid or alkali.
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Buffers and
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.
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6.3.2 Typical
buffers and their action.
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Buffering
action example 6.3.2a
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A mixture of
a weak acid and the salt of the weak acid with a strong base.
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Organic acids
like methanoic, ethanoic, propanoic, citric, benzenedicarboxylic
etc. are frequently used in buffer mixtures i.e. those with the
carboxylic acid functional group –COOH
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The salts are
usually those of the strong base–alkalis sodium and potassium
hydroxide.
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e.g. ethanoic
acid CH3COOH and sodium ethanoate CH3COO–Na+
gives buffers in the range pH 3.7–5.6
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CH3COOH
and CH3COO– constitute a conjugate
acid–base pair.
-
In solution
most of the weak acid is NOT ionised and the relatively high
concentration of the CH3COO– ion actually
inhibits ionisation.
-
The salt is
fully ionised in solution to give a relatively high concentration of
the ethanoate ions.
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How to choose the best
weak acid and its corresponding salt is explained in
section 6.4.1
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Buffering
action example 6.3.2b
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A mixture of
a weak base and the salt of the weak base with a strong acid.
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e.g. ammonia NH3
and ammonium chloride NH4+Cl–
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NH4+
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).
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The salt is
fully ionised in solution giving relatively high concentrations of
the ammonium ion.
-
–
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6.3.3 Preparing
buffer solutions.
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Quite often
several solutions of salts, weak acids/bases are prepared and then
mixed in different ratios to provide buffers of a wide pH range.
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Sometimes a
single salt will do to give a single accurate pH value for calibrating a pH
meter. (see Case study 6.5.1)
Chemical Equilibrium Notes Parts 5 & 6 Index
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Chemical Equilibrium Notes Parts 5 & 6 Index |