Brown's Chemistry Advanced A Level Notes - Theoretical–Physical
Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 6
6.5 Case studies of uses of buffers in aqueous media
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Part 6 sub–index
Acid–base indicator theory, pH curves and
Buffers – definition, formulation
of buffer function (this page)
Several examples of using of buffers are
described and explained e.g. buffers for calibrating pH meters,
controlling the aqueous media supporting enzyme action, the carbonate
buffering action in blood.
6.5 Case studies of the function and
uses of buffers in aqueous media
6.5.1 Other common buffer solutions and their use in the
hydrogen benzene–1,2–dicarboxylate is an 'all in one' buffer
solution of pH 4.0
+ OH–(aq) –OOC–C6H4–COO–(aq)
(i) removes hydrogen ions
and (ii) removes hydroxide ions.
Buffers can be made by
mixing the salt with the original benzene–1,2–dicarboxylic acid to
give buffers in the range 2.2–3.8
A mixture of salts of a
polybasic/polyprotic acid e.g. the salts KH2PO4,
and Na3PO4 from phosphoric(V) acid (a
tribasic/triprotic acid) can give buffer solutions in the range pH
(removes hydrogen ions)
+ H2O(l) (removes hydrogen ions)
+ H2O(l) (removes hydroxide ions)
+ H2O(l) (removes hydroxide ions)
ion is amphoteric, acting both as a proton donor and acceptor
and phosphate(V) ions are important in the buffering of
intracellular fluids in living organisms (see Case study 6.5.2
Buffer solutions are used to accurately calibrate pH meters.
6.5.2 The importance of buffering in biological systems
6.5.2b Inside cells
hydrogenphosphate(V) ions act as the major intracellular buffer
system, with contributions from organic phosphates such as
glucose–6–phosphate and ATP.
The major extracellular buffer is the 'carbonic acid'–'bicarbonate'
or hydrogencarbonate system which enables e.g. blood, to function as
an extraordinary effective buffer operating at about pH 7.
A whole series of
linked equilibria control the pH of blood ...
CO2(aq) + H2O(l)
the dissociation of
carbonic acid into the hydrogencarbonate ion and the hydrogen ion –
so decreasing pH
OR, the reverse reaction
to remove hydrogen ions – so increasing pH
carbonic acid removing
hydroxide ions, so lowering the pH
OR, the reverse, the
production of hydroxide ion to increase the pH
- (aq) = intra/extracellular
Note the negative anions
must be counter balanced by positive ions such as sodium, Na+ otherwise an unwanted potential would be set up.
(iv) to (iii) removes
hydrogen ions and (v) to (vi) removes hydroxide ions.
The effectiveness of the
system depends on the reservoir of dissolved carbon dioxide in the
blood plasma and the gas in the lungs.
If hydroxide ions are
removed via reaction (v) to (iv), the depleted H2CO3
is readily replaced via the reaction sequence (i) to (ii) and (ii)
If hydrogen ions are
removed via (iv) to (iii) the reverse sequence of 1. can restore the
system to the original pH.
The ability of mammals to
maintain a fairly constant [HCO3–]/[H2CO3]
ratio in blood plasma is reflected in the rate of CO2
production in the cell oxidation reactions of respiration and the rate
of CO2 loss by expiration.
The blood plasma of man is
about 7.4 and any deviation below 7.0, or above 7.8, as can happen
in disease, can cause irreparable damage.
extracellular systems are very pH sensitive and small changes in pH
can produce ill–effects in living organisms, hence, e.g. the bodies
irritation by all except the very weakest of acids and alkalis in
contact with the skin.
Case study 6.5.3
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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