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I-1
EXPERIMENT I: BUFFER SOLUTIONS
OBJECTIVES
In this experiment, the properties
of buffer solutions will be studied.
THEORETICAL CONSIDERATIONS
The pH of normal human blood is approximately
7.3. The addition of small quantities of a strong acid
or a strong base to blood, for example
0.01 mole to one litre of blood, leads to a change in pH of only
about 0.1 pH units. By comparison,
the addition of 0.01 mole of hydrochloric acid (HCl) to one litre of
pure water causes the pH to drop from
about pH 7 to near pH 2. Blood along with several other body
fluids are said to be buffered.
These buffered solutions resist changes in the hydronium ion
concentration ([H3O
+
]) and, hence, pH when small quantities
of strong acids or strong bases are added
to them.
Buffers are important for several
reasons:
1.
The pH of these solutions change very
slightly on addition of small amounts of either hydronium
or hydroxide ions. Maintaining
a constant pH is vital in most biological systems and in many
chemical reactions. (Our blood
is mainly buffered by the carbonic acid-hydrogen carbonate
equilibrium which maintains the [H3O
+
] at approximately 5 x 10
-8
M
, a pH of about 7.3).
2.
They allow us to prepare solutions
of known pH quite simply.
Buffers are solutions in which a
weak acid and its conjugate base are both present in concentrations
that are high compared with the concentrations
of [H3O
+
] or [OH
-
] being added. The mechanism
by
which buffer solutions resist changes
in [H3O
+
] can be explained by Le Châtelier's
Principle: the
position of the acid-base equilibrium
shifts in order to minimize the stress caused by the added reagent.
The weak acid (HA) - conjugate base
(A
-
) buffer system is used below to illustrate
this resistance to pH
changes.
HA
(aq)
+ H2O
(l)
H3O
+
(aq)
+ A
-
(aq)
(3)
When both HA and A
–
are added to pure water equilibrium
(3) is established. If H3O
+
is added to this
solution, the position of the equilibrium
shifts to form more reactant, consuming most of the added
H3O
+
. Therefore, no dramatic change
in the pH is observed. Similarly, addition of OH
-
ion to the
buffer solution shifts the position
of equilibrium to form more product, thus, consuming most of the
