Page 302 - Physical chemistry understanding our chemical world

pH BUFFERS 269

water as the river passes over the limestone floor of river basins.
Calcium hydroxide is a fairly strong base. Limestone or chalk dis-
solve in water to a lim-
Figure 6.5 shows a buffering action since the pH does not change
ited extent. The CaCO 3
particularly while adding alkali to the solution. In fact, as soon as
decomposes naturally
the alkali mixes with the acid in the lake, its hydroxide ions are to form Ca(OH) ,
2
neutralized by reaction with solvated protons in the lake, thereby thereby generating
resisting changes in the pH. Figure 6.5 shows how little the lake pH alkaline water.
changes; we term the relatively invariant range of constant pH the
buffer region of the lake water. The mid pH of the buffer region cor-
The natural buffers in
responds quite closely to the pK a of the weak acid (here H 2 CO 3 ),
the lake ‘mop up’ any
where the pK a is a mathematical function of K a , as defined by
additional alkali enter-
ing the lake from the
(6.47)
pK a =− log K a tributary rivers, thereby
10
restricting any changes
As a good generalization, the buffer region extends over the range to the pH.
of pK a ± 1.
Only when all the acid in the lake has been consumed will the pH A buffer is only really
rise significantly. In fact, the end point of such a titration is gauged effective at restrict-
when the pH rises above pH 7, i.e. the pH of acid–base neutrality. ing changes if the pH
The pH of the lake water fluctuates when not replenished by the remains in the range
alkaline river water in the tributary rivers. In fact, the pH of the pK a ± 1.
lake water drops significantly each time it rains (i.e. when more
H 2 CO 3 enters the lake). If the amounts of acid and alkali in the
water remain relatively low, then the slight fluctuations in water pH will not be great
enough to kill life forms in the lake.
The system above describes the addition of alkali to a lake containing a weak
acid. The reverse process also occurs, with acid being added to a base, e.g. when the
tributary rivers deliver acid rain to a lake and the lake basin is made of limestone or
chalk. In such a case, the lake pH drops as the acid rain from the rivers depletes the
amounts of natural Ca(OH) dissolved in the lake.
2
As a further permutation, adding a strong acid to a weak base also yields a buffer
solution, this time with a buffer region centred on the pK a of the base. The pH at the
end point will be lower than 7.

Buffers

Each species within a buffer solution participates in an equilibrium reaction, as char-
acterized by an equilibrium constant K. Adding an acid (or base) to a buffer solution
causes the equilibrium to shift, thereby preventing the number of
protons from changing, itself preventing changes in the pH. The A buffer is a solution
change in the reaction’s position of equilibrium is another mani- of a weak acid mixed
festation of Le Chatelier’s principle (see p. 166). with its conjugate base,
which restricts changes
One of the most common buffers in the laboratory is the so-
to the pH.
called ‘phosphate buffer’, which has a pH of 7.0. It comprises salts