Buffer solutions
Buffer solutions are solutions which resist change in hydrogen ion and the hydroxide
ion concentration (and consequently pH) upon addition of small amounts of acid or
base, or upon dilution. Buffer solutions consist of a weak acid and its conjugate base
(more common) or a weak base and its conjugate acid (less common). The resistive
action is the result of the equilibrium between the weak acid (HA) and its conjugate
base (A−):
HA(aq) + H2O(l) → H3O+(aq) + A−(aq)
Any alkali added to the solution is consumed by hydrogen ions. These ions are mostly
regenerated as the equilibrium moves to the right and some of the acid dissociates into
hydrogen ions and the conjugate base. If a strong acid is added, the conjugate base is
protonated, and the pH is almost entirely restored.
Applications
Their resistance to changes in pH makes buffer solutions very useful for chemical
manufacturing and essential for many biochemical processes. The ideal buffer for a
particular pH has a pKa equal to the pH desired, since a solution of this buffer would
contain equal amounts of acid and base and be in the middle of the range of buffering
capacity.
Buffer solutions are necessary to keep the correct pH for enzymes in many organisms
to work. Many enzymes work only under very precise conditions; if the pH strays too
far out of the margin, the enzymes slow or stop working and can denature, thus
permanently disabling its catalytic activity. A buffer of carbonic acid (H2CO3) and
bicarbonate (HCO3−) is present in blood plasma, to maintain a pH between 7.35 and
7.45.
Industrially, buffer solutions are used in fermentation processes and in setting the
correct conditions for dyes used in colouring fabrics. They are also used in chemical
analysis and calibration of pH meters
Calculating pH of a buffer
The equilibrium above has the following acid dissociation constant:
Simple manipulation with logarithms gives the Henderson-Hasselbalch equation,
which describe pH in terms of pKa:
In this equation
1. [A−] is the concentration of the conjugate base.
This may be considered as coming completely
from the salt, since the acid supplies relatively
few anions compared to the salt.
2. [HA] is the concentration of the acid. This may
be considered as coming completely from the
acid, since the salt supplies relatively few
complete acid molecules (A − may extract H +
from water to become HA) compared to the
added acid.
Maximum buffering capacity is found when pH = pKa, and buffer range is considered
to be at a pH = pKa ± 1.
Illustration of buffering effect: Sodium acetate/acetic acid
The acid dissociation constant for acetic acid-sodium acetate is given by the equation:
Since this equilibrium only involves a weak acid and base, it can be assumed that
ionization of the acetic acid and hydrolysis of the acetate ions are negligible. In a
buffer consisting of equal amounts of acetic acid and sodium acetate, the equilibrium
equation simplifies to
Ka = [H + ],
and the pH of the buffer as is equal to the pKa.
To determine the effect of addition of a strong acid such as HCl, the following
mathematics would provide the new pH. Since HCl is a strong acid, it is completely
ionized in solution. This increases the concentration of H+ in solution, which then
neutralizes the acetate by the following equation.
The consumed hydrogen ions change the effective number of moles of acetic acid and
acetate ions:
After accounting for volume change to determine concentrations, the new pH could
be calculated from the Henderson-Hasselbalch equation. Any neutralization will result
in a small change in pH, since it is on a logarithmic scale..
Making buffer solutions
In general, preparing a buffer solution requires:



a weak acid
a salt of the acid's conjugate base
both of which in sufficient amounts to maintain
the ability to buffer
Citric acid-phosphate buffer
Make up 0.1M citric acid and 0.2M Disodium hydrogen phosphate solutions then mix
as follows to make a 100 ml solution:
Citric acid-phosphate buffers
pH 0.2M Na2HPO4 0.1M Citric Acid
3.0 20.55 ml
79.45 ml
4.0 38.55 ml
61.45 ml
5.0 51.50 ml
48.50 ml
6.0 63.15 ml
36.85 ml
7.0 82.35 ml
17.65 ml
8.0 97.25 ml
2.75 ml