Page 76 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

The hydrogen ions from the hydrochloric acid react with acetate ions forming
practically undissociated acetic acid, and neglecting the change in volume from
1000 mL to 1010 mL we can say

and pH = 4.76 + log 0.09/0.11 = 4.76 - 0.09 = - 4.67
Thus the pH of the acetic acid-sodium acetate buffer solution is only altered
by 0.09 pH unit on the addition of the hydrochloric acid. The same volume of
hydrochloric acid added to 1 litre of water (pH = 7) would lead to a solution
with pH = -log(O.Ol) = 2; a change of 5 pH units. This example serves to
illustrate the regulation of pH exercised by buffer solutions.
A solution containing equal concentrations of acid and its salt, or a
half-neutralised solution of the acid, has the maximum 'buffer capacity'. Other
mixtures also possess considerable buffer capacity, but the pH will differ slightly
from that of the half-neutralised acid. Thus in a quarter-neutralised solution of
acid, [Acid] = 3 [Salt] :

pH = pKa + log 4 = pKa + 1.52 = pKa - 0.48
For a three-quarter-neutralised acid, [Salt] = 3 [Acid]

In general, we may state that the buffering capacity is maintained for mixtures
within the range 1 acid:lO Salt and 10 acid:l Salt and the approximate pH
range of a weak acid buffer is:

The concentration of the acid is usually of the order 0.05-0.2 mol L-'. Similar
remarks apply to weak bases. It is clear that the greater the concentrations of
acid and conjugate base in a buffer solution, the greater will be the buffer
capacity. A quantitative measure of buffer capacity is given by the number of
moles of strong base required to change the pH of 1 litre of the solution by
1 pH unit.
The preparation of a buffer solution of a definite pH is a simple process once
the acid (or base) of appropriate dissociation constant is found: small variations
in pH are obtained by variations in the ratios of the acid to the Salt concentration.
One example is given in Table 2.2.
Before leaving the subject of buffer solutions, it is necessary to draw attention
to a possible erroneous deduction from equation (21), namely that the
hydrogen-ion concentration of a buffer solution is dependent only upon the
ratio of the concentrations of acid and Salt and upon Ka, and not upon the
actual concentrations; otherwise expressed, that the pH of such a buffer mixture
should not change upon dilution with water. This is approximately although
not strictly true. In deducing equation ( l8), concentrations have been substituted
for activities, a step which is not entirely justifiable except in dilute solutions.
The exact expression controlling buffer action is: