Synthetic Inorganic Chemistry

134 THE THEORY OF IONIZATION

Control of pH. Values of pH near the top and near the bottom
of the scale may be definitely controlled by adjusting the nor-
mality of a strong acid or base. For values of 4 through 10 the
amount of the strong acid or strong base which would be added to
water is so small that its effect is commonly exceeded by that of
uncontrollable impurities. However, by the use of buffers the
pH value can be established precisely at any desired point and held
there with extremely little change against large accidental addi-
tions of either acid or base.
Buffers. A mixture of a weak acid and a salt of that acid or a
mixture of a weak base and a salt of that weak base serves as a
buffer. Let us consider a liter of solution containing 0.1 F.W. of
HAc and 0.1 F.W. of NaAc. The ionization constant of acetic
acid is given in the expression

[Ac"] _

[HAc]

which may be transposed:

[H+] = 0.000,018 X t~i} (1)
IA° J
Since [HAc] = 0.1 (the ionization of HAc is repressed to almost
zero by the Ac" ions of the salt) and [Ac"] = 0.1 (the 0.1 mole of
salt is completely ionized)

0 1 18 10

[H+] = 0.000,018 X - = 0.000,018 = ^^ = ^- = 10—

The pH value of this buffered solution is therefore 4.74.
To appreciate the effectiveness of this buffer solution let us
compare the effect upon the pH value of adding small amounts of
strong acid or base to pure water and then to the buffered solution.
If we add 1 cc. of 1 iV HC1 to 1 liter of pure water we obtain a
solution 0.001 N in H+ ions with a pH of 3. If, on the other hand,
we add 1 cc. of normal HC1 to 1 liter of the buffered solution, the
reaction
H+ + Ac" -* HAc

increases [HAc] by 0.001 and decreases [Ac"] by 0.001 so that

[H+] = 0.000018 X !!', + n'^. = 0.0000184 = 10-*-™

0.1 — 0.001