Synthetic Inorganic Chemistry

HYDROGEN ION CONCENTRATION; THE pH SCALE 133

important to have a convenient scale by which to measure this

activity.

In liV strong acid, assuming complete ionization, the con-

centration of the H+ ion is 1 mole per liter ([H+] = 1). In pure

water [H+] = 0.000,000,1 = 10~^7 , [OH"] = 0.000,000,1 = 10~^7.

Since the law of molecular concentration gives for the ionization

of water

[H+] [OH-] R

[H 2 O]

and since [H 2 O] is constant for all dilute solutions it follows that

[H+] X [OH"] = 10~^7 X 10-^7 = 10~^14

and 10~^14 is the ion product of water which is constant. In a liV
solution of a strong base since [OH~] = 1 the value of [H+]
must be 10~^14.
With a range of hydrogen ion concentrations from 1 to 10~^14
in solutions commonly used it has been found advantageous to
adopt a logarithmic notation for expressing the concentration of
the hydrogen ions. This is the so-called pH scale. The pH
value is the logarithm to the base 10 of the number of liters of the
solution in question which would contain 1 mole of active H+ ions.
When the concentration is expressed by the exponential method,
the pH value is simply the exponent to the base 10 with the
negative sign omitted.

pH

0 1 2 3 4 5 6 7 8 9

10

11

12

13

14

THE

[H+]

10°

io-i

io-^2

10-'

10-^4

io-^6

10~«

10-^7

IO-^8

IO-^9

lO-io

10-u

10-12

IO-1^3

1Q-14

pH SCALE

Normality of strong acid or base

1 N acid

0.1JV acid

0.01 N acid

0.001 N acid

exact neutrality

Buffer

Range

0.001 2V base

0.01 N base

0.1 A^ base

1 N base