Page 65 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

THE IOMIC PROOUCT OF WATER 2.16

therefore be slightly ionised in accordance with the equation:

Applying the Law of Mass Action to this equation, we obtain, for any given
temperat ure:
a,+ x a,,- [H+].[OH-1 YH+.YoH-
- x = a constant
-
a ~ , ~ CH201 YH~O
Since water is only slightly ionised, the ionic concentrations will be small, and
their activity coefficients may be regarded as unity; the activity of the un-ionised
molecules may also be taken as unity. The expression thus becomes:
CH+] x [OH-]
= a constant
CH201
In pure water or in dilute aqueous solutions, the concentration of the
undissociated water may be considered constant. Hence:

CH+] x [OH-] = K,
where K, is the ionic product of water. It must be pointed out that the
assumption that the activity coefficients of the ions are unity and that the activity
coefficient of water is constant applies strictly to pure water and to very dilute
solutions (ionic strength < 0.01); in more concentrated solutions, i.e. in solutions
of appreciable ionic strength, the activity coefficients of the ions are affected
(compare Section 2.5), as is also the activity of the un-ionised water. The ionic
product of water will then not be constant, but will depend upon the ionic
strength of the solution. It is, however, difficult to determine the activity
coefficients, except under specially selected conditions, so that in practice the
ionic product K,, although not strictly constant, is employed.
The ionic product varies with the temperature, but under ordinary
experimental conditions (at about 25 OC) its value may be taken as 1 x 10-l4
with concentrations expressed in mol L-'. This is sensibly constant in dilute
aqueous solutions. If the product of [H+] and [OH-] in aqueous solution
momentarily exceeds this value, the excess ions will immediately combine to
form water. Similarly, if the product of the two ionic concentrations is
momentarilv less than 10-14. more water molecules will dissociate until the
equilibrium value is attained.
The hydrogen and hydroxide ion concentrations are equal in pure water;
therefore CH+] = [OH-] = z=
mol L-' at about 25 OC. A solution
in which the hydrogen and hydroxide ion concentrations are equal is termed
an exactly neutral solution. If [H+] is greater than the solution is acid,
and if less than the solution is alkaline (or basic). It follows that at
ordinary temperatures [OH-] is greater than in alkaline solution and
less than this value in acid solution.

*Strictly speaking the hydrogen ion H + exists in water as the hydroxonium ion H30+ (Section
2.4). The electrolytic dissociation of water should therefore be written:
2H20eH30+ +OH-
For the sake of simplicity, the more familiar symbol H+ will be retained.