Page 66 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 ANDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

In al1 cases the reaction of the solution can be quantitatively expressed by
the magnitude of the hydrogen ion (or hydroxonium ion) concentration, or,
less frequently, of the hydroxide ion concentration, since the following simple
relations between CH+] and [OH-] exist:

Kw Kw
and
CH+] = - [OH-] =-
[OH -1 ' CH+]
The variation of Kw with temperature is shown in Table 2.1.

Table 2.1 Ionic product of water at various temperatures
Temp. (OC) K, x 1014 Temp. (OC) K, x 1014

2.17 THE HYDROGEN ION EXPONENT
For many purposes, especially when dealing with small concentrations, it is
cumbersome to express concentrations of hydrogen and hydroxyl ions in terms
of moles per litre. A very convenient method was proposed by S. P. L. S~rensen
(1909). He introduced the hydrogen ion exponent pH defined by the relationships:

The quantity pH is thus the logarithm (to the base 10) of the reciprocal of the
hydrogen ion concentration, or is equal to the logarithm of the hydrogen ion
concentration with negative sign. This method has the advantage that al1 States
of acidity and alkalinity between those of solutions containing, on the one hand,
1 mol L - ' of hydrogen ions, and on the other hand, 1 mol L - ' of hydroxide
ions, can be expressed by a series of positive numbers between O and 14. Thus
a neutral solution with CH+] = has a pH of 7; a solution with a
hydrogen ion concentration of 1 mol L-' has a pH of O ([H +] = IO0); and a
solution with a hydroxide-ion concentration of 1 mol L-' has [H+] =
Kw/[OH -1 = 10-'4/100 = 10-14, and possesses a pH of 14. A neutral solution
is therefore one in which pH = 7, an acid solution one in which pH < 7, and an
alkaline solution one in which pH > 7. An alternative definition for a neutral
solution, applicable to al1 temperatures, is one in which the hydrogen ion and
hydroxide ion concentrations are equal. In an acid solution the hydrogen ion
concentration exceeds the hydroxide ion concentration, whilst in an alkaline or
basic solution, the hydroxide ion concentration is greater.

Example IO. (i) Find the pH of a solution in which [H +] = 4.0 x IO-' mol L-'.