Page 49 - Soil and water contamination, 2nd edition

36 Soil and Water Contamination

2.9 ACID–BASE REACTIONS
2.9.1 Introduction

Acid–base reactions are of great significance for the formation, alteration, and dissolution
of minerals and, therefore, the composition of natural waters. Usually, these reactions
proceed very fast, so equilibrium is reached quickly. Therefore, equilibrium constant s are
very effective for predicting the composition of solutions susceptible to acid –base reactions.
+
10
The strength of an acidic solution is usually measured in terms of its pH (= - log[H ]; see
Section 2.2). The pH of the solution is a major variable that largely determines the direction
and intensity of the alteration of minerals. The pH in an aqueous solution is the resultant
equilibrium of all interrelated reactions involving conjugate acid–base pairs, including the
dissociation , i.e. the self-ionisation, of water:
H 2 O H OH (2.49a)

K w [H ][OH ] 10 14 ; pK w 14 (2.49b)
where K = equilibrium dissociation constant for water at 25 °C. An aqueous solution is said
w
-
+
-7
to be neutral if [H ] equals [OH ] = 10 , so the pH of a neutral solution is equal to 7. Since
K changes with temperature, the pH of neutrality also changes slightly with temperature. As
w
mentioned earlier, acid solutions have a pH less than 7 and basic solutions have a pH more
than 7.
2.9.2 Acids
+
An acid is a chemical which produces hydrogen ions, i.e. protons (H ), when dissolved in
water. When an acid loses a proton it forms its conjugate base :

HA H A (2.50)
where HA = an acid and A- = the conjugate base . The equilibrium constant for the reaction
(2.50) is:
[H ][A ]
K a (2.51)
[HA ]

where K = the equilibrium dissociation constant for the acid , also referred to as the acidity
a
constant , which, like all equilibrium constants , is usually reported for a constant temperature
60
of 25 °C. Because acidity constants for different acids vary enormously (by over 10 ), a log
scale is often used, analogous to the pH :
K
pK log (2.52)
a a
A strong acid dissociates completely in an aqueous solution (e.g. hydrochloric acid: HCl) and
has a large K (K >> 1; pK << 0), whereas a weak acid only dissociates partially (e.g. acetic
a a a
acid; CH COOH) and has a small K (K < 0.001; pK > 3). The weaker an acid, the stronger
3 a a a
is its conjugate base ; the stronger an acid, the weaker is its conjugate base. At a pH above the
pK of an acid, the conjugate base will predominate and at a pH below the pK the conjugate
a a
acid will predominate.

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