Geology Reference
In-Depth Information
When acids and bases react together, they tend to
neutralize
each other, forming
salts
(plus water).
Acids, bases and salts form
electrolyte
solutions, in
which the solute is partly or completely ionized, result-
ing in electrical conductivity through migration of
charged ions. Compounds like HCl, which are more or
less completely ionized in solution, are called '
strong
electrolytes' (in this case a
strong acid
).
Weak
electro-
lytes (like carbonic acid, H
2
CO
3
, a
weak acid
) are those
that exhibit only slight ionization in aqueous solution.
Salts are almost always strong electrolytes, but acids
and bases may be strong or weak, depending on the
bond holding the compound together (Chapter 7).
The pH of a solution can be measured in two alterna-
tive ways:
(a) using a paper treated with a pH-sensitive dye
whose colour indicates the pH of the solution (lit-
mus paper is the traditional acid/base indicator,
but more specific pH papers are available, whose
colours relate to a range of pH values);
(b) using a special electrical meter called a pH meter,
which, when a sensing electrode is dipped into a
solution, gives a direct digital reading of its pH.
Figure B1 shows the pHs of some familiar solutions.
To summarize, an acid solution is one whose H
+
con-
centration is greater than that found in pure water (10
−7
mol kg
−1
). An acid solute is one that raises the H
+
con-
centration of a solution, and a base is one that depresses
it. An equivalent definition of a base is a solute that
increases the concentration of hydroxyl (OH
−
) ions in
solution: because additional OH
−
will associate with
Ionization of water: pH
Pure water at room temperature undergoes partial
self-ionization:
aa
a
..
+
+
+
−
Natural waters
and precipitation
Food and domestic
HO HOH
→+ =
K
H H
=
aa
(B1)
2
HO
+
−
H H
2
HO
2
14
14
since the activity
a
HO
2
of pure water = 1.00. Water is a
weak electrolyte: the equilibrium constant (Chapter 4)
for this reaction,
K
HO
2
13
Household bleach
13
, has a value of 10
−14
at room tem-
perature. It follows that the activities (concentrations)
of free H
+
and OH
−
ions in pure water are both about
10
−7
mol kg
−1
. This can be expressed most concisely by
saying that the
pH
of pure water is 7.0, where
12
12
11
11
Milk of magnesia
[suspension of Mg(OH)
2
]
10
10
9
9
pH
=−
log
m
(B2)
Limestone
aquifers
+
NaHCO
3
solution
H
Seawater
8
8
pH
Egg white
So if a solution has a pH of 2 it means that the con-
centration of free hydrogen ions (
m
H
+
) is 10
−2
mol kg
−1
.
The pH notation may be used to describe the acidity of
a solution. Values of pH less than 7.0 denote higher
concentrations of the H
+
ion than are found in pure
water (
acidic
behaviour), whereas values above 7.0
indicate lower H
+
concentrations than pure water
(basic behaviour). Other equilibrium constants such
as
K
1
in Equation 4.21 can be expressed in an analo-
gous way:
7
7
Milk
6
Egg yolk
6
Unpolluted rain
Most ground waters
5
Beer, black coffee
5
Soil water
Acid
rain
Tomato juice
Peat water
4
4
3
Orange juice
Vinegar
Lemon juice
3
Acid mine waters
2
2
Photochemical smog
aa
a
+
−
H CO
−
64
.
K
=
=
10
thereforepK
=
64
..
3
Figure B1
pH values of geological environments and
everyday solutions.
1
1
HCO
2
3
Search WWH ::
Custom Search