Geoscience Reference
In-Depth Information
Neutral
Acidic
Basic
01
2
3
4
5
6
7
8
9
10 1
12 3 4
Seawater
Urine
Oranges
Blood
1-
M
HCl
1-
M
NaOH
Gastric
juices
Household
ammonia
Tomatoes
Pure
water
FIGURE 14.3
pH of selected liquids. (From Spellman, F.R.,
Handbook of Water and Wastewater Treatment
Plant Operations
, 3rd ed., Lewis Publishers, Boca Raton, FL, 2013.)
14.1.2.7.11 pH
pH is a measure of the hydrogen ion (H
+
) concentration. Solutions range from very acidic (having a
high concentration of H
+
ions) to very basic (having a high concentration of OH
-
ions). The pH scale
ranges from 0 to 14, with 7 being the neutral value (see Figure 14.3).
The pH of water is important to the chemical reactions that take place within water, and pH val-
ues that are too high or low can inhibit growth of microorganisms. High pH values are considered
basic, and low pH values are considered acidic. Stated another way, low pH values indicate a high
level of H
+
concentration, while high pH values indicate a low H
+
concentration. Because of this
inverse logarithmic relationship, there is a tenfold difference in H
+
concentration. The pH is the
logarithm of the reciprocal of the molar concentration of the hydrogen ion. In mathematical form,
1
pH
=
log
(14.7)
+
H
Natural water varies in pH depending on its source. Pure water has a neutral pH, with an equal
number of H
+
and OH
-
. Adding an acid to water causes additional positive ions to be released so the
H
+
ion concentration goes up and the pH value goes down:
HCl → H
+
+ Cl
-
Changing the hydrogen ion activity in solution can shift the chemical equilibrium of water. Thus,
pH adjustment is used to optimize coagulation, softening, and disinfection reactions and for corro-
sion control. To control water coagulation and corrosion, it is necessary for the waterworks operator
to test for the hydrogen ion concentration of the water to obtain its pH. In coagulation tests, as more
alum (acid) is added, the pH value is lowered. If more lime (alkali, base) is added, the pH value
is raised. This relationship is important—and if good floc is formed, the pH should then be deter-
mined and maintained at that pH value until there is a change in the new water.
■
EXAMPLE 14.6
Problem:
A 0.1-
M
solution of acetic acid has a hydrogen ion concentration of 1.3 × 10
-3
M
. Find the
pH of the solution.
Solution:
3
1
1
13 10
10
13
3
pH
=
log
=
log
=
log
log
10
−
log.
13
= −
3
log.
1 33011289
= −=
.
.
+
−
3
H
.
×
.
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