Civil Engineering Reference
In-Depth Information
TABLE 5-2. Ions Commonly Found in Water
Molecular
Weight
Common
Valence
Combining
Weight
Name
Formula
Ammonium
NH
4
18.0
1
18.0
Hydroxyl
OH
17.0
1
17.0
Bicarbonate
HCO
3
61.0
1
61.0
Carbonate
CO
2
3
60.0
2
30.0
Orthophosphate
PO
3
4
95.0
3
31.7
Orthophosphate, mono-hydrogen
HPO
2
4
96.0
2
48.0
Orthophosphate, di-hydrogen
H
2
PO
4
97.0
1
97.0
Bisulfate
HSO
4
97.0
1
97.0
Sulfate
SO
2
4
96.0
2
48.0
Bisulfite
HSO
3
81.0
1
81.0
Sulfite
SO
2
3
80.0
2
40.0
Nitrite
NO
2
46.0
1
46.0
Nitrate
NO
3
62.0
1
62.0
Hypochlorite
OCl
51.5
1
51.5
mg/L
valence
meq/L
(5-2)
atomic weight
pH, Acidity, and Alkalinity
The pH is an expression of the acid or base condition of a solution. Pure water dis-
associates into hydrogen ion (H
) and hydroxide ion (OH
), as shown in Equation
5-3:
HO
H
OH
(5-3)
2
pH is defined as the negative log of the hydrogen ion molar concentration. At neutral,
the pH is negative log of 10
7
, or 7. The pH scale ranges from 0 (strong acid) to 14
(strong base). Most natural waters tend to have pH values of 5.5 to 8.0. Most soft
waters have a pH between 6.0 and 7.0. A pH less than 7 is often due to the presence
of carbon dioxide and other acid constituents. Alkaline waters have pH values between
7 and 8. For instance, waters of the Great Lakes have pH values of approximately 7.8.
Figure 5-1 shows the pH of some common water sources and the compound typically
used to adjust the water pH.
Alkalinity
Alkalinity is a measure of the ability to accept hydrogen ions or neutralize acid without
changing the pH of the water. The ability of a solution to accept hydrogen ions is
called
buffering capacity.
The three main chemical forms that contribute to alkalinity
in natural water are bicarbonates, carbonates, and hydroxides. Bicarbonates represent
the major form of alkalinity, because they originate naturally from the reaction between
carbon dioxide and water.
