Civil Engineering Reference
In-Depth Information
carbonate is several times more soluble in seawater than in freshwater as a result of
the secondary salt effect.
Chemical Reactions Changing Equilibrium
Chemical reactions in water treatment rely on shifting equilibria to achieve desired
results. The most common methods for competing reactions are by formation of in-
soluble substances, weakly ionized compounds, gaseous end products, and oxidation
and reduction. The best example is the precipitation of calcium by adding lime. At a
pH of approximately 10, hydroxyl ions convert bicarbonates to carbonates to allow
the formation of calcium carbonate precipitation as follows:
Ca(HCO )
Ca(OH)
2CaCO
2H O
→
(5-4)
32
2
3
2
lime slurry
→
solid precipitation
water
bicarbonate hardness
Gas Solubility
Henry's law is used to calculate the amount of a gas that will dissolve in water. It
states that the concentration of any gas dissolved in a liquid, at a constant temperature,
is directly proportional to the pressure that the gas exerts above the liquid. Equation
5-5 is a practical form of Henry'slaw.
Note that Henry's law can be presented in different forms, depending on the units
of measurement and approximating assumptions used.
C
HP
(5-5)
eq
gas
where:
C
eq
the equilibrium gas solubility, mg / L
P
gas
the partial pressure of the gas above liquid, atm
H
Henry's law constant for the gas at a given temperature, mg / L-atm
Table 5-7 shows the Henry's law constant for some gases commonly used in water
treatment.
The solubility of oxygen, called dissolved oxygen (DO), in water is greatly affected
by temperature and to a lesser extent by barometric pressure:
TABLE 5-7. Henry's Law Constants at 20
C
Compound
H, mg / L-atm
Oxygen
41
Carbon dioxide
16,200
Hydrogen sulfide
3,700
Ozone
535
Source:
Adapted from Reference 3.
