Page 719 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

674 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

depositing or corrosive, which is defined (Babbitt and Neutralization: A reaction in which water is a product, as
Doland, 1949, p. 558) as when an acid reacts with a base. Soluble metal
oxides act as strong bases in solution because when

pH ¼ pK pK þ pCa þ pAlk (21:G:5) the oxide ion dissociates, it reacts with water to form
0
0
2
s
s
hydroxide ion (Silberberg, 1996, p. 148), that is,
in which
pH s is the pH that the water should have in order O (aq) þ H 2 O(l) ! 2OH (aq) (21:G:9)
2

to be in equilibrium with CaCO 3 .
pK is the negative logarithm of the second Ammonia gas when dissolved in water also acts
0
s
dissociation constant for carbonic acid, that is, as a weak base, that is,
K ¼ 6:35
0
s
pK isthenegativelogarithmoftheactivityproduct þ
0
2 NH 3 (g) þ H 2 O(l) ! NH 4 (aq) þ OH (aq) (21:G:10)
of CaCO 3 , that is, K ¼ 10:33
0
2
pCa is the negative logarithm of molal concentra-
Normality: Equivalents of solute dissolved in 1 L of solution,
tion of Ca
that is, M equivalents of solute=L solution
pAlk is the negative logarithm of equivalent
Precipitation: Two soluble ionic compounds react to form an
concentration of titratable base
insoluble product, that is, a precipitate. For example,
when silver nitrate and sodium chromate are mixed,
The saturation index is obtained by subtracting
a brick-red precipitate, Ag 2 CrO 4 forms, which can
pH s ¼ from the pH of the water in question. A nega-
be represented by any of three types of balanced
tive index indicates that the water is undersaturated
equations, that is, molecular, total ionic, and net
with respect to CaCO 3 . A positive index indicates
ionic (Silberberg, 1996, p. 145).
that the water is supersaturated with respect to
The ‘‘molecular equation’’ shows the reactants
CaCO 3 and is depositing. The water analysis
and products as if they were undissociated com-
includes data on total solids, temperature, calcium,
pounds:
and alkalinity. With these data, the Langelier index
may be determined from tables, a nomograph, or a
2AgNO (aq) þ Na 2 CrO 4 (aq)
3
computation wheel.
! Ag CrO 4 (s) þ 2NaNO 3 (aq) (21:G:11)
Ligand: Ions or molecules that are attached by a covalent bond 2
to a central atom or ion as a part of a complex.
Lime: Calcium oxide, CaO. Slaked lime is CaO that has The more realistic depiction is the ‘‘total ionic equa-
reacted with water to form Ca(OH) 2 . The former is tion,’’ that is,
cheaper and if used must be slaked at the plant.
Lime-soda process: Softening by lime, Ca(OH) 2 , to remove 2Ag (aq) þ 2NO 3 (aq) þ 2Na (aq) þ CrO 4 2

þ
þ
temporary hardness, followed by soda, NaCO 3 ,to ! Ag CrO 4 (s) þ 2Na (aq) þ 2NO 3 (aq)

þ
2
remove permanent hardness.
(21:G:12)
Mass percent: Mass of solute per 100 g solution (Silberberg,
1996, p. 481), that is,
The number of Na (aq) ions and NO 3 (aq) are
þ

unchanged on both sides and are called ‘‘spectator
mass of solute
100 ions,’’ because they are not involved in chemical
mass of solute þ mass of solvent change, but are present as a part of the reactants.
Mass percent
(21:G:6) The ‘‘net ionic equation’’ depicts only the actual
mass of solute change taking place, that is, with no spectator ions:
100 (21:G:7)

mass of solution
þ 2 ! Ag CrO 4 (s) (21:G:13)
2Ag (aq) þ CrO 4 2
Molality: Moles of solute dissolved in 1000 g of solvent, that
is, m mol solute=kg solvent (Silberberg, 1996, Re-carbonation: The addition of lime increases the tendency
p. 480). of a water to deposit calcium carbonate on filter sand,
Molarity: Moles of solute dissolved in 1 L of solution, that is, pipes, boiler tubes, etc. The carbonate balance may
M mol solute=L solution (Silberberg, 1996, be partially or completely restored by re-carbonation.
p. 480). The process involves diffusing carbon dioxide gas
Mole fraction: Ratio of solute moles to total moles of solute through water. Reaction time should be u 20 min.
plus solvent (Silberberg, 1996, p. 482), that is, Equipment for production of carbon dioxide includes
a burner, a scrubber, a compressor, and a diffuser.
moles of solute Salt: An ionic compound that results from the reaction
(21:G:8)
X
moles of solute þ moles of solvent between an acid and a base (Silberber, 1996, p. 148).