Civil Engineering Reference
In-Depth Information
Title
Description
Formulae
Interpretation
Comments
Reference
Langelier Index
(LI)
The most common calcite sat-
uration index. Compares the
measured pH (pH
m
)ofa
water with a hypothetical
calcite saturation pH (pH
s
).
That is, the equilibrium pH
at saturation with calcite as-
suming equivalent alkalinity
and hardness levels. pH
s
can
be calculated several differ-
ent ways. The method be-
low is a simplified approach
developed by Larson and
Buswell.
LI
pH
s
pH
m
where:
pH
s
A
B
log
10
[Ca
2
]
log
10
[Alk]
[Ca
2
] expressed as mg / L CaCO
3
[Alk] is alkalinity expressed as mg /
L CaCO
3
A and B are constants specific to the
alkalinity and hardness level—see
reference for tabulated values.
LI
O water is poten-
tially scale-forming
(supersaturated with
respect to CaCO
3
)
LI
O water is in
equilibrium with
CaCO
3
LI
O water will dis-
solve CaCO
3
deposits
(undersaturated with
respect to CaCO
3
)
The presence of a cal-
cite scale is assumed
to be protective of
the underlying metal.
Although there is lit-
tle evidence to sup-
port this assumption,
many utilities main-
tain a slightly posi-
tive LI as a corrosion
control objective.
7
Calcium Carbon-
ate Precipita-
tion Potential
(CCPP)
Predicts the milligram per liter
of CaCO
3
that will dissolve
or precipitate based on solu-
tion chemistry. Requires a
rigorous interpretation of
CaCO
3
solubility with ionic
strength corrections.
CCPP
50,045 (Alk
i
Alk
eq
)
where:
50,045 is a unit conversion factor
Alk
i
measured total alkalinity
Alk
eq
equilibrium total alkalinity
Calculation of Alk
eq
is complicated
and usually achieved by iterative
solution of the following equation:
1
2
KrP
sp
This approach gives a
direct estimate of the
mass of calcite that
will either dissolve
or deposit per liter of
water at equilibrium
conditions.
Frequently used in soft-
ening operations to
predict potential scal-
ing.
8
eq
eq
Alk
i
t
(Acy
S
)
eq
i
eq
t
(Acx
S
)
eq
i
eq
S
2[Ca
2
]
eq
i
P
eq
For a definition of terms and sug-
gested mathematical approach, see
reference.
