Determine the Langelier & Ryznar indexes for
the Denver water supply
Constituent
Conc. (mg/L)
Conc. (mol/L)
TDS
179
-
Ca+2
42
1.05 x 10 -3
HCO3-
115
1.89 x 10 -3
pH = 7.9, Temp = 20°C
Determine the value of pHS
Determine the Langelier index
Determine the Ryznar index
We determined ionic strength two classes ago
I = (2.5 × 10 −5 ) × TDS(mg/L)
I = 0.00448
We also determined activity coefficient for HCO3-
logγi = −
logγHCO − = −
3
0.5(z i )2 I 1/2
1+ I 1/2
0.5( −1)2 (0.00448)1/2
= −0.0313
1+ (0.00448)1/2
γHCO − = 0.93
3
And activity coefficient for Ca+2
logγCa +2 = −
0.5( +2)2 (0.00448)1/2
= −0.125
1+ (0.00448)1/2
γCa +2 = 0.75
1
Carbonate Equilibrium Constants as a Function of Temperature
K1
K2
Ksp
5
3.02 x 10-7
2.75 x 10-11
8.13 x 10-9
10
3.46 x 10-7
3.24 x 10-11
7.08 x 10-9
15
3.80 x 10-7
3.72 x 10-11
6.03 x 10-9
20
4.17 x 10-7
4.17 x 10-11
5.25 x 10-9
4.47 x 10-7
4.68 x 10-11
4.57 x 10-9
40
5.07 x 10-7
6.03 x 10-11
3.09 x 10-9
60
5.07 x 10-7
7.24 x 10-11
1.82 x 10-9
T, °C
25
Km
1.58 x 10-3
−
Km =
−2
[H 2 CO 3 ]
[H + ][HCO 3 ]
[H + ][CO 3 ]
K1 =
K2 =
−
[CO 2 ]aq
[H 2 CO 3 ]
[HCO 3 ]
Ksp = Solubility product for CaCO3
Calculated activities:
γCa +2
= 0.75 γHCO − = 0.93
3
pH = 7.9
Conc. (mol/L)
Ca+2
1.05 x 10 -3
HCO3
-
1.89 x 10 -3
Now calc:
⎛ K 2 γ + 2 [Ca + 2 ] γ
[HCO
Ca
HCO 3 −
pH S = − log ⎜
⎜
K SP
⎝
−
3
]⎞
⎟
⎟
⎠
Determine the the value of pHS
−
⎛ K 2 γ +2 [Ca +2 ] γ
[HCO3 ] ⎞
Ca
HCO3 −
⎜
⎟
pHS = −log
⎜
⎟
K SP
⎝
⎠
⎛ (4.17 × 10 −11 )(0.75)(1.05 × 10 −3 ) (0.93)(1.89 × 10 −3 ) ⎞
⎟⎟
pHS = −log⎜⎜
(5.25 × 10 −9 )
⎝
⎠
(
)
pHS = −log 1.1× 10 −8 = 7.96
14
5.5 6.2 6.8
RI
very
corrosive
pHs –no
no precipitate
corrosive
scale
heavy
scale
Ryznar index
RI = 2 x 7.96 – 7.9 = 8.02
Water is corrosive
pH
neutral
1
Langelier index
pHs –precipitate
precipitate
LI = 7.9 – 7.96 = - 0.06
Scale formation is not expected
8.5
2
Chemical Characteristics: NITROGEN (N)
Sources:
Fertilized areas; Sewage disposal; Feed lots; N cycle
Potential Problems:
Infants <6mo convert nitrate to nitrite due to higher pH in their
digestive system & could become seriously ill, and may die if
untreated because the nitrite diminishes oxygen carrying capacity
of their blood
E
Excessive
i concentrations
t ti
can lead
l d to
t eutrophication
t
hi ti
Chemical Characteristics: NITROGEN (N)
Maximum Contaminant Levels (MCL):
nitrite-N : 1 mg/L
nitrate-N : 10 mg/L
nitrite + nitrate (as N) : 10 mg/L
For all in mg/L or ppm:
To express concentration of
Nitrates and Nitrites as N
in terms of
Nitrate (N03) or Nitrite (N02)
multiply by
4.4 and 3.3 respectively
At Wt
At Wt Ratio
14
N
NO3
62
4.4
16
O
NO2
46
3.3
3