The Islamic University of Gaza
Faculty of Engineering
Civil Engineering Department
M.Sc. Water Resources
Water Quality Management
(ENGC 6304)
Lect. 3- Chemical Water Quality- Continue
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Alkalinity
Alkalinity is a measure of the substances (ions) in water that
react to neutralize acid (hydrogen ions) and resist changes in pH
Sources
• Natural water systems include CO32-, HCO32-, OH-, HPO42- ,
H2PO4-, HS-, NH3
• CO32-, HCO32- , OH- ………( dissolution of mineral substances,
from CO2),
• HPO42- , H2PO4- …………. ( detergents in wastewater, fertilizers
and insecticides)
• HS-, NH3 ……………(products of microbial decomposition of
organic material).
• Groundwater has higher alkalinity than surface water
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•
CO2 influences the carbonate system in water as follows:
Carbon dioxide dissolves in water and produces carbonic acid
CO2 + H2O ⇔ H2CO3 (dissolved CO2 and H2CO3)
Carbonic acid dissociates producing H+
1.
H2CO3 ⇔ H+ + HCO3- (HCO3- can absorb another H+ to become
H2CO3)
2.
HCO3- ⇔ H+ + CO32- (CO3-2 can absorb one H+ to become HCO3-)
•
•
The ability of water to absorb H+ ions (anions) without a change in pH
is known as its alkalinity.
In freshwater, alkalinity typically is due to the presence of excess
carbonate anion (from the weathering of silicate or carbonate rocks)
that when hydrolyzed produces OH- (and neutralizes H+) as follows:
Hydrolysis of carbonate and carbonate produces OHCO32- + H2O = HCO3- + OH٣
The relative quantities of the alkalinity species are pH dependent.•
See the next figure•
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Alkalinity species at various pH levels, (values calculated for water
with a total alkalinity of 100 mg/l at 25oC.
CO2 is acidic and lowers pH as •
the amount of CO2 in the water
increases.
alkaline
alkaline
acidic
As pH increases from 7, •
HCO3- is formed and the water
becomes slightly alkaline.
As pH continues to raise, •
CO32- becomes the dominant
source of alkalinity in most
waters
pH increases when CO2 is •
removed from the water by
photosynthesis and decreases
when CO2 is added to the water
by respiration, especially at
night when photosynthesis has
stopped.
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Remember
pH is a measure of acidity (hydrogen ion concentration) in water or
soil.
or pH is a measure of the acidic or basic characteristics of water
Specifically,
pH = -log [H+]
So a pH of 7 means the [H+]=10-7
Waters with a low pH below 7 have a high hydrogen ion
concentration and are termed acid and water with a high pH above
7 has a low hydrogen ion concentration and are termed alkaline
pH = - log [ H+ ]
0
1
2
3
4
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acid
5
6
7
neutral
8
9
10 11 12 13
alkaline
14
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Impacts
• Bitter taste to water,
• Reaction can occur between the alkalinity species and cations,
resulting in precipitated substances that can foul pipes.
Measurement
By titration the water with an acid and determining the hydrogen
equivalent…. Expressed as mg/L of CaCO3
For example:
Each mL of 0.02 N H2SO4 will neutralize 1mg of alkalinity as
CaCo3
Normality is the number of gram equivalents per liter = nM
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Hydrogen ions from the acid react with the alkalinity according to:
H+ + OH- ⇔ H2O
CO32- + H+ ⇔ HCO3HCO3- + H+ ⇔ H2CO3
If the acid is added slowly to water and the pH is recorded for
each addition, a titration curve is obtained as shown:
8.3
4.5
OH- + (1/2 ) CO32-
(½)CO32- + HCO3-
P
M
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• If P is the amount of acid required to reach pH 8.3 ,
• M is the total quantity of acid required to reached pH 4.5,
The following generalizations can be made:
P=M
Alkalinity = OH-
P = M/2
Alkalinity = CO32-
P=0
Alkalinity = HCO3-
P < M/2
Alkalinity = HCO3- +CO32-
P > M/2
Alkalinity = OH- + CO32-
(Initial pH is below 8.3)
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Example
• A 200 ml sample of water has an initial pH of 10.
• 30 mL of 0.02 N H2SO4 is required to titrate the sample to pH
4.5.
• What is the total alkalinity of the water in mg/L as CaCO3?
Solution:
1 mL of 0.02 N H2SO4 will neutralize 1mg as CaCO3 of alkalinity,
So, 30 mL of 0.02 N H2SO4 will neutralize 30 mg as CaCO3 of
alkalinity.
The concentration of alkalinity = 30 mg / 200mL = 150mg/L as
CaCO3
Example
• Determine the species and the quantity of each specie, of the
alkalinity in Example 2-6 if the 8.3 equivalence point is reached
at 11 mL of acid.
Solution:
From example 2-6:
• Initial pH = 10  [H+] = 10-10 mole /L,
 [OH-] = 10-4 mole /L = (10-4 *1)*50,000
= 5 mg/L as CaCo3
•So, 5 mL of 0.02 N H2SO4 will neutralize 1 mg as CaCO3 of OH- in 1-L
sample.
• The given sample of 200 mL will require 5 *(200/1000) = 1 mL of
acid to neutralize total OH-.
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Solution (cont.):
Then, the remained 10 mL of acid measures (neutralizes) one half of
CO3 2- and the same volume (10 mL) of acid is required of the
remaining one half CO3 2-,
• So, 20 mL of acid measures 20 mg of CO3 2- in a sample of 200 mL
the concentration of CO3 2- = 20 mg * (1000/200)
= 100 mg/L as CaCo3
This will leave 9 mL of acid to measure HCO3-,
• The concentration of HCO3- = 9 mg * (1000/200)
= 45 mg/L as CaCo3
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Hardness
• Hardness is the concentration of multivalent metallic cations in
solution (mg/L), which includes mainly Ca2+ and Mg2+
• The units are, like alkalinity, mg/L as CaCO3
• Hardness is classified as carbonate hardness and non-carbonate
hardness, depending upon the anion with which it associated.
• Anions of alkalinity (e.g., CO3-) and cations of hardness (e.g., Ca2+)
are normally derived from the same carbonate minerals – and
this is the reason for the observed general association between
alkalinity and hardness
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1. Carbonate hardness - the portion of total
hardness that is chemically equivalent to the
CO32- and HCO3- alkalinity present in the
water.
2. Non-carbonate hardness - that hardness
which is in excess of carbonate hardness; will
only occur in water where Total Hardness >
alkalinity
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Sources
Caused by the presence of multivalent cations, mostly Ca2+
and Mg2+; (Fe2+, Mn2+, Sr2+, Al3+ may be present in much
smaller amounts).
Impacts
• Hardness determines how hard or easy it is to lather soap
• Hard water is water that requires considerable amounts of
soap to produce foam or lather; the precipitates formed by
the hardness and soap adheres to surfaces of tubes, sinks and
dishwashers, produces scale in hot water pipes, etc.
• Not a health concern, but have economic concern
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Measurement
Hardness can be measured using chemical titration to
determine the quantity of calcium and magnesium ions.
Use
• Analysis for hardness is commonly made on natural waters and
on waters for potable supplies and for certain industrial uses.
• Soft
• Moderately hard
• Hard
• Very hard
< 50 mg/L as CaCO3
50-150 mg/L as CaCO3
150-300 mg/L as CaCO3
>300 mg/L as CaCO3
• Maximum hardness of 500 mg/L in drinking water (puplic
health service standards)
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Fluoride
• Associated in nature with few types of sedimentary or igneous
rocks.
• Fluoride is seldom found in surface waters,
• Fluoride appears in groundwater in only few geographical
regions,
• Toxic to humans and animals in large quantities,
• Beneficial with small concentrations,
• 1 mg/L concentration in drinking water help to prevent dental
cavities in children … harder, stronger teeth.
• Added to water supplies for good dental formation
• Excessive fluoride can result in discoloration of teeth (< 2mg/L)
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Metals
1. Nontoxic Metals
Include calcium, magnesium, sodium, iron, manganese,
aluminum, copper and zinc.
Sodium: - in natural waters, earth crust.
- reactive , soluble in water.
- corrosive to metal surfaces,
- Toxic to plants in large concentrations.
Iron and manganese :
- in natural waters,
- (0.3 mg/L and 0.05 mg/L respec.) concentrations cause
color problems and may cause taste and odor problems in
the presence of some bacteria.
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• Iron associate with chloride or bicarbonate or sulfate,
• In the presence of oxygen, Fe2+ is oxidized to Fe3+ and forms
an insoluble compounds with hydroxide (Fe(OH)3) .
• Manganese (Mn 2+) associated with chloride or nitrate or
sulfate are soluble, while oxidized (Mn3+ , Mn5+ ) are insoluble.
Toxic Metals
• Are harmful to humans and other organisms in small
quantities
• include arsenic, barium, cadimuim, chromium, lead, mercury,
and silver
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