Transcript IV. Water Chemistry A. pH, hardness, and other affecting water quality

IV. Water Chemistry
A. pH, hardness, and other
ionic compounds and gases
affecting water quality
Cations (positively charged ions):
Iron (Fe2+) - causes taste, discoloration,
hardness.
 Manganese (Mn2+) - causes taste,
discoloration, hardness, but less
common than iron.
 Calcium (Ca2+) - often occurs with Mg,
causes hardness.
 Magnesium (Mg2+) - Mg2+ & Ca2+ are the
primary contributors to water hardness.
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Anions (negatively charged ions):
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Chloride (Cl-) - reduces palatability,
contributes to permanent hardness.
Fluoride (F-) - may occur naturally with
chloride or added to prevent cavities.
– High concentrations can cause tooth
mottling or bone damage.
Nitrate (NO3- ) - inorganic plant nutrient, that
may cause “blue baby” syndrome in
children.
Carbonates (CO3-) & bicarbonates
(HCO3-).
Dissolved gases in water:
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Nitrogen (N2) - common atmospheric gas.
Ammonia (NH3) – indicate “fresh pollution”.
Oxygen (O2) - from atmosphere, plants.
– improves taste of water, increases corrosiveness.
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Carbon dioxide (CO2) - from atmosphere,
animals, organic decomposition.
– increases solvent properties of water.
– Improves taste of water, increases corrosiveness.
Dissolved gases in water (cont.):
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Hydrogen sulfide (H2S) - affects taste
and odor, contributes to
corrosiveness.
– Often found in groundwater, seldom in
surface water due to volatility.
– Produced by bacterial conversion of
sulfur-bearing minerals and/or
decomposition of organic matter.
pH (power of the Hydrogen ion):
The pH scale is used to measure the
relative concentration of OH- and H+
ions in solution (0 = most acidic, 14 =
most basic).
 pH of natural waters may vary widely
(6-9) due to organic acids (e.g. humic
acids) or alkaline mineral deposits
(e.g., limestone).
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pH (power of the Hydrogen ion):
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Alkalinity is defined as the ability
to neutralize acids and may be
due to:
– Carbonates and bicarbonates (to be
discussed later) or,
– Hydroxides released from bases.
Water hardness:
Water hardness neutralizes soaps.
 Scale at high temperature
 Hardness may be caused by divalent
metallic cations (Ca2+ , Mg2+ , Fe2+).
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– Temporary hardness (carbonate hardness) is
usually due to carbonates or bicarbonates and
can be reduced by boiling (CO3- & HCO3-).
– Permanent hardness (non carbonate hardness)
is due to Cl- , SO4- , NO3- , and silicate ions.
Water softening
(Removal of Hardness)
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Precipitation - get Ca2+ and Mg2+ into
their least soluble forms (CaCO3 and
Mg(OH)2 ).
– Straight lime (Ca(OH)2) addition (applies
when water is high in carbonate
hardness)
– Excess Lime - soda ash (Na2CO3)
addition (applies when water is high in
non carbonate hardness)
Water softening
(Removal of Hardness)
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Ion-Exchange (Ions of a non-soluble
resin are used to displace the Ca2+ and
Mg2+ ions in water)
– resin (naturally occurring sodium
aluminosilicate)
– 2NaAlSi2O6 + Ca2+ = Ca(AlSi2O6)2 +2Na+
- 2NaAlSi2O6 + Mg2+ = Mg(AlSi2O6)2 +2Na+
Regenerate by treating with a concentrated
solution of NaCl.
Summary:
Specific ions, ionic compounds and
gases in water may affect acidity,
taste, odor, color, hardness, and
corrosiveness of water in a variety of
different ways.
 Examples include H+, OH-, Ca2+, Mg2+,
Fe2+, Mn2+, Cl-, SO4-, F-, NO3-, CO3-,
HCO3-, N2, O2, CO2, CH4, and H2S.
 Water softening can be achieved by
precipitation or ion-exchange.
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