IV. Water ChemistryIV. Water Chemistry

A. pH, hardness, and other A. pH, hardness, and other ionic compounds and gases ionic compounds and gases

affecting water quality affecting water quality

Cations (positively charged ions): Iron (FeIron (Fe2+2+) - causes taste, discoloration, ) - causes taste, discoloration,

hardness.hardness. Manganese (MnManganese (Mn2+2+) - causes taste, ) - causes taste,

discoloration, hardness, but less discoloration, hardness, but less common than iron.common than iron.

Calcium (CaCalcium (Ca2+2+) - often occurs with Mg, ) - often occurs with Mg, causes hardness.causes hardness.

Magnesium (MgMagnesium (Mg2+2+) - Mg) - Mg2+2+ & Ca & Ca2+2+ are the are the primary contributors to water hardness.primary contributors to water hardness.

Anions (negatively charged ions): Chloride (ClChloride (Cl--) - reduces palatability, ) - reduces palatability,

contributes to permanent hardness.contributes to permanent hardness. Fluoride (FFluoride (F--) - may occur naturally with ) - may occur naturally with

chloride or added to prevent cavities.chloride or added to prevent cavities.– High concentrations can cause tooth High concentrations can cause tooth

mottling or bone damage.mottling or bone damage. Nitrate (NONitrate (NO33

-- ) - inorganic plant nutrient, that ) - inorganic plant nutrient, that

may cause “blue baby” syndrome in children.may cause “blue baby” syndrome in children. Carbonates (COCarbonates (CO33

--) & bicarbonates (HCO) & bicarbonates (HCO33--).).

Dissolved gases in water: Nitrogen (NNitrogen (N22) - common atmospheric gas.) - common atmospheric gas. Ammonia (NH3) – indicate “fresh pollution”.Ammonia (NH3) – indicate “fresh pollution”. Oxygen (OOxygen (O22) - from atmosphere, plants. ) - from atmosphere, plants.

– improves taste of water, increases improves taste of water, increases corrosivenesscorrosiveness..

Carbon dioxide (COCarbon dioxide (CO22) - from atmosphere, ) - from atmosphere,

animals, organic decomposition.animals, organic decomposition.– increases solvent properties of water. increases solvent properties of water. – Improves taste of water, increases Improves taste of water, increases corrosivenesscorrosiveness..

Dissolved gases in water (cont.):

Hydrogen sulfide (HHydrogen sulfide (H22S) - affects taste S) - affects taste

and odor, contributes to and odor, contributes to corrosiveness.corrosiveness.– Often found in groundwater, seldom in Often found in groundwater, seldom in

surface water due to volatility.surface water due to volatility.– Produced by bacterial conversion of Produced by bacterial conversion of

sulfur-bearing minerals and/or sulfur-bearing minerals and/or decomposition of organic matter.decomposition of organic matter.

pH (power of the Hydrogen ion): The pH scale is used to measure the The pH scale is used to measure the

relative concentration of OHrelative concentration of OH-- and H and H++ ions in solution (0 = most acidic, 14 = ions in solution (0 = most acidic, 14 = most basic).most basic).

pH of natural waters may vary widely pH of natural waters may vary widely (6-9) due to organic acids (e.g. humic (6-9) due to organic acids (e.g. humic acids) or alkaline mineral deposits acids) or alkaline mineral deposits (e.g., limestone).(e.g., limestone).

pH (power of the Hydrogen ion):

AlkalinityAlkalinity is defined as the ability is defined as the ability to neutralize acids and may be to neutralize acids and may be due to:due to:– Carbonates and bicarbonates (to be Carbonates and bicarbonates (to be

discussed later) or,discussed later) or,– Hydroxides released from bases.Hydroxides released from bases.

Water hardness: Water hardness neutralizes soaps.Water hardness neutralizes soaps. Scale at high temperatureScale at high temperature HardnessHardness may be caused by divalent may be caused by divalent

metallic cations (Cametallic cations (Ca2+2+ , Mg , Mg2+2+ , Fe , Fe2+2+).).– TemporaryTemporary hardness ( hardness (carbonatecarbonate hardness) is hardness) is

usually due to carbonates or bicarbonates and usually due to carbonates or bicarbonates and can be reduced by boiling (COcan be reduced by boiling (CO33

-- & HCO & HCO33--).).

– PermanentPermanent hardness ( hardness (non carbonatenon carbonate hardness) hardness) is due to Clis due to Cl-- , SO , SO44

-- , NO , NO33-- , and silicate ions. , and silicate ions.

Water softeningWater softening(Removal of (Removal of HardnessHardness))

PrecipitationPrecipitation - get Ca - get Ca2+2+ and Mg and Mg2+2+ into into their least soluble forms (CaCOtheir least soluble forms (CaCO3 3 and and Mg(OH)Mg(OH)2 2 ).).– Straight lime (Ca(OH)Straight lime (Ca(OH)22) addition) addition (applies (applies

when water is high in carbonate when water is high in carbonate hardness)hardness)

– Excess Lime - soda ash (NaExcess Lime - soda ash (Na22COCO33) ) additionaddition (applies when water is high in (applies when water is high in non carbonate hardness)non carbonate hardness)

Water softeningWater softening(Removal of (Removal of HardnessHardness))

Ion-Exchange (Ions of a non-soluble resin Ion-Exchange (Ions of a non-soluble resin are used to displace the Caare used to displace the Ca2+2+ and Mg and Mg2+2+ ions ions in water)in water)– resin (naturally occurring sodium aluminosilicate)resin (naturally occurring sodium aluminosilicate)

– 2NaAlSi2NaAlSi22OO66 + Ca + Ca2+ 2+ = Ca(AlSi= Ca(AlSi22OO66))22 +2Na +2Na++

- - 2NaAlSi2NaAlSi22OO66 + Mg + Mg2+ 2+ = Mg(AlSi= Mg(AlSi22OO66))22 +2Na +2Na++

Regenerate by treating with a concentrated Regenerate by treating with a concentrated solution of NaCl.solution of NaCl.

Summary: Specific ions, ionic compounds and gases Specific ions, ionic compounds and gases

in water may affect acidity, taste, odor, in water may affect acidity, taste, odor, color, hardness, and corrosiveness of color, hardness, and corrosiveness of water in a variety of different ways.water in a variety of different ways.

Examples include HExamples include H++, OH, OH--, Ca, Ca2+2+, Mg, Mg2+2+, Fe, Fe2+2+, , MnMn2+2+, Cl, Cl--, SO, SO44

--, F, F--, NO, NO33--, CO, CO33

--, HCO, HCO33--, N, N22, O, O22, ,

COCO22, CH, CH44, and H, and H22S.S. Water softening can be achieved by Water softening can be achieved by

precipitation or ion-exchange.precipitation or ion-exchange.