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IV. Water Chemistry

IV. Water Chemistry. A. pH, hardness, and other ionic compounds and gases affecting water quality . Cations (positively charged ions):. Iron (Fe 2+ ) - causes taste, discoloration, hardness. Manganese (Mn 2+ ) - causes taste, discoloration, hardness, but less common than iron.

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IV. Water Chemistry

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  1. IV. Water Chemistry A. pH, hardness, and other ionic compounds and gases affecting water quality

  2. 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.

  3. Anions (negatively charged ions): • 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-).

  4. Dissolved gases in water: • Nitrogen (N2) - common atmospheric gas. • Ammonia (NH3) – indicate “fresh pollution”. • Oxygen (O2) - from atmosphere, plants. • improves taste of water, increases corrosiveness. • Carbon dioxide (CO2) - from atmosphere, animals, organic decomposition. • increases solvent properties of water. • Improves taste of water, increases corrosiveness.

  5. Dissolved gases in water (cont.): • 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.

  6. 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).

  7. pH (power of the Hydrogen ion): • 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.

  8. Water hardness: • Water hardness neutralizes soaps. • Scale at high temperature • Hardness may be caused by divalent metallic cations (Ca2+ , Mg2+ , Fe2+). • 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.

  9. Water softening(Removal of Hardness) • 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)

  10. Water softening(Removal of Hardness) • 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.

  11. 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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