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

Cations (positively charged ions): n Iron (Fe 2+ ) - causes taste, discoloration, hardness. n Manganese (Mn 2+ ) - causes taste, discoloration, hardness, but less common than iron. n Calcium (Ca 2+ ) - often occurs with Mg, causes hardness. n Magnesium (Mg 2+ ) - Mg 2+ & Ca 2+ are the primary contributors to water hardness.

Anions (negatively charged ions): n Chloride (Cl - ) - reduces palatability, contributes to permanent hardness. n Fluoride (F - ) - may occur naturally with chloride or added to prevent cavities. –High concentrations can cause tooth mottling or bone damage. n Nitrate (NO 3 - ) - inorganic plant nutrient, that may cause “blue baby” syndrome in children. n Carbonates (CO 3 - ) & bicarbonates (HCO 3 - ).

Dissolved gases in water: n Nitrogen (N 2 ) - common atmospheric gas. n Ammonia (NH3) – indicate “fresh pollution”. n Oxygen (O 2 ) - from atmosphere, plants. –improves taste of water, increases corrosiveness. n Carbon dioxide (CO 2 ) - from atmosphere, animals, organic decomposition. –increases solvent properties of water. –Improves taste of water, increases corrosiveness.

Dissolved gases in water (cont.): n Hydrogen sulfide (H 2 S) - 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): n The pH scale is used to measure the relative concentration of OH - and H + ions in solution (0 = most acidic, 14 = most basic). n pH of natural waters may vary widely (6-9) due to organic acids (e.g. humic acids) or alkaline mineral deposits (e.g., limestone).

pH (power of the Hydrogen ion): n 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: n Water hardness neutralizes soaps. n Scale at high temperature n Hardness may be caused by divalent metallic cations (Ca 2+, Mg 2+, Fe 2+ ). –Temporary hardness (carbonate hardness) is usually due to carbonates or bicarbonates and can be reduced by boiling (CO 3 - & HCO 3 - ). –Permanent hardness (non carbonate hardness) is due to Cl -, SO 4 -, NO 3 -, and silicate ions.

Water softening (Removal of Hardness) n Precipitation - get Ca 2+ and Mg 2+ into their least soluble forms (CaCO 3 and Mg(OH) 2 ). –Straight lime (Ca(OH) 2 ) addition (applies when water is high in carbonate hardness) –Excess Lime - soda ash (Na 2 CO 3 ) addition (applies when water is high in non carbonate hardness)

Water softening (Removal of Hardness) n Ion-Exchange (Ions of a non-soluble resin are used to displace the Ca 2+ and Mg 2+ ions in water) –resin (naturally occurring sodium aluminosilicate) –2NaAlSi 2 O 6 + Ca 2+ = Ca(AlSi 2 O 6 ) 2 +2Na + - 2NaAlSi 2 O 6 + Mg 2+ = Mg(AlSi 2 O 6 ) 2 +2Na + - 2NaAlSi 2 O 6 + Mg 2+ = Mg(AlSi 2 O 6 ) 2 +2Na + Regenerate by treating with a concentrated solution of NaCl.

Summary: n 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. n Examples include H +, OH -, Ca 2+, Mg 2+, Fe 2+, Mn 2+, Cl -, SO 4 -, F -, NO 3 -, CO 3 -, HCO 3 -, N 2, O 2, CO 2, CH 4, and H 2 S. n Water softening can be achieved by precipitation or ion-exchange.