Water Chemistry: Hardness

Hardness Related to the dissolved minerals in contains General Hardness (GH): caused be divalent ions (valence of two, can form 2 covalent bonds) Calcium and Magnesium Carbonate Hardness (KH): due to carbonate/bicarbonate ions Main buffering capacity High KH, stable pH Low KH, unstable pH

Hardness Calcium and Magnesium acquired in aquatic environments through contact with rocks and sediments Positive Ions Bicarbonate and Carbonate acquired from limestone sediments and carbon dioxide Negative ions

Hardness Carbon dioxide reacts with water to form carbonic acid (1) which at ordinary environmental pH exists mostly as bicarbonate ion (2). Microscopic marine organisms take this up as carbonate (4) to form calcite skeletons which, over millions of years, have built up extensive limestone deposits. Groundwaters, made slightly acidic by CO2 (both that absorbed from the air and from the respiration of soil bacteria) dissolve the limestone (3), thereby acquiring calcium and bicarbonate ions and becoming "hard". If the HCO3– concentration is sufficiently great, the combination of processes (2) and (4) causes calcium carbonate ("lime scale") to precipitate out on surfaces such as the insides of pipes. (Calcium bicarbonate itself does not form a solid, but always precipitates as CaCO3.)

Hardness Water Hardness is read as parts per million (ppm) or weight/volume (mg/L) of calcium carbonate in the water Hard water (higher concentration): toothpaste is gritty when brushing (not as much lather), harder to get soap off Soft water (lower concentration): toothpaste is smooth and lathers easily, soap rinses off easily