pH, Alkalinity, and Water Hardness
When water molecules are ionized, they produce hydrogen (H+) and hydroxyl (OH-) ions, which carry an electrical charge. These ions in the water determine its fundamental character-whether it is acid (excess H+) or alkaline (excess OH-). The term "pH" refers to the hydrogen cation (H+) concentration in water and is defined as the logarithm of the reciprocal of the hydrogen-ion concentration.
Alkalinity is a measure of the buffering capacity of the bicarbonate ions and, to some extent, the carbonate and hydroxide ions of water. These three ions all react with hydrogen ions to reduce acidity and raise pH. Alkalinity is normally given in mg/l as calcium carbonate (CaCO3) for all three ions.
The result of the competition between the pH increasing and lowering properties of water is determined by the residual alkalinity (RA). The residual alkalinity is the difference between carbonate (carbonate + bicarbonate) hardness and non-carbonate hardness, which is shown by the following relationship:
Total water hardness is the measure of the bicarbonate, calcium, and magnesium ions present in the water. Total hardness is expressed as mg/l of calcium carbonate (CaCO3), which determines the degree of softness or hardness. Generally, a measurement of fewer than 50 mg/l is considered very soft water, 50 to 100 mg/l is considered soft water, 100 to 200 mg/l is considered medium-soft water, 200 to 400 mg/l is considered moderately hard water, 400 to 600 mg/l is considered hard water, and greater than 600 mg/l is considered very hard water.
In the United States, temporary hardness results from calcium, magnesium, and sodium salts of carbonates and bicarbonates in the water. Most of the hardness is attributed to calcium bicarbonate.
Permanent hardness is that portion of total hardness remaining after the water has been boiled. Permanent hardness results from calcium and magnesium salts of sulfates and chlorides remaining in the water.
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