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Alkalinity measures the ability of water to resist changes in the H+ ion concentration (or pH) when either acid (H+) or base (OH-) is added to the water. The ability to resist changes in pH mainly depends on the amount of dissolved carbon dioxide (CO2) in the water. A measurement of alkalinity represents the total amount of bicarbonate and carbonate in the river. Like hardness, alkalinity is measured in mg/L of calcium carbonate.
If H+ ions (acid) are added to a system, carbonate ions will associate with them so they are not left free in solution. If OH- ions (base) are added to the system, H+ ions dissociate, or split apart, from the carbonate ion and combine with the OH- ions to form water. By this constant adjustment in the concentrations of carbonate and bicarbonate ions, the concentration of free H+ ions is kept relatively constant – that is the pH will change very little. A system that can resist changes in pH is a "buffered" system, referring to its ability to absorb acidic ions without major changes in pH. Such a system is highly alkaline.
Living cells utilize this buffering system to maintain a uniform internal environment where the biochemical reactions necessary to sustain life can proceed. High alkalinity also protects fish in streams that contain high concentrations of dissolved metals.
The same minerals in soils and rocks that influence hardness also affect alkalinity. Surface water or groundwater that has contact with limestone or dolomite will have high values for alkalinity. Large amounts of acid from mine drainage will lower the alkalinity because carbonate and bicarbonate ions will be removed from solution as they associate with the H+ ions. Alkalinity fluctuates daily and seasonally but to a lesser degree than hardness. Also, alkalinity values are always less than hardness values except in the presence of certain geologic influences. There is no state standard for alkalinity. |
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