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Analytical Methods

Chemical Analyses: Water: Alkalinity

Alkalinity is the capacity of a system to neutralize acid. How much acid can be added to a system before the pH of the solution is brought down to a certain level? The alkalinity of a natural water source is, for the most part, defined by the carbonate system. The carbonate system is the main group of molecules that determine how well a natural water source can "buffer" the addition of acid without the pH dropping rapidly. The molecules of the carbonate system that largely attribute to this ability to buffer large drops in pH are HCO3-, CO32-, and OH-. These molecules are bases and when H+ (acid) is added to the water source, the H+ ions will chemically bond with the bases. For example:

OH- + H+ —-> H2O;
CO32- + H+ —-> HCO3-;
HCO3- + H+ —-> H2CO3

it is shown here how the H+ ions chemically bond with the carbonate minerals. When the only carbonate mineral in solution is H2CO3, all the alkalinity has been used up since there are no carbonate molecules left for the H+ ions to chemically bond with.

When there are mostly basic carbonate molecules in solution (HCO3-, CO32-, and OH-), the pH is correspondingly basic (almost always >7). In contrast, if the carbonate molecules in solution are mostly H2CO3, the pH is correspondingly acidic (< 7). The larger the concentration of basic molecules, the more H+ ions that can be added to the water source without the pH dropping rapidly to a low level. If more acid is added to the system than can chemically react with the basic molecules, all carbonate molecules will be in the form of H2CO3 and the pH will drop to a very low level. This is classically represented by acid mine drainage with very low pHs = 2 or 3.

The presence of large quantities of alkalinity in water sources that are impacted by mining is very important in preventing highly acidic conditions. It is quite common to add alkalinity to a water source to raise the pH. Alkalinity is commonly added in the solid form of calcium carbonate (CaCO3), also known as limestone. When limestone dissolves in water, the calcium carbonate molecule dissociates and large concentrations of CO32- are released into solution which will chemically bond with H+ ions. When all the H+ ions have chemically bonded with the CO32- molecules, CO32- and HCO3- will accumulate in solution, increasing the alkalinity of the water source and also raising the pH (Snoeyink and Jenkins, 1980). See the water treatment section of the tech guide to learn more about the different water treatment techniques used to add alkalinity and reduce acidity in mine contaminated wastewaters.

The general method for measuring alkalinity is the potentiometric titration technique. This method involves continuously adding volumes of acid with a certain concentration to a water sample until the pH of the water reaches a specified endpoint. The total volume of acid required to drop the pH to a certain level is called a titration. "Potentiometric" refers to the use of a pH meter to identify when the desired pH has been reached. The amount of acid added is converted to equivalent mg CaCO3/L and reported along with the titrated pH endpoint. mg CaCO3/L is the common way to express the concentration of alkalinity (MEND, 2001). See Section 2320 in Standard Methods, 1998 for details on the titration method.

For a more in depth understanding of alkalinity and the carbonate system, see Snoeyink and Jenkins, 1980.

 

Chemical Analysis | Physical Properties

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