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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
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