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Chemical Analyses: Water: Acidity
Acidity is the capacity of a system to neutralize base. The more
acid a solution, the more base that must be added to raise the pH
to an acceptable level. It is important to know the acidity of a
solution since the creation of highly acidic waters is one of the
largest problem faced by the mining industry. By monitoring acidity
in runoff, receiving waters, and mine waste, it can be determined
when acid is being produced. Many different measures are taken at
a site to prevent the production of acidic waters or treat waters
that are acidic. For more information on the different technologies
used to mitigate acid mine drainage formation, see Technologies
for Project Construction.
The concept of acidity is opposite that of alkalinity and is also
based on 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 a base without raising the pH rapidly.
The acidity of a water source is generally attributable to the carbonate
molecules H2CO3 and HCO3- and sometimes to strong acids, namely,
H+. These molecules are acids and when OH- (a base) is added to
the water source, they will chemically bond with the acids. For
example:
H+ + OH- –-> H2O;
H2CO3 + OH- –-> HCO3-;
HCO3- + OH- –-> CO32-
it is shown here how the OH- ions chemically bond with the carbonate
minerals. After adding large quantities of OH-, eventually all the
carbonate molecules will be in the form of CO32-. When there are
mostly acidic carbonate molecules in solution (H2CO3, HCO3-, and
H+), the pH is correspondingly acidic (< 7). In contrast, if the
carbonate molecules in solution are mostly HCO3-, CO32-, and OH-,
the pH is correspondingly basic (< 7). The larger the concentration
of acidic molecules, the more OH- ions that must be added to the
water source in order to raise the pH significantly. In addition,
based on the concentration of acidic carbonate minerals and H+,
the concentration and volume of base that must be added in order
to raise the pH to a specified level can be determined (Snoeyink
and Jenkins, 1980).
The ability to define the amount of base (based on the acidity
of the solution) that must be added to raise the pH to a certain
level is important for remediation of acid mine drainage. For more
information on water treatment technologies used to raise the pH
of acidic solutions, see the water
treatment section.
In the laboratory, the determination of acidity is fairly similar
to the determination of alkalinity,
using a titration method. See section 2310 (Standard
Methods, 1998) for specifics on the titration method for acidity.
A specified concentration of base is continuously added to the acidic
solution until the solution pH is raised to a specified level. The
total volume of base added is recorded and expressed as mg CaCO32-/L
(MEND,
2001). For additional information on the carbonate system and
a more in depth discussion of acidity, see (Snoeyink
and Jenkins, 1980).
Chemical
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