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Analytical Methods

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 Analysis | Physical Properties

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