U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of
the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3
of 3--Subsurface Contamination From Point Sources, Water-Resources
Investigations Report 99-4018C
Equilibrium Vapor Method to Determine the Concentration of Inorganic Carbon
And Other Compounds in Water Samples
by Ronald J. Baker, Arthur L. Baehr, and Matthew A. Lahvis
ABSTRACT
The total concentration of dissolved carbon dioxide, bicarbonate, and carbonate
(CT) in water is a fundamental parameter in studies
of natural and contaminated systems. A new method (the equilibrium vapor method)
for determining CT that involves the measurement of carbon dioxide concentration
in the headspace of a water sample by gas chromatography and calculation of
CT by using the equilibrium relations of the carbonate system is presented.
The method is most accurate when sample pH is low (near the alkalinity titration
endpoint, about 4.3) when the standard titration method is least accurate or
inapplicable. The method is also advantageous over the standard titration method
when the water contains other alkaline species, such as salts of organic acids,
which can occur as metabolites of microbial activity. The presence of such species
can result in overestimation of CT with the standard titration method because
they contribute to alkalinity. An additional advantage of the equilibrium vapor
method is that aqueous concentrations of volatile compounds, such as hydrocarbons,
methyl-tert-butyl ether (MTBE), and dissolved gases, such as oxygen, nitrogen,
methane, and carbon disulfide, can be calculated from the headspace analysis
of a single sample by using Henry's law.
Ground-water samples from two gasoline-spill sites were analyzed
to demonstrate the method and compare the results with those obtained
by using the standard titration method. The two methods provided comparable
estimates of CT for samples
with pH values above 5.0. For low-pH samples, CT
was obtainable only with the equilibrium vapor method. Filtration
and acidification of samples did not affect the accuracy of the equilibrium
vapor method for samples from these sites; however, acidification
of samples from sites with significant amounts of carbonate minerals
may result in overestimation of CT.
The concentrations of gasoline hydrocarbons in several samples also
were determined.
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