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Chemical Analyses: Water: Dissolved Oxygen
The levels of dissolved oxygen (DO) in natural waters and wastewaters
depend on the physical, chemical, and biochemical activities in
the water source. Low DO concentrations can have harmful effects
on aquatic life that require oxygen for survival. Low DO concentrations
result from oxygen-demanding pollutants that serve as a food source
for aquatic microorganisms. These aquatic microorganisms use oxygen
in their metabolism and are capable of surviving at lower DO levels
than higher life forms thereby, lowering DO levels in the water
creating a harmful environment for aquatic life of higher life forms.
Most oxygen-demanding pollutants are organic compounds (i.e. municipal
waste), but ammonia nitrogen is an important inorganic pollutant.
The two most common methods for measuring DO are the Winkler (iodiometric)
and electrometric methods. The iodiometric method, section 4500-O
B (Standard
Methods, 1998) can be used on a routine basis to provide very
accurate estimates for DO in the microgram-per-liter range provided
that interfering particulate matter, color, and chemical interferences
are absent. This method is not applicable to many industrial and
domestic wastewaters or to field testing and cannot be adapted easily
for continuous monitoring of DO in situ. Electrometric methods,
section 4500-O G (Standard
Methods, 1998) utilize a membrane-covered electrode that measures
the rate of diffusion of molecular oxygen across the membrane. Membrane
electrodes provide an excellent method for DO analysis in polluted
waters, highly colored waters, and strong waste effluents. They
are recommended for use especially under conditions that are unfavorable
for use of the iodometric method, or when the iodometric method
and its modifications are subject to serious errors caused by interferences.
The portability, ease of operation and maintenance make membrane
electrodes particularly convenient for field applications.
Chemical
Analysis | Physical
Properties |
