EXERCISE 6
Dissolved Oxygen
The measurement of dissolved oxygen is one of
the most frequently used and the most important
of all chemical methods available for the investigation of the aquatic environment. Dissolved
oxygen provides valuable information about the
biological and biochemical reactions going on in
waters; it is a measure of one of the important
environmental factors affecting aquatic life and
of the capacity of water to receive organic matter
without causing nuisance.
Oxygen gas dissolves freely in fresh waters.
Oxygen may be added to the water from the
atmosphere or as a by-product of photosynthesis
from aquatic plants and is utilized by many
respiratory biochemical, as well as by inorganic
chemical reactions. The concentration of dissolved oxygen in water depends also on temperature, pressure, and concentrations of various ions
[cf., Hutchinson (1957), Wetzel (1983)].
To be successful, a method for measuring
dissolved oxygen must meet two requirements.
First, owing to the small amount of substance to
be determined (a few mg/l), it must be exact;
second, it must be done with apparatus suited for
field operation.
The method least subject to chemical errors,
and probably the first to be proposed, is that of
Bunsen, in which the gases are boiled out under
either atmospheric pressure or diminished
pressure. The amount of gas collected then is
measured by absorption methods. However,
the Bunsen method is too cumbersome for
field work and requires considerable skill for
accurate manipulation.
A few colorimetric methods have been proposed, but most have been found to be quite
69
inaccurate, particularly at low concentrations.
Although a number of chemical methods have
been employed for dissolved oxygen measurement, the Winkler method, or some modification
of it, is the most frequently used in limnology.
In recent times, major advances have occurred
in the development and application of oxygensensitive electrodes for the rapid and sensitive
measurement of dissolved oxygen. The Clarktype polarographic oxygen sensors often consist
of platinum anode and a gold-plated cathode,
encased in an electrolyte-filled housing and separated from the water by an oxygen-permeable
membrane. Oxygen must diffuse through the
membrane and electrolytic solution to the
electrodes. The quantity of oxygen reduced per
unit time is directly proportional to the oxygen
concentration in the water, and the resulting
electrical current is measured with a meter [cf.,
Gnaiger and Forstner (1983)].
Oxygen electrodes have the advantages of
speed of measurement and the potential for
continuous measurement in remote places. Commercially available macro electrodes (ca. \- 5-mm
diameter) require a rapid flow of water across the
membrane; without such exchange, measurements are inaccurate and unreliable. Simple up
and down movements of the electrodes in the
water are insufficient to provide the conditions
necessary for accurate measurements. Nearly all
macroelectrodes are unreliable at dissolved oxygen concentrations between 0 and 1 mg/1. This
low range is critical for many major chemical
transformations and dissociation reactions, as
well as crucial for microbial metabolism. Macroelectrodes are not satisfactory for studies of
Dissolved Oxygen
The measurement of dissolved oxygen is one of
the most frequently used and the most important
of all chemical methods available for the investigation of the aquatic environment. Dissolved
oxygen provides valuable information about the
biological and biochemical reactions going on in
waters; it is a measure of one of the important
environmental factors affecting aquatic life and
of the capacity of water to receive organic matter
without causing nuisance.
Oxygen gas dissolves freely in fresh waters.
Oxygen may be added to the water from the
atmosphere or as a by-product of photosynthesis
from aquatic plants and is utilized by many
respiratory biochemical, as well as by inorganic
chemical reactions. The concentration of dissolved oxygen in water depends also on temperature, pressure, and concentrations of various ions
[cf., Hutchinson (1957), Wetzel (1983)].
To be successful, a method for measuring
dissolved oxygen must meet two requirements.
First, owing to the small amount of substance to
be determined (a few mg/l), it must be exact;
second, it must be done with apparatus suited for
field operation.
The method least subject to chemical errors,
and probably the first to be proposed, is that of
Bunsen, in which the gases are boiled out under
either atmospheric pressure or diminished
pressure. The amount of gas collected then is
measured by absorption methods. However,
the Bunsen method is too cumbersome for
field work and requires considerable skill for
accurate manipulation.
A few colorimetric methods have been proposed, but most have been found to be quite
69
inaccurate, particularly at low concentrations.
Although a number of chemical methods have
been employed for dissolved oxygen measurement, the Winkler method, or some modification
of it, is the most frequently used in limnology.
In recent times, major advances have occurred
in the development and application of oxygensensitive electrodes for the rapid and sensitive
measurement of dissolved oxygen. The Clarktype polarographic oxygen sensors often consist
of platinum anode and a gold-plated cathode,
encased in an electrolyte-filled housing and separated from the water by an oxygen-permeable
membrane. Oxygen must diffuse through the
membrane and electrolytic solution to the
electrodes. The quantity of oxygen reduced per
unit time is directly proportional to the oxygen
concentration in the water, and the resulting
electrical current is measured with a meter [cf.,
Gnaiger and Forstner (1983)].
Oxygen electrodes have the advantages of
speed of measurement and the potential for
continuous measurement in remote places. Commercially available macro electrodes (ca. \- 5-mm
diameter) require a rapid flow of water across the
membrane; without such exchange, measurements are inaccurate and unreliable. Simple up
and down movements of the electrodes in the
water are insufficient to provide the conditions
necessary for accurate measurements. Nearly all
macroelectrodes are unreliable at dissolved oxygen concentrations between 0 and 1 mg/1. This
low range is critical for many major chemical
transformations and dissociation reactions, as
well as crucial for microbial metabolism. Macroelectrodes are not satisfactory for studies of