Dissolved Oxygen
73
acidification the manganic hydroxide acts on any reducing agents that may be present.
Iodide, oxalate, and other reducing agents compete with one another. When oxalate is
present in substantial amounts and the iodide concentration is low, a considerable
amount of manganic hydroxide will react with oxalate, causing low results. When a
high concentration of iodide is present, more iodide and less oxalate will be used. After
adding the acid, there may be a slow reaction of iodine with organic matter. This
problem is diminished by a high concentration of iodide.
Loss of iodine vapors may cause appreciable error; this error is diminished by
increasing the iodide concentration, since iodide holds the iodine in solution by the
equilibrium equation: 1- + 12 ~ 13.
An additional advantage is that the high concentration of iodide results in a sharper
end point in titration.
7. In saline waters with high carbonate concentrations, the Winkler method cannot be
used because of CO 2 effervescence upon acidification. Variations of the Miller
method of oxygen determination should be used [cf., Walker et al. (1970) and Ellis
and Kanamori (1973)].
PROCEDURES
The primary difficulty in determining the concentration of dissolved oxygen in fresh
waters is related to the collection of the water sample. Of necessity many samples must
be taken at a particular time, place, and depth, under a specified set of conditions, which
mayor may not represent the average conditions over a period of time or the average
cross-section of the lake, stream, or other body of water.
Moreover, the samples should represent the actual conditions at the point of
sampling and not be aerated or de-aerated artificially. Samples should not be allowed
to stand for long periods or at high temperature to permit algae, bacteria, and other
organisms to change the dissolved oxygen content by their metabolism, nor should the
dissolved gases be allowed to escape.
1. Using a Van Dorn or similar in situ water sampler, collect unmodified water
samples from depth.
2. Remove the sample from the water bottle immediately.
3. Transfer the sample from the water bottle by placing the delivery tube to the
bottom of a 250- or 300-ml BOD (= biological oxygen demand) bottle. Allow the
water to flow continuously through the bottle, excluding all entrapped bubbles,
until at least three times the capacity ofthe sample bottle has overflowed (count the
seconds needed to fill the BOD bottle initially and then repeat twice). Withdraw the
delivery tube gently at the end of sample flushing, without stopping flow, so that the
bottle is filled completely. Do not stopper the bottle when the following step can be
done immediately.
4. Immediately and gently add, just below the surface, 1 ml of MnS0 4 reagent and
1 ml of NaOH + KI. Be careful not to mix the automatic pipets (do not mouth pipet
these reagents). Carefully stopper the bottle without introducing any air bubbles
and mix vigorously by inversion.
5. Allow the precipitate to settle. Shake vigorously again and allow the precipitate to
settle to at least the bottom third of the bottle volume.
6. Add 1 ml of concentrated H 2 S0 4 with an automatic pipet by inserting the tip just
below the surface of the sample. Carefully stopper without introducing air bubbles
and shake the bottle until all of the precipitate has dissolved.
73
acidification the manganic hydroxide acts on any reducing agents that may be present.
Iodide, oxalate, and other reducing agents compete with one another. When oxalate is
present in substantial amounts and the iodide concentration is low, a considerable
amount of manganic hydroxide will react with oxalate, causing low results. When a
high concentration of iodide is present, more iodide and less oxalate will be used. After
adding the acid, there may be a slow reaction of iodine with organic matter. This
problem is diminished by a high concentration of iodide.
Loss of iodine vapors may cause appreciable error; this error is diminished by
increasing the iodide concentration, since iodide holds the iodine in solution by the
equilibrium equation: 1- + 12 ~ 13.
An additional advantage is that the high concentration of iodide results in a sharper
end point in titration.
7. In saline waters with high carbonate concentrations, the Winkler method cannot be
used because of CO 2 effervescence upon acidification. Variations of the Miller
method of oxygen determination should be used [cf., Walker et al. (1970) and Ellis
and Kanamori (1973)].
PROCEDURES
The primary difficulty in determining the concentration of dissolved oxygen in fresh
waters is related to the collection of the water sample. Of necessity many samples must
be taken at a particular time, place, and depth, under a specified set of conditions, which
mayor may not represent the average conditions over a period of time or the average
cross-section of the lake, stream, or other body of water.
Moreover, the samples should represent the actual conditions at the point of
sampling and not be aerated or de-aerated artificially. Samples should not be allowed
to stand for long periods or at high temperature to permit algae, bacteria, and other
organisms to change the dissolved oxygen content by their metabolism, nor should the
dissolved gases be allowed to escape.
1. Using a Van Dorn or similar in situ water sampler, collect unmodified water
samples from depth.
2. Remove the sample from the water bottle immediately.
3. Transfer the sample from the water bottle by placing the delivery tube to the
bottom of a 250- or 300-ml BOD (= biological oxygen demand) bottle. Allow the
water to flow continuously through the bottle, excluding all entrapped bubbles,
until at least three times the capacity ofthe sample bottle has overflowed (count the
seconds needed to fill the BOD bottle initially and then repeat twice). Withdraw the
delivery tube gently at the end of sample flushing, without stopping flow, so that the
bottle is filled completely. Do not stopper the bottle when the following step can be
done immediately.
4. Immediately and gently add, just below the surface, 1 ml of MnS0 4 reagent and
1 ml of NaOH + KI. Be careful not to mix the automatic pipets (do not mouth pipet
these reagents). Carefully stopper the bottle without introducing any air bubbles
and mix vigorously by inversion.
5. Allow the precipitate to settle. Shake vigorously again and allow the precipitate to
settle to at least the bottom third of the bottle volume.
6. Add 1 ml of concentrated H 2 S0 4 with an automatic pipet by inserting the tip just
below the surface of the sample. Carefully stopper without introducing air bubbles
and shake the bottle until all of the precipitate has dissolved.