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- Pickling Oxygen Samples using
Carpenter’s Modified Winkler Titration Method:
- Seawater samples are drawn into a
calibrated volumetric flasks using tygon tubing. The flask is
rinsed three times then overflowed with twice the sample
volume. Carefully removing the sampling tube to prevent the
influx of bubbles, the sample is then fixed with manganous chloride
(MnCl2) and sodium iodate/sodium hydroxide (NaI/NaOH). A
stable precipitate forms. The flask is stoppered and shaken
vigorously to homogenize. A sample label is filled out and
the case is covered. After settling for several minutes, a second shake
is performed to insure all the oxygen is fixed. The case is
added to the O2 sample queue. Autotitrator oxygen samples are
drawn and fixed in the same manner but the temperature of the sample is
measured and recorded.
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The relevant chemical reactions occurring
throughout the procedure are outlined below:
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Mn2+ + 2OH-
+ 1/2 O2
 oxygen-manganese complex + H2O
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(1)
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oxygen-manganese complex + 4H+
+ 2I-
I2 + Mn2+
+ 2H2O
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(2)
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I2 + 2Na2S2O3
Na2S4O6
+ 2NaI
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(3)
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- Addition of the manganous chloride and the
alkaline-iodide results in the formation of an insoluble
oxygen-manganese complex (1), the precipitate in step 2. The oxygen is
stable in this form for several days. Both the manganous chloride and
the alkaline-iodide are added in excess to ensure reaction with all of
the oxygen. Treatment with the sulfuric acid dissolves the complex and
liberates free iodine (2), imparting the distinctive yellow-gold color.
The amount of free iodine is proportional to the amount of oxygen
dissolved in the original sample. By titrating a measured portion of
the sample against a standardized sodium thiosulfate solution (3), the
amount of free iodine—and the corresponding amount of
oxygen—is determined. The starch "indicator" (which forms a
distinctly colored complex with the free iodine) is used to provide an
unmistakable visual endpoint for the titration.
- The concentration of dissolved oxygen (DO) is one of
the most important indicators of the overall health of a body of water.
Waters with consistently high levels of DO (> 6 mg/L) typically
support the most diverse biological communities. Waters with
consistently low DO levels (< 3 mg/L) may be virtually devoid of
aquatic life or may harbor only a few species adapted to such
conditions.
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