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Dissolved Oxygen Essay

3465 words - 14 pages

12.097 Environmental Chemistry of Boston Harbor – IAP 2006

Lab 1:
DETERMINATION OF DISSOLVED OXYGEN BY WINKLER TITRATION
1. Background
Knowledge of the dissolved oxygen (O2) concentration in seawater is often necessary in
environmental and marine science. It may be used by physical oceanographers to study
water masses in the ocean. It provides the marine biologist with a means of measuring
primary production - particularly in laboratory cultures. For the marine chemist, it
provides a measure of the redox potential of the water column.
The concentration of dissolved oxygen can be readily, and accurately, measured by the
method originally developed by Winkler in 1888 (Ber. Deutsch ...view middle of the document...

Oxygen in the water sample oxidizes iodide ion (I-) to iodine (I2)
quantitatively. The amount of iodine generated is then determined by titration with a
standard thiosulfate (S2O3-2) solution. The endpoint is determined by using starch as a
visual indicator. The amount of oxygen can then be computed from the titer: one
mole of O2 reacts with four moles of thiosulfate.

12.097 Environmental Chemistry of Boston Harbor – IAP 2006
At the time of sampling, dissolved oxygen is fixed by the addition of Mn(II) under
basic conditions, resulting in a brown precipitate, manganic hydroxide (MnO(OH)2).
Prior to analysis, the sample is acidified to pH 1.0-2.5. This causes the precipitated
hydroxides to dissolve, liberating Mn(III) ions. Mn(III) ions oxidize previously added
iodide ions to iodine. Iodine forms a complex (I3-) with surplus iodide ions. Iodine
and the complex exist in equilibrium; thus, I3- serves as a reservoir of I2. The iodine
is then titrated with thiosulfate; iodine is reduced to iodide and the thiosulfate is
oxidized to tetrathionate. The stoichiometric equations for the reactions described
above are:
Mn +2 + 2OH − → Mn (OH )2
1
oxidation of Mn(II) to Mn(III)
2 Mn(OH )2 + O2 + H 2 O → 2 MnO (OH )2
2
2 Mn(OH )3 + 2 I − + 6 H + → 2 Mn +2 + I 2 + 6 H 2 O oxidation of I- to I2

I 2 + I − ↔ I 3−
I 3− + 2 S 2 O3−2 → 3I − + S 4 O6−2

oxidation of S2O3-2 to S4O6-2; reduction of I3- to I-

The thiosulfate solution is not stable and therefore must be standardized with a
primary standard, typically potassium iodate (KIO3). Standardization is based on the
co-proportionation reaction of iodide with iodate, thereby forming iodine. As
described above, the iodine binds with excess iodide, and the complex is titrated with
thiosulfate. One mole of iodate produces three moles iodine, which are consumed by
six moles of thiosulfate.
IO3− + 8 I − + 6 H + → 3I 3− + 3H 2 O

I 3 + 2 S 2 O3

2−

→ 3 I − + S 4 O6

2−

A note to the student:
The entire method is presented below. It is important that you understand the
complete method from sample collection to analysis. In addition, you might need/
want to repeat this measurement in the future. All reagents will be made before the
lab period. Additionally, the sodium thiosulfate solution will be standardized for you.
If time allows, you may get a chance to do this. You will be responsible for
measuring your own blank and samples. You are responsible for the procedures
described in section 9c and onward.

12.097 Environmental Chemistry of Boston Harbor – IAP 2006
5. Sampling apparatus
a. Sample flasks: Glass stoppered dissolved oxygen bottles (115 mL nominal
capacity – also called “BOD bottles”). 1 clean and dry bottle per sample, blanks
and standard.
b. Volumetric dispensers (or manual volumetric pipettes (disposable, glass)).
c. Four dispensers capable of accurately dispensing 1 mL aliquots. These should be
labeled “Reagent #1”, “Reagent #2” and...

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