70
Exercise 6
oxygen distribution near interfaces, particularly
in zones of steep oxygen gradients, as at the
sediment-water interface. Problems of slow diffusion rates and inaccuracy at low oxygen concentrations are circumvented to a significant
extent with oxygen microelectrodes [less than
100-,um diameter; cf., Revsbech and Jorgensen
(1986)]. The advantages of small size, where
sensing surfaces are only a few micrometers in
diameter, and rapid response times are counterbalanced by the difficulty of construction and
fragility. Nonetheless, microelectrodes are providing unprecedented understanding about the
distribution and dynmics of oxygen microgradients and about the ecology of the organisms
generating these gradients [e.g., Carlton and
Wetzel (1987,1988)]. Repeated calibration of
oxygen sensors by chemical methods of analysis
is required with solutions containing known
quantities of dissolved oxygen; calibration in air
is not satisfactory. Thus, although oxygen sensors are being improved constantly and will
dominate measurements of dissolved oxygen in
the future, the need still exists for chemical
methods of measuring dissolved oxygen.
THE WINKLER METHOD
The Winkler method for measuring dissolved oxygen was introduced in 1888 by L.W.
Winkler of Budapest and is a cleverly conceived as well as a very accurate procedure.
The method depends on the oxidation of manganous hydroxide (bivalent manganese)
by the oxygen dissolved in the water, resulting in the formation of a tetravalent
compound. When the water containing the tetravalent compound is acidified, free
iodine is liberated from the oxidation of potassium iodide. The free iodine is equivalent
chemically to the amount of dissolved oxygen present in the samples and is determined
by titration with a standard solution of sodium thiosulfate (e.g., 0.005 M). The reactions
involved by the addition of reagents (KI, KOH, MnSO 4' and H 2 S04) to the water are
as follows.
Manganous sulfate reacts with the potassium hydroxide-potassium iodide mixture
to produce a white flocculent precipitate of manganous hydroxide:
MnS0 4 + 2KOH ----> Mn(OH)2 + K 2S04
When the white precipitate is obtained, there is no dissolved oxygen in the sample. A
brown precipitate indicates that oxygen was present and reacted with the manganous
hydroxide, forming manganic basic oxide:
2Mn(OH)2 + O 2 ----> 2MnO(OHh
Upon the addition of sulfuric acid, this precipitate is dissolved, forming manganic
sulfate:
2MnO(OH)2 + 4H 2S04 ---->2Mn(S04h + 6H 20
There is an immediate reaction between Mn(S04)2 and the potassium iodide added
previously, liberating iodine and resulting in the typical iodine coloration (brown) of
the water:
2Mn(S04h + 4KI ---->2MnS04 + 2K 2S04 + 212
The number of moles of iodine liberated by this reaction is equivalent to the number
of moles of oxygen present in the sample. The quantity of iodine is determined by
titrating a portion of the solution with a standard solution of sodium thiosulfate:
4Na 2 S20 3 + 212 ----> 2Na2S40 6 + 4NaI
Exercise 6
oxygen distribution near interfaces, particularly
in zones of steep oxygen gradients, as at the
sediment-water interface. Problems of slow diffusion rates and inaccuracy at low oxygen concentrations are circumvented to a significant
extent with oxygen microelectrodes [less than
100-,um diameter; cf., Revsbech and Jorgensen
(1986)]. The advantages of small size, where
sensing surfaces are only a few micrometers in
diameter, and rapid response times are counterbalanced by the difficulty of construction and
fragility. Nonetheless, microelectrodes are providing unprecedented understanding about the
distribution and dynmics of oxygen microgradients and about the ecology of the organisms
generating these gradients [e.g., Carlton and
Wetzel (1987,1988)]. Repeated calibration of
oxygen sensors by chemical methods of analysis
is required with solutions containing known
quantities of dissolved oxygen; calibration in air
is not satisfactory. Thus, although oxygen sensors are being improved constantly and will
dominate measurements of dissolved oxygen in
the future, the need still exists for chemical
methods of measuring dissolved oxygen.
THE WINKLER METHOD
The Winkler method for measuring dissolved oxygen was introduced in 1888 by L.W.
Winkler of Budapest and is a cleverly conceived as well as a very accurate procedure.
The method depends on the oxidation of manganous hydroxide (bivalent manganese)
by the oxygen dissolved in the water, resulting in the formation of a tetravalent
compound. When the water containing the tetravalent compound is acidified, free
iodine is liberated from the oxidation of potassium iodide. The free iodine is equivalent
chemically to the amount of dissolved oxygen present in the samples and is determined
by titration with a standard solution of sodium thiosulfate (e.g., 0.005 M). The reactions
involved by the addition of reagents (KI, KOH, MnSO 4' and H 2 S04) to the water are
as follows.
Manganous sulfate reacts with the potassium hydroxide-potassium iodide mixture
to produce a white flocculent precipitate of manganous hydroxide:
MnS0 4 + 2KOH ----> Mn(OH)2 + K 2S04
When the white precipitate is obtained, there is no dissolved oxygen in the sample. A
brown precipitate indicates that oxygen was present and reacted with the manganous
hydroxide, forming manganic basic oxide:
2Mn(OH)2 + O 2 ----> 2MnO(OHh
Upon the addition of sulfuric acid, this precipitate is dissolved, forming manganic
sulfate:
2MnO(OH)2 + 4H 2S04 ---->2Mn(S04h + 6H 20
There is an immediate reaction between Mn(S04)2 and the potassium iodide added
previously, liberating iodine and resulting in the typical iodine coloration (brown) of
the water:
2Mn(S04h + 4KI ---->2MnS04 + 2K 2S04 + 212
The number of moles of iodine liberated by this reaction is equivalent to the number
of moles of oxygen present in the sample. The quantity of iodine is determined by
titrating a portion of the solution with a standard solution of sodium thiosulfate:
4Na 2 S20 3 + 212 ----> 2Na2S40 6 + 4NaI