72
Exercise 6
Iron affects the measurement of dissolved oxygen in two ways [e.g., Buswell and
Gallaher (1923)]. First, when present in large amounts, iron requires so much of an even
stronger oxidant to oxidize it from the ferrous condition that the dissolved oxygen in
the reagent itself is appreciable; second, when oxidized to the ferric state it serves as an
oxidizing agent itself and will oxidize the iodide ion to free iodine, which in turn will
give high values of dissolved oxygen:
In the Rideal-Stewart modification, the ferrous iron and some of the nitrites and
organic matter are first oxidized by potassium permanganate, and then excess
permanganate is removed with potassium oxalate. Care must be taken not to add too
great an excess of the oxalate or error will be introduced into the final result. The
addition of potassium fluoride as the first reagent, or after the Rideal-Stewart treatment
for ferrous ion, will eliminate this interference as the more active fluoride ions will be
liberated rather than iodine.
The extent of interference by organic matter, which causes low results, varies with the
technique of the procedure. If the precipitate of manganous and manganic hydroxide
settles and compacts on the bottom of the bottle, and if the acid is added without
prompt and complete mixing, local depletion of iodide occurs, thus permitting
more extensive oxidation of organic matter. Interference is minimized by adding the
acid promptly and mixing rapidly.
In addition to the Winkler method, which should be used in measuring the dissolved
oxygen of relatively pure water, there are several modifications which are described in
detail in American Public Health Association et al. (1989):
1. Winkler method, unmodified: It should be noted that 1 ml of each reagent is
recommended in 250- to 300-ml bottles (Welch, 1948).
2. Alsterberg (Azide) modification: This modification is used for water containing
more than 0.05 mg/l ferrous iron. Other reducing or oxidizing agents should be
absent. If 1 ml of potassium fluoride solution were added to the sample before
acidifying and there were no delay in titration, the method also would be applicable
in the presence of 100 to 200 mg/l ferric iron.
3. Rideal-Stewart (permanganate) modification: This modification should be used
only on samples containing ferrous iron. It is ineffective for the oxidation of sulfates,
thiosulfates, and polythionates, or of the organic matter in sewage.
4. Alum flocculation modification: Samples high in suspended solids may absorb
appreciable quantities of iodine in acid solution. This interference may be removed
by alum flocculation.
5. Copper sulfate-sulfamic acid flocculation modification: This modification is used for
biological flocs such as activated sludge mixtures, which have high rates of oxygen
consumption.
6. Pomeroy-Kirschman-Alsterherg modification: This modification is designed for
samples containing more than 15 mg/I dissolved oxygen or which have a high
content of organic matter, such as domestic sewage. It uses an alkalinc-iodide
solution that is 6N in sodium iodide and ION in sodium hydroxide. This solution is
saturated and provides sufficient iodide for oxygen-enriched samples. K I cannot be
used because of limited solubility.
There are several arguments in fa vor of increasing the iodide concentration as is done
in the Pomeroy-Kirschman-Alsterberg modification above. It is known that an excess
of oxalate in the permanganate method causes low results, since at the time of
Exercise 6
Iron affects the measurement of dissolved oxygen in two ways [e.g., Buswell and
Gallaher (1923)]. First, when present in large amounts, iron requires so much of an even
stronger oxidant to oxidize it from the ferrous condition that the dissolved oxygen in
the reagent itself is appreciable; second, when oxidized to the ferric state it serves as an
oxidizing agent itself and will oxidize the iodide ion to free iodine, which in turn will
give high values of dissolved oxygen:
In the Rideal-Stewart modification, the ferrous iron and some of the nitrites and
organic matter are first oxidized by potassium permanganate, and then excess
permanganate is removed with potassium oxalate. Care must be taken not to add too
great an excess of the oxalate or error will be introduced into the final result. The
addition of potassium fluoride as the first reagent, or after the Rideal-Stewart treatment
for ferrous ion, will eliminate this interference as the more active fluoride ions will be
liberated rather than iodine.
The extent of interference by organic matter, which causes low results, varies with the
technique of the procedure. If the precipitate of manganous and manganic hydroxide
settles and compacts on the bottom of the bottle, and if the acid is added without
prompt and complete mixing, local depletion of iodide occurs, thus permitting
more extensive oxidation of organic matter. Interference is minimized by adding the
acid promptly and mixing rapidly.
In addition to the Winkler method, which should be used in measuring the dissolved
oxygen of relatively pure water, there are several modifications which are described in
detail in American Public Health Association et al. (1989):
1. Winkler method, unmodified: It should be noted that 1 ml of each reagent is
recommended in 250- to 300-ml bottles (Welch, 1948).
2. Alsterberg (Azide) modification: This modification is used for water containing
more than 0.05 mg/l ferrous iron. Other reducing or oxidizing agents should be
absent. If 1 ml of potassium fluoride solution were added to the sample before
acidifying and there were no delay in titration, the method also would be applicable
in the presence of 100 to 200 mg/l ferric iron.
3. Rideal-Stewart (permanganate) modification: This modification should be used
only on samples containing ferrous iron. It is ineffective for the oxidation of sulfates,
thiosulfates, and polythionates, or of the organic matter in sewage.
4. Alum flocculation modification: Samples high in suspended solids may absorb
appreciable quantities of iodine in acid solution. This interference may be removed
by alum flocculation.
5. Copper sulfate-sulfamic acid flocculation modification: This modification is used for
biological flocs such as activated sludge mixtures, which have high rates of oxygen
consumption.
6. Pomeroy-Kirschman-Alsterherg modification: This modification is designed for
samples containing more than 15 mg/I dissolved oxygen or which have a high
content of organic matter, such as domestic sewage. It uses an alkalinc-iodide
solution that is 6N in sodium iodide and ION in sodium hydroxide. This solution is
saturated and provides sufficient iodide for oxygen-enriched samples. K I cannot be
used because of limited solubility.
There are several arguments in fa vor of increasing the iodide concentration as is done
in the Pomeroy-Kirschman-Alsterberg modification above. It is known that an excess
of oxalate in the permanganate method causes low results, since at the time of