Dissolved Oxygen
71
Four moles of thiosulfate are titrated for each mole of molecular oxygen (0 2 ), Thus
1 ml of 0.025 M sodium thiosulfate is equivalent to 0.025 meq of oxygen. This value
commonly is multiplied by 8 mg/meq to convert to mg O 2 , When 200 ml of the original
sample is titrated, then 1 ml 0.025 M Na 2 S 2 0 3 = 1 mg dissolved oxygen/l.
The iodine should be distributed uniformly throughout the bottle before decanting
the amount needed for titrating. The volume decanted should corespond to the volume
of the original sample. Experienced analysts can detect the visual endpoint of the
titration with a precision of ± 50 j.lg/l (American Public Health Association, 1989). A
correction for the dilution of sample with the reagents can be made when high accuracy
is required. Thus, when a total of 2 ml, 1 ml each of the manganous sulfate and alkalineiodide reagent, is added to a 300-ml bottle, the volume taken for titration should be
300
100 x 300 _ 2
300
100.7ml or 200x 300 _ 2 =201.3ml
If the sodium thiosulfate were other than 0.025M, ifthe bottles were of another size, or if
a different volume were to be titrated, the appropriate correction factor must be
computed and applied to the calculation.
If the results were desired in cubic centimeters of oxygen gas, the value in mg/l should
be multiplied by 0.698.
The amount of dissolved oxygen is distilled water (lOOml) can be calculated with a
precision expressed as a standard deviation of about 0.043 ml of 0.025M sodium
thiosulfate; in sewage and secondary effluents the precision is about 0.058 ml. In the
presence of appreciable interferences, even with the proper Winkler modification, the
standard deviation may be as high as 0.1 ml. Even greater errors may occur in waters
containing suspended organic solids or in heavily polluted waters. Large errors may be
introduced in the measurement of dissolved oxygen by neglecting proper precautions
in the presence of interfering substances such as nitrites, iron salts, and organic matter,
which are common in natural waters, or by an improper application of the various
modifications of the Winkler method that are designed to overcome these interferences.
Errors from nitrites are introduced at the time the solution is made acidic with
sulfuric acid. In an acid medium, nitrites react with the potassium iodide, liberating
iodine:
2KI + H 2S04 ---+2HI + K 2S04
2HN0 2 + 2HI ---+2H 2 0 + N 20 2 + 12
If the reaction were complete at this point, the error due to the presence of nitrites in
most cases would not be significant. However, if the sample were allowed to stand
exposed to the air, the dissolved oxygen would react with N 2 0 2 , again producing the
nitrite:
2N20 2 + 2H 2 0 + O 2 ---+4HN0 2
This reaction will again liberate more iodine. Should this cycle be repeated a significant
number of times, the error introduced would soon become very large. The continuous
reaction can be minimized by an immediate and rapid titration of the sample after
exposure to the air. The effect of nitrites is eliminated when sodium azide is added to the
sample along with the alkaline potassium iodide. The reactions are as follows:
2NaN3 + H 2S04 ---+2HN3 + Na2S04
HN02 + HN3 ---+N20 + N2 + H 20
71
Four moles of thiosulfate are titrated for each mole of molecular oxygen (0 2 ), Thus
1 ml of 0.025 M sodium thiosulfate is equivalent to 0.025 meq of oxygen. This value
commonly is multiplied by 8 mg/meq to convert to mg O 2 , When 200 ml of the original
sample is titrated, then 1 ml 0.025 M Na 2 S 2 0 3 = 1 mg dissolved oxygen/l.
The iodine should be distributed uniformly throughout the bottle before decanting
the amount needed for titrating. The volume decanted should corespond to the volume
of the original sample. Experienced analysts can detect the visual endpoint of the
titration with a precision of ± 50 j.lg/l (American Public Health Association, 1989). A
correction for the dilution of sample with the reagents can be made when high accuracy
is required. Thus, when a total of 2 ml, 1 ml each of the manganous sulfate and alkalineiodide reagent, is added to a 300-ml bottle, the volume taken for titration should be
300
100 x 300 _ 2
300
100.7ml or 200x 300 _ 2 =201.3ml
If the sodium thiosulfate were other than 0.025M, ifthe bottles were of another size, or if
a different volume were to be titrated, the appropriate correction factor must be
computed and applied to the calculation.
If the results were desired in cubic centimeters of oxygen gas, the value in mg/l should
be multiplied by 0.698.
The amount of dissolved oxygen is distilled water (lOOml) can be calculated with a
precision expressed as a standard deviation of about 0.043 ml of 0.025M sodium
thiosulfate; in sewage and secondary effluents the precision is about 0.058 ml. In the
presence of appreciable interferences, even with the proper Winkler modification, the
standard deviation may be as high as 0.1 ml. Even greater errors may occur in waters
containing suspended organic solids or in heavily polluted waters. Large errors may be
introduced in the measurement of dissolved oxygen by neglecting proper precautions
in the presence of interfering substances such as nitrites, iron salts, and organic matter,
which are common in natural waters, or by an improper application of the various
modifications of the Winkler method that are designed to overcome these interferences.
Errors from nitrites are introduced at the time the solution is made acidic with
sulfuric acid. In an acid medium, nitrites react with the potassium iodide, liberating
iodine:
2KI + H 2S04 ---+2HI + K 2S04
2HN0 2 + 2HI ---+2H 2 0 + N 20 2 + 12
If the reaction were complete at this point, the error due to the presence of nitrites in
most cases would not be significant. However, if the sample were allowed to stand
exposed to the air, the dissolved oxygen would react with N 2 0 2 , again producing the
nitrite:
2N20 2 + 2H 2 0 + O 2 ---+4HN0 2
This reaction will again liberate more iodine. Should this cycle be repeated a significant
number of times, the error introduced would soon become very large. The continuous
reaction can be minimized by an immediate and rapid titration of the sample after
exposure to the air. The effect of nitrites is eliminated when sodium azide is added to the
sample along with the alkaline potassium iodide. The reactions are as follows:
2NaN3 + H 2S04 ---+2HN3 + Na2S04
HN02 + HN3 ---+N20 + N2 + H 20