3.10 Analytical Methods
57
W 2 = weight of filter paper including filtrates after filtration in gm.
3.10.7 Determination of Dissolved Oxygen (DO)
Dissolved oxygen (DO) is measured by azide modification of Winkler’s method based
on iodometric principle. Oxygen oxidizes Mn
2+ to a higher state of valency (+4) under
alkaline conditions. If no oxygen is present, a white precipitate of Mn(OH) 2 is formed.
In the presence of oxygen, Mn
2+ is stoichiometrically oxidized to MnO 2 , which forms
brown hydrated oxide at high pH. This step is called fixation of oxygen. Addition of
H 2 SO 4 to sample in the presence of iodide results in the oxidized manganese (Mn
4+ )
to revert to the divalent state, with the liberation of iodine equivalent to the original
DO content. Liberated iodine is titrated against N/40 sodium thiosulfate solution.
Certain oxidizing agents such as nitrate and ferric ions are capable of oxidizing I to
I 2 and thus interfere with the measurement. Nitrite interference can be overcome by
introducing sodium azide, which reacts with nitrite under acidic conditions to form
N 2 , N 2 O and H 2 O.
For measurement of DO, remove the stopper and add 1 mL of MnSO 4 solution,
followed by 1 mL of alkali-iodide-azide solution by inserting the pipette well below
the surface of the liquid. Put the stopper with care to exclude air bubbles completely
and mix by inverting the bottle several times. When precipitate has settled sufficiently
to leave a clear supernatant above manganese hydroxide floc, add 1 mL conc. H 2 SO 4 .
Stopper was put again and mixed by inverting several times until dissolution of the
precipitate is complete and there is uniform distribution of liberated iodine. Titrate a
volume (201 mL) corresponding to 200 mL of the original sample after correction for
sample loss by displacement with reagents. Titration with standard N/40 Na 2 S 2 O 3
solution is done to a pale straw color, and then, a few drops of starch solution is
added to proceed titration up to the first disappearance of the blue color. DO in mg/l
is determined from the volume (mL) of the standard Na 2 S 2 O 3 solution consumed in
titration.
3.10.8 Ammonium Nitrogen (NH4 + -N)
50 mL of sample or its suitable aliquot was placed in 800-mL Kjeldahl flask. The
aliquot was diluted to 300 mL in the flask. 25 mL of borate buffer and 6(N) NaOH
were added until the content in the flask is reduced to pH 9.5. By putting the glass
beads, the sample solution in the flask was boiled. In the distillation process, a
conical flask containing 50 mL of boric acid was kept at the receiving end. Now, the
distillation of the sample solution was continued till the volume of the conical flask
reached about 250 mL. Then, the content of the conical flask is titrated with N/50
conc. H 2 SO 4 till the end point color changes from green to blue, and the ammonium
nitrogen was calculated as follows:
57
W 2 = weight of filter paper including filtrates after filtration in gm.
3.10.7 Determination of Dissolved Oxygen (DO)
Dissolved oxygen (DO) is measured by azide modification of Winkler’s method based
on iodometric principle. Oxygen oxidizes Mn
2+ to a higher state of valency (+4) under
alkaline conditions. If no oxygen is present, a white precipitate of Mn(OH) 2 is formed.
In the presence of oxygen, Mn
2+ is stoichiometrically oxidized to MnO 2 , which forms
brown hydrated oxide at high pH. This step is called fixation of oxygen. Addition of
H 2 SO 4 to sample in the presence of iodide results in the oxidized manganese (Mn
4+ )
to revert to the divalent state, with the liberation of iodine equivalent to the original
DO content. Liberated iodine is titrated against N/40 sodium thiosulfate solution.
Certain oxidizing agents such as nitrate and ferric ions are capable of oxidizing I to
I 2 and thus interfere with the measurement. Nitrite interference can be overcome by
introducing sodium azide, which reacts with nitrite under acidic conditions to form
N 2 , N 2 O and H 2 O.
For measurement of DO, remove the stopper and add 1 mL of MnSO 4 solution,
followed by 1 mL of alkali-iodide-azide solution by inserting the pipette well below
the surface of the liquid. Put the stopper with care to exclude air bubbles completely
and mix by inverting the bottle several times. When precipitate has settled sufficiently
to leave a clear supernatant above manganese hydroxide floc, add 1 mL conc. H 2 SO 4 .
Stopper was put again and mixed by inverting several times until dissolution of the
precipitate is complete and there is uniform distribution of liberated iodine. Titrate a
volume (201 mL) corresponding to 200 mL of the original sample after correction for
sample loss by displacement with reagents. Titration with standard N/40 Na 2 S 2 O 3
solution is done to a pale straw color, and then, a few drops of starch solution is
added to proceed titration up to the first disappearance of the blue color. DO in mg/l
is determined from the volume (mL) of the standard Na 2 S 2 O 3 solution consumed in
titration.
3.10.8 Ammonium Nitrogen (NH4 + -N)
50 mL of sample or its suitable aliquot was placed in 800-mL Kjeldahl flask. The
aliquot was diluted to 300 mL in the flask. 25 mL of borate buffer and 6(N) NaOH
were added until the content in the flask is reduced to pH 9.5. By putting the glass
beads, the sample solution in the flask was boiled. In the distillation process, a
conical flask containing 50 mL of boric acid was kept at the receiving end. Now, the
distillation of the sample solution was continued till the volume of the conical flask
reached about 250 mL. Then, the content of the conical flask is titrated with N/50
conc. H 2 SO 4 till the end point color changes from green to blue, and the ammonium
nitrogen was calculated as follows: