References:- 1) Standard Methods for the examination of water & wastewater.
by A.P.H.A. ( 16th Edition - 1985.)
2) Chemistry for Environmental Engineering.
By Sawyer, Mccarty & Parkin (4th Edition – 1988.)

Aim:- To determine Dissolved Oxygen content in given water sample.

Reagents:-

Manganous sulphate
Alkali-Iodide Azide solution
Concentrated sulphuric acid
Starch indicator
Sodium thiosulphate (0.025 N) Na2S2O3
Potassium Dichromate solution (0.025N)

Significance:-

Dissolved Oxygen measurement is important for maintaining aerobic conditions in aerobic biological treatment units.
Determination of dissolved Oxygen is the basis of the BOD test.
Dissolved Oxygen values are used to control corrosion of steel pipes.
Dissolved Oxygen determination at various points along a river course gives pollutional status of the river, Dissolved oxygen level of more than
4 mg/litre is desirable for existence & growth of aquatic life such as fish.

Theory :-

Solubility of atmospheric oxygen in water depends on altitude (atmospheric pressure), temperature & salt concentration in water. At 00c, 200c & 350c saturation. Dissolved oxygen concentration is 14.62, 9.17 & 7.0 mg/litre, respectively in clean water at 1.0 atmospheric pressure.


Higher temperature, higher salt concentration, higher altitude and higher organic matter content reduce the dissolved oxygen in water.

Principles: -

When manganese sulphate solution is added to sample followed by strong alkali D.O. rapidly oxidizes an equivalent amount of the dispersed divalent manganous ions, which precipitate to hydroxides of higher valency state. In the presence of iodide ion in an acidic solution the oxidized manganese reverts back to divalent state with the liberation of iodine equivalent to the original D.O. contents. The iodine is then titrated with Thiosulphate of iodine equivalent to the original D.O. contents. The iodine is then titrated with Thiosulphate.

Collection and preservation of sample (D.O. fixation) :-

While monitoring the river water samples or wastewater samples in treatment plants, it is not convenient to determine dissolved oxygen in field. As DO values change with time during transit from field to laboratory, due to variation in temperature and biological action, it is necessary to fix DO in the samples at the site. DO fixation is done by adding 2ml of alkali-iodide-azide and 2ml of Manganous Sulphate solution at site after collecting the sample in BOD bottle. Remaining procedure is done in the laboratory for best results; the samples may be stored below 10 0c during transit and titrated within 6 hours after fixation.

Procedure :-

A) Standardization of Sodium Thiosulphate: -

Necessity: - Standardisation of Sodium Thiosulphate is required because it’s Normality changes with time. K2Cr2O7 is used as primary standard for the determination of normality of Na2S2O3.
1) Place 2gm of KI in a flask.
2) Add 100ml distilled water.
3) Add 10ml H2SO4 (1+9)
4) Add 10 ml of standard potassium dichromate solution (0.025N)
5) Again add 100ml-distilled water.
6) Titrate against sodium thiosulphate using starch indicator.

Normality of Sodium Thiosulphate = (0.025 x 10) / ml of titrant required

B) DO Determination (Winkler Method): -

1) Fill the BOD bottles (300ml) without any turbulence and stopper the bottle.
2) Remove the stopper and first add 2ml of MnSO4, and then add 2ml of alkali- iodide-Azide using a
pipette below liquid level in BOD bottle. Stopper the bottle.
3) Formation of white precipitate indicates, absence of DO. If DO is present there will be brown
precipitate.
4) Remove the stopper and add 2ml of concentrated H2SO4 (36N) below liquid level in B.O.D.bottle.
Stopper the bottle and shake the bottle until the brown precipitate completely dissolves to give a
uniformly yellow colored solution. Yellow color is due to formation of free iodine from oxidation
of iodide.
5) Take 203ml solution from 300ml BOD bottle. Extra 3ml is the correction for 4ml of
added reagents.
300x200/(300-4) = 203ml
6) Titrate it with Sodium thiosulphate using starch indicator, until blue colour changes
to colourless.

Chemical Reactions :-

MnSO4 + 2 KOH - Mn(OH)2 + K2SO4
(White ppt)

2 Mn(OH)2 + O2 - MnO(OH)2 (Magnus Oxy Hydroxide)
(brown ppt)

MnO(OH)2 + 2 H2SO4 - Mn(SO4)2 + 3H2O

Mn(SO4)2 + 2 KI - K2SO4 + MnSO4 + I2

I2 liberated = Dissolved Oxygen present in water sample.

2Na2S2O3 + I2 - 2NaI + Na2S4O6










Observation Table :-

Sr. No.
Volume of Na2S2O3 (ml)

D.O. (mg/litre)
1



2



3



Calculation:

D.O = (A x N x 8 x 1000)/ ml of sample taken
(mg/litre )
where, A = ml of sodium thiosulphate
N = Normality of Sodium thiosulphate

Result:





Assignment:-



1) Name the titrant and indicator used in the method of DO determination.
2) What colour change is observed at the end point in the DO determination?
3) What are the factors which affect the level of DO in the sample?
4) What is the saturated DO value of pure water at 00c, 200c & 300c?
5) What do you mean by the term oxygen deficit?
6) What conclusion can be drawn, if after addition of MnSO4 & Alkali iodide Azide in
the BOD bottle-
a) White ppt. is formed-
b) Brown ppt. is formed-
7) Whether Winkler’s method of DO determination is direct or indirect one? Why?
8) Why fixation of DO is necessary for the sample collected from the site of
ETP/STP?
9) What are the precautions that should be observe while conducting the BOD test?
10) What is the principle of Determination of DO?