Dissolved Oxygen in a water sample
The
amount of oxygen present in water is called as Dissolved Oxygen (D.O). The
Principle involved in the determination of Dissolved Oxygen (D.O) as about the
oxidation of KI to I2 with dissolved oxygen present in water sample,
after addition of MnSO4, KOH and KI. The basic manganic oxide
MnO(OH)2 formed acts as an oxygen carrier to enable the dissolved
oxygen in the molecular form to take part in the reaction. The dissolved oxygen
(D.O) in water oxidized KI to I2 and liberated I2 is
titrated with (N/10) Na2S2O3 solution.

PRINCIPLE :-
Oxygen
present in water oxidises Mn+2 to Mn+4 in presence of
alkali iodide-azide solution and these Mn+4 in presence liberates I2
from KI in acidic medium. The amount of I2 liberated is equivalent
to dissolved oxygen O2 present in water and the liberated I2
is estimated by titration with Na2S2O3
solution using starch as indicator.
MnSO4+2KOH
®Mn(OH)2+H2SO4
2Mn(OH)2
+H2SO4®2MnO(OH)2[basic
manganic oxide]
MnO(OH)2+H2SO4®
MnSO4+2H20+[O]
2KI
+H2SO4+[O]®K2SO4+H20+I2
I2+2Na2SO3®Na2S4O6+2NaI
Starch+I2®Coloured
complex
CHEMICALS :-
- Water sample
- MnSO4 solution
- Alkali-iodide-azide solution(NaOH,KI,NaN3)
- Conc.H2SO4 (v)(N/10)Na2SO3 solution
- Starch indicator
APPARATUS :-
- Conical flask
- Burette
- Measuring cylinder
- Beaker
- Volumetric flask
- Funnel
- BOD bottle
- Glass rod etc.
PROCEDURE :-
About
200 m.l of water sample is taken in a BOD bottle(~300 m.l capacity).To it 1m.l of
alkali-azide-iodide solution is added and stoppered the bottle immediately and
shaken well. Appearance the brown ppt. of manganic oxide indicates the presence
of Dissolved Oxygen. The ppt was allowed to settle down. Then add 1 m.l conc.H2SO4
to dissolve the ppt.Now, total solution is taken into a conical flask and liberated I2 is titrated
with (N/10) Na2S2O3 solution, until the colour
of the solution turns pale yellow.2 m.l of starch solution (indicator) is added
and the titration is continued. The end point is colourless.
OBSERVATION :-
Table:1, Estimation of Dissolved Oxygen in H2O
CALCULATION :-
The amount of Dissolved Oxygen in the supplied water sample = V X Conc. of Na2S2O3 X 8 X 1000/Total volume of sample
Where, V = Volume of Na2S2O3
solution Conc. Of Na2S2O3 = (N/10)=0.1(N)
8
is the equivalent wt .of O2
Total volume of sample = 203 m.l
PRECAUTION:-
- After the addition of reagents in water sample taken in BOD bottle there should not be any air bubble present because it will give high Dissolved Oxygen value.
- As far as possible, the sample should not be allowed to come in contact with air.
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