Dissolved Oxygen

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 :-

WINKLER METHOD :

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:-

1.  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.
2. As far as possible, the sample should not be allowed to come in contact with air.