Tuesday, September 15, 2020

Dissolved Oxygen

Determination of dissolved oxygen in a water sample

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.

Dissolved oxygen title image



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


Starch+I2®Coloured complex


  • Water sample
  • MnSO4 solution
  • Alkali-iodide-azide solution(NaOH,KI,NaN3)
  • Conc.H2SO4 (v)(N/10)Na2SO3 solution
  • Starch indicator


  • Conical flask
  • Burette
  • Measuring cylinder
  • Beaker
  • Volumetric flask
  • Funnel
  • BOD bottle
  • Glass rod etc.


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.


Table:1,  Estimation of Dissolved Oxygen in H2O

Dissolved oxygen table image


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 Na2S2O= (N/10)=0.1(N)

                                                            8 is the equivalent wt .of O2

Total volume of sample = 203 m.l


  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.

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